[advances in food research] advances in food research volume 6 volume 6 || tunnel dehydrators for...

Tunnel Dehydrators for Fruits and Vegetables

BY P . W . KILPATRICK. E . LOWE. AND W . B . VAN ARSDEL

Western Utilization Research Branch. Agricultural Research Service. U . S . Department of Agriculture. Albany. California

Page I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

1 . The Development of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . 314 2 . Production of Dehydrated Fruits and Vegetables . . . . . . . . . . . . . . . . . . . 315

I1 . Classification of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . 316 1 . General Discussion, Characteris d Arrangements . . . . . . . . . . . 316 2 . Longitudinal Air Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

a . Counterflow Circulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 b . Parallel-Flow Circulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 c . Two-Stage Tunnels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

3 . Transverse Air Circulation 324 a . Combination Compartment and Tunnel . . . . . . . . . . . . . . . . . . . . 324

4 . Other Tunnel Arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 I11 . Mechanical Elements of Tunnel Construction . . . . . . . . . . . . . . . . . . . . . . . . . 326

1 . Fans and Blowers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 2 . Heating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 3 . Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 4 . Materials of Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 5 . TraysandTrucks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

IV . Typical Commercial Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 1 . Twin-Tunnel Counterflo-w Dehydrator . . . . . . . . . . . . . . . . . . . . . . . . . . 339 2 . The Miller Tunnel Dehydrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 3 . The Carrier Compartment Drier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

V . Criteria for Selection of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . 345 VI . Basic Theory of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

1 . Theoretical Tunnel Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 2 . Optimum Tray-Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 3 . Optimum Recirculation of Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 4 . Product Temperature in the D rator . . . . . . . . . . . . . . . . . . . . . . . . . 359 5 . Departures from Theory 362

VII . Operating Procedures for Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . 363 VIII . Recent Trends in Tunnel Dehydration of Fruits and Vegetables . . . . . . . . . 367

IX . List of Symbols Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

313

314 P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

I. INTRODUCTION

This discussion of tunnel dehydrators, as used to dehydrate certain fruits and vegetables, is intended to provide a general introduction to the subject, primarily for the use of students, food technologists, and engineers. Although i t goes into a number of matters concerned with the design and operation of this kind of equipment, i t is in no sense a manual either of design or of operation. Emphasis is laid on discussion of under- lying principles and the more recent advances in application of these principles. An effort has been made to bring together published information from many different sources, some of which are not widely available. Unfortunately, i t has not been possible for the authors to survey publica- tions in other than English-language journals and books.

1. The Development of Tunnel Dehydrators

The germ of the idea of the tunnel dehydrator is a t least a century old. Various features of present-day, typical tunnels were undoubtedly added one a t a time to as simple a basic idea as that described by Yule (1845) in an English patent for “improvements in preserving animal and vegetable matters.” Yule, a “preserved provision manufacturer,” placed the cooked or uncooked animal or vegetable product on shelves in “ a chamber of oblong form,” and passed through the chamber a current of air which had been dried by passage through a receptacle of lump calcium chloride or other chemical absorbent of moisture. Yule says nothing in his patent about heating the air stream, but Prescott and Proctor (1937) say that Eisen, in 1795, dried vegetables on racks arranged around a stove in a dry-room, so Yule undoubtedly was acquainted with warm-air drying. No records have come to light about the kind of equipment used to dry the dehydrated vegetables used by the Union Army in the Civil War. Something strongly resembling a counterflow tunnel dehydrator was being used later in the 19th century to dry glue, according to Thorp (1905). Cruess (1938) says the Oregon tunnel drier was invented by Allen about 1890; this drier, which consists of a long sloping box with spaced ledges on the interior vertical sides to support trays of fruit, mas originally ventilated only by convection of warm air from a furnace room a t the lower end, but this was later supplanted by fan ventilation. The trays of fruit were pushed downhill through this tunnel, counterflow to the air movement. Still later, as described by Wiegand (1923)) the design was modified to permit recirculation of part of the air, and finally the tunnels were constructed level and wheeled trucks replaced the sliding trays.

During the First World War a considerable flurry of interest in vegetable dehydration had led to the construction and operation of a number

TUNNEL DEHYDRATORS FOR FRUITS AND VEGETABLES 315

of dehydration plants. Some of the driers were extremely elaborate and complex, and were abandoned at the end of the war. Soon thereafter, however, untimely rains in the prune-drying area of California led to a great demand for practical farm fruit dehydrators. Within the next few years the work of investigators like Cruess (1919) and Cruess and Christie (1921a,b), engineers like Ridley (1921), and designers and builders like Chapman (1922a,b), Rees (1922), Puccinelli (1923), and Pearson (1923) resulted in the rapid development of simple tunnel dehydrators basically similar to those used today. The design of the Oregon tunnel had con- verged toward a similar pattern. Many hundreds of these dehydrators went into regular use in central California, Oregon, and Washington for drying fruits. A few plants gradually built up a steady business in dehydrated onions, garlic, peppers, and several other vegetables.

A number of years later Eidt (1938) described two-stage tunnels which had been designed and built in the Canadian Maritime Provinces for use in dehydrating apples. With the outbreak of the European war in the following year the British Ministry of Foods (1946), after intensive investigation, decided upon a two-stage tunnel dehydrator for its emer- gency vegetable dehydration plants. The same pattern was followed in most of the British Commonwealth countries. When the United States entered upon its own wartime dehydration program, individual operators were left free to select the dehydration system they thought most suitable. Some of them adapted existing fruit dehydrating tunnels to the faster evaporation rates obtainable from cut vegetables. Many new tunnels of the same basically simple design were built. Several of the larger plants installed two-stage tunnels, others purchased the more elaborate multistage transverse-flow “compartment tunnels.” At the end of the war, all three of these types were operating successfully in the United States. Today simple counterflow tunnels handle most of the prune and raisin dehydration and a considerable proportion of the apple dehydration; counterflow or two-stage tunnels are used for most of the current vegetable dehydration. The authors do not know of any count of the dehydration tunnels now in use in the United States, but the number must be several thousand. The vast majority of these are used for drying fruit.

2. Production of Dehydrated Fruits and Vegetables

Raisins, prunes, and apples make up by far the bulk of the dried fruits. Although most raisins are still sun-dried in California, golden-bleached raisins are dehydrated, and during the war there was extensive building of dehydrators for the Thompson Seedless grapes which are the main source of ordinary raisins. The annual production of raisins varies from about 150,000 t o about 400,000 tons.


Substantially all of the American prunes are dehydrated. The production has declined somewhat since the early 1930’s, but is fairly steady around 200,000 tons.

The production of dehydrated apples, peaches, and pears has gradually declined from peaks reached in the 1920’s and ’30’s. About 15,000- 20,000 tons of dried apples, something less than 10,000 tons of dried peaches, and 1000-2000 tons of dried pears have been produced during recent years. All of the dried apricots, and most of the dried peaches, figs, and pears are processed by sun-drying, rather than dehydrating; on the other hand, all of the dried apples are dehydrated.

The production of dehydrated vegetables, in contrast to the production of dried fruits, has fluctuated widely in response to demands brought about by wartime emergencies. Total United States production in 1941 is estimated by Rasmussen and Shaw (1953) to have been 13,000,000 lb.; only 3 years later it had increased 15-fold1 to 209,000,000 lb. In another 2 years it dropped back to 55,000,000 lb. The European crisis of 1948 boosted it steeply, and 3 years later the Korean war produced another upsurge. The production of 60,000,000 lb. in 1950 was composed 36 of potatoes, 15 % of onions and garlic, 13 % of peppers, and the remainder distributed between many vegetables. Except for the fairly large proportion of mashed potato powder (“potato granules”) in this 1950 production, nearly all of the product was made in tunnel dehydrators. A growing demand for high-quality dehydrated vegetables in a variety of processed foods (canned hashes and stew, catsup, cottage cheese, “ a la king” products, meat pies, etc.) has resulted in a steady and diversified post-war growth of the civilian market.

11. CLASSIFICATION OF TUNNEL DEHYDRATORS

I. General Discussion, Characteristics, and Arrangements

The fruit and vegetable dehydration industries, both in the United States and the British Commonwealth countries, have used the tunnel drier far more extensively than any other type of dehydrator. Tunnel dehydrators, as a class, are frequently called tunnel-and-truck, truck-and- tray, or simply tunnel driers. Principles pertaining to their use have been discussed by Van Arsdel (195la,b) and Perry and associates (1946). Abstracts from these sources have been freely incorporated into the material contained in this part of the chapter. Likewise, Figures 1 through 7 have been reproduced from these same sources.

All tunnel-and-truck dehydrators for fruits and vegetables have a common feature which distinguishes them from other kinds of drier. This characteristic is the method of handling the commodity. Normally, the


prepared commodity in piece form is spread thinly on special trays fabricated of wood or metal. Depending on the commodity, tray loading for vegetables may range from 1 t o 3 lbs. per sq. ft.; for fruit, theloading may be in the range of 1 to 5 lbs. per sq. f t . Loaded trays are then stacked, one above another, on a portable low-bed truck or dolly. Height of the stacked trays may range from about 5 to 7 ft., depending on operating conditions.

SIDE EXIT DIRECTION OF AIR FLOW ,FOR DRY TRUCKS -

I BLOWER’ WET TRUCKS TRUCKS PROGRESS IN

FIG. 1. Simple counterflow tunnel (elevation) (from Iran Arsdel, 1951b, Fig. I). INSERTED THIS DIRECTION

The trays are so designed that, when loaded and stacked, there is a clear air passage left between successive trays. The loaded trucks are pushed, one a t a time, into one end (usually called the “wet end” or loading end) of the dehydrator’s drying section. The drying section or “tunnel” is a straight passageway with a cross-section just large enough to accommodate the loaded trucks. Tunnel lengths vary; some may hold only 4 or 5

EXHAUST SIDE ENTRANCE

FOR WET TRU

TRUCKS PROGRESS IN THIS DIRECTION

FIG. 2. Simple parallel-flow tunnel (elevation) (from Van Arsdel, 1951b, Fig. 2).

trucks, whereas others may contain as many as 15 to 20. During operation, a truck of dried material is removed from the “dry end” of the tunnel, the remaining trucks are pushed forward one truck length, and a truck of wet material is rolled into the vacant space a t the “wet end” of the tunnel. It is obvious that operation is only quasi-continuous (not truly continuous as it would be in the case of most conveyer driers), and this is known as “progressive” operation. Primarily, the flow of hot air used for drying is across the horizontal surface of the layer of wet material. Very little air circulates through the layer of wet material while it is in a tunnel drier. This is‘distinctly different from the air flow in most conveyer driers in which the air flows up (or down) through the product layer.

I n commercial use, there are 3 basic arrangements of tunnel driers,


plus several different combinations of these basic arrangements. The essential difference between the various types is mainly the direction of air flow relative to truck movement through the tunnel. The three basic types are illustrated by the simplified sketches shown in Figs. 1 to 3, and will be referred to later. Actually, all three types are much more compli- cated than indicated, since working units have provisions for recirculating

H,E ATE TS

0 WET TRUCK ___.

TRUCKS PROGRESS I N T H I S DIRECTION EXHAUST

AIR

0- -HEATER

-FRESH AIR INLET

‘BLOWER

FIG. 3. Simple combination compartment and tunnel (plan view) (from Van Arsdel, 1951b, Fig. 3).

part of the drying air. The paragraphs which follow describe the counterflow, parallel-flow, and compartment tunnels, as well as combinations of the parallel and counterflow units (two-stage tunnels) and other tunnel arrangements.

2. Longitudinal A i r Circulation

a. CounterJlow Circulation. I n the counterflow tunnel (Fig. l), the hot drying air is blown into the dry end of the tunnel and moves straight through it, in a direction opposite t o the movement of material being dried. The (‘wet” air is discharged a t the wet end of the tunnel where the prepared fruit or vegetable enters. In actual operation, in order t o increase fuel economy, or to raise the air humidity in the tunnel, provisions are made for recirculating a part of the air discharged from the wet end. As the hot air passes through the line of loaded trucks, i t picks up moisture from the fruit or vegetables on the trays, and in so doing the air becomes cooler. I n the counterflow tunnel, the warmest, driest air comes in contact with the nearly dry product while the cooler, more humid air is in contact with the wet material entering the tunnel. The maximum air temperature which can be used is determined by the commodity being dried, and is that temperature which the nearly dried product will tolerate for several hours without perceptible damage. In the counterflow tunnel, the best conditions for drying are a t the end of the tunnel where the product is nearly dry. Reasonably good drying conditions can be secured at the wet


end of the tunnel if “light tunnel loading” is used. (Light tunnel loading refers to a suitable balance between mass air flow rate, air temperature, and total water evaporated per unit of time, so that air a t the wet end of the tunnel has a reasonably high evaporative capacity, i.e., a wet-bulb depression of a t least 15 t o 25” F. for most commodities.)

Industry has used several different arrangements of the counterflow tunnel. If production capacity requires more than one such tunnel, the

TRUCKS

I


RECIRCULATION

FIG. 4. Direct-fired twin counterflow tunnels (plan view) (from Van

DAMPERS J 1951b, Fig. 4).

FRESH AIR

Arsdel,

\SIDE ENTRANCE \SIDE EXIT FOR FOR WET TRUCKS DRY TRUCKS

FIG. 5. Side-entrance counterflow tunnel (elevation) (from Van Arsdel, 1951b, Fig. 5 ) .

initial investment in tunnel cost can be kept a t a minimum by using a common blower, heater, and recirculation return for two tunnels. Such an arrangement (see Fig. 4) is known as the “twin tunnel.”

Arrangements such as the one illustrated in Fig. 4 sometimes prove unsatisfactory because of uneven air distribution. To correct this difficulty, a further modification of the basic counterflow design (using doors in the side walls of the dehydrator near the tunnel ends through which the trucks are pushed) is shown in Fig. 5. This side entry principle was


used for many two-stage dehydrators operating in Great Britain during World War 11. However, the side entrance and exit method adds to the complexity of truck movement, requires additional tunnel length, and increased floor space.

b. Parallel-Flow Circulation.. In the parallel-flow tunnel (Fig. 2), the air moves straight through the tunnel in the same direction as that of truck movement or progression. The name “concurrent,” instead of parallel-flow, is usually used in the British Commonwealth countries for this type of tunnel arrangement.

The parallel-flow tunnel is very similar to the counterflow unit in the general arrangement and layout. Basically, the only difference is that the product loading and unloading ends are interchanged, resulting in a reversal of the direction of truck travel with respect to air movement. With these changes in mind, Figs. 4 and 5 are applicable to either parallel- flow or counterflow tunnels.

There are marked differences in the behavior of parallel-flow and counterflow tunnels. For example, if prepared vegetables are dehydrated in a parallel-flow tunnel, difficulty may be encountered in drying the product sufficiently to assure satisfactory stability in subsequent storage. On the other hand, in dehydrating a whole fruit, for example prunes, a parallel-flow tunnel may cause cracking of the skins and excessive loss of juice. These problems do not arise in proper counterflow tunnel operation. Nevertheless, the parallel-flow tunnel can be and is used very satisfactorily if operated in conjunction with an auxiliary or finishing drier. The following facts explain why it is impractical t o use the parallel-flow tunnel by itself for the dehydration of fruits and vegetables.

The hot drying air entering the parallel-flow tunnel comes in contact with the very wet product at the loading end. As drying progresses, the wet product is warmed up by contact with the hot air. At the discharge end of the tunnel, the relatively dry product is in contact with moisture laden air which has been greatly cooled and has a very low evaporative capacity. Thus it is difficult t o dry a commodity to reasonably low moisture levels in a parallel-flow tunnel.

Let us now re-examine the loading end of the parallel-flow tunnel. The hot drying air which enters the tunnel will always have a wet-bulb temperature very much lower than its dry-bulb temperature. If the commodity being dehydrated is a prepared vegetable, the temperature of the wet material (near the loading end) will not exceed the wet-bulb temperature of the hot drying air. This condition will prevail for an appreciable length of time. During this initial drying period, the evaporation process is quite similar to that which would occur if a wick were moistened with water and placed in the air stream. As the prepared vegetable dries down


t o a moisture content somewhere in the region of 50 to 65%, the water has more and more difficulty in traversing the internal structure to the outer surface of the pieces where i t vaporizes. The surface of the product then becomes relatively dry. At this stage, the temperature of the vegetable pieces slowly rises above the wet-bulb temperature of the air. Therefore, the hot air supplied to a parallel-flow tunnel can be raised to a higher temperature than would be safe for counterflow tunnel operation. A different situation arises if the commodity being dehydrated is uncut fruit. I n this case, since the moisture diffuses slowly to the surface, the temperature of the wet material will rapidly rise above the wet-bulb temperature of the air. Drying under excessively high temperature conditions will tend to make the fruit crack and bleed, and there may be an appreciable loss of juice,

When in proper use, evaporation is very rapid in the wet-end zone of the parallel-flow tunnel drier. Compared to the counterflow tunnel, evaporation in this zone is at least 3 times as fast. Therefore, while the material is still very wet, excellent drying conditions prevail in the parallel-flow tunnel. Due to the evaporative cooling effect, it is possible to use a relatively high hot-end temperature without scorching the product. Consequently, parallel-flow tunnels have a high potential evaporative capacity. On the other hand, if a very dry product must be produced without aid of an auxiliary drier or finisher, the parallel-flow tunnel must be operated in such manner that heat economy and capacity are very low. Nonuniformity of drying may also be a serious difficulty in the use of a parallel-flow t*unnel drier. The tray edge closest t o the hot-air end of any tunnel is always exposed to more severe drying conditions than the down-stream edge. I n the parallel-flow tunnel this condition is most pronounced.

c. Two-Stage Tunnels. The fruit dehydration industry has sometimes modified the design of the standard counterflow tunnel to take advantage of the good characteristics of the parallel-flow unit. One of these modifications, called the " hot-center " arrangement, has been used successfully in the drying of prunes. In a typical unit, the hot drying air is blown into the center of a long tunnel. The air stream divides and moves toward both ends. This modification provides counterflow operation in the wet half and parallel-flow in the dry half of the tunnel. However, it does not take full advantage of the high wet-end evaporative capacity of the parallel- flow tunnel.

Another tunnel arrangement, with a parallel-flow wet end followed by a counterflow dry end, capitalizes on both the high wet-end evaporative capacity of the parallel-flow tunnel and the good final drying characteristics of the counterflow unit. This arrangement is best suited for the

322 P. w. KILPATRICK, E. LOWE, AND w. B. VAN ARSDEL

dehydration of commodities which are able to withstand the high wet-end temperature characteristics of a parallel-flow first stage. There is ample evidence to indicate that most vegetables are in this category; also some cut fruits, for example, apples. In general, the determining criterion is that the prepared commodity shall be a relatively fast-drying material.

The dehydration industry has also used an arrangement called the “ center-exhaust ” dehydrator, illustrated by the simplified diagrammatic sketch of Fig. 6. This type unit operates by drawing heated air into both ends of the tunnel. The hot air (under a slight negative pressure) travels through the loaded trucks and is sucked out of the tunnel chamber (near

FRESH AIR TO FIRST STAGE

FRESH AIR TO RECIRCULATION SEC\oND STAGE

I I I DAMPERS ,HEATER

HEATER, EXHAUST AIR

EXHAUSTEI~ h l R EXIT BELOW TRUCK) TRUCK LEVEL DOOR

c TRUCKS PROGRESS IN

THIS DIRECTION FIG. 6. Center-exhaust tunnel dehydrator (plan view) (from Van Arsdel, l95lb,

Fig. 11).

its center) by a blower acting as an exhauster. Although called a center- exhaust arrangement, the exhaust port is usually located about one-third of the tunnel length from the loading end. An advantage of the “center- exhaust” system lies in the fact that trucks can be pushed straight through the tunnel, and therefore do not require rehandling during the transition from first to second stage drying. However, there are some serious design and operating problems. It is hard t o balance the air flow through the two ends, particularly if the two sections contain an unequal number of trucks. It is also difficult t o secure good air-flow distribution through the trucks as they approach and leave the vicinity of the air exhaust section of the tunnel.

Another arrangement of the two-stage type of dehydrator is shown diagrammatically in Fig. 7. Essentially this consists of separate blowers and heaters a t each end of a single tunnel. A sliding partition near the tunnel center divides the parallel-flow section from the counter-flow section, but permits truck movement from one to the other. I n this type of divided single-tunnel, the trucks are pushed straight through from the parallel-flow section to the counterflow section. Sometimes, to economize

TUNNEL DEHYDRATOHS FOR FRUITS AND VEGETABLES 323

on labor, provisions are made for automatic synchronization of the truck- advancing and partition-moving mechanisms.

In the United States, a widely used two-stage arrangement has the parallel-flow and counterflow stages physically separated. This requires handling of the trucks during their transfer from the parallel-flow tunnel to the counterflow unit. The usual practice is to have a bank of parallel- flow tunnels arranged side by side. Special transfer tracks with turntables permit manual transfer of the trucks from the “dry end” of the parallel- flow units to the “wet end” of the counterflow tunnels. The latter are also arranged in banks side by side, and usually located behind the bank of

\ HEATER, INLETS ,HEATER /’ BLOWER, FRESH AIR BLOWER

I I

TRUCK) DOOR


FIG. 7. Two-stage, single-tunnel dehydrator (plan view) (from Van Arsdel, 1951b, Fig. 12).

first-stage driers. This arrangement permits considerable flexibility in the dehydration plant’s operation and drier load capacity. The number of first- and second-stage driers in operation ran be varied, so as to gear the plant’s drying capacity to the commodity being processed in the preparation line ahead of the driers. Trucks from any of the operating first stage tunnels can be routed to any of the counterflow second stage driers.

Since most of the evaporative load takes place in the parallel-flow or first-stage tunnels, there is only a relatively light evaporative load requirement in the counterflow sections. The heater and blower capacities in the two sections will usually differ accordingly. The air exhausted from the counterflow stage is relatively warm and dry. For reasons of heat economy, i t is desirable to use this air as a part of the supply to the parallel-flow section. This requires duct-work of fairly large dimensions when the two drying stages are physically separated.

The two-stage arrangement, standardized by the British Ministry of Foods (1946)) uses primary and secondary stage tunnels of equal length placed side by side. Trucks progress in one direction through the parallel- flow tunnel. As they leave that section, they are turned around and pro-


gress in the opposite direction through the counterflow tunnel. In this arrangement, the loading and unloading of a group of dehydrators are done at the same end. Air recirculation is controllable in each stage. Nor- mally, the exhaust air from the secondary tunnel furnishes the entire " fresh-air '' supply for the parallel-flow first-stage unit. Extensive performance data and plant experience are given in the bulletin issued by the Great Britain, Ministry of Foods (1946).

The two-stage method of dehydration offers some distinct advantages over a single-stage drier. The reversal of air-flow direction, with respect to movement of material, tends to give a more uniformly dried product. Drying times are shorter, and good drying conditions prevail a t both ends of a two-stage unit. These conditions tend to favor product quality. The shorter drying time also allows an increased output from a dehydrator of a given size.

Three-stage tunnels have been used successfully in a t least one plant, but the dehydration industry generally favors one- or two-stage driers. Perhaps the chief advantage of the three-stage unit is its flexibility which permits the drying (under nearly optimum conditions) of a large variety of different commodities. As the number of stages increases, control becomes rather complex, and more labor is needed for operation (unless truck handling is completely automatic).

3. Transverse Air Circulation

a. Combination Compartment and Tunnel. In direct contrast to the dehydrators previously mentioned, the combination compartment and tunnel drier operates with the drying air moving back-and-forth through the trucks transversely to the axis of the tunnel. The principle is illustrated in simplified form by Fig. 3. As is evident, the material advancing through the tunnel is subjected to reversals of air-flow direction, an advantage which tends to equalize drying. There are many possible variations of the basic arrangement.

The combination compartment and tunnel drier can be equipped with controlled air reheaters and provisions for air recirculation for each corn- partment (each truck position). The large number of independent controls makes the combination compartment tunnel very flexible in operation. As the material is passed through the tunnel, the commodity can be subjected to almost any desired time-temperature-humidity drying condition.

Such units are also well suited as general-purpose, experimental, continuous driers, and can produce the relatively large quantities of dehydrated material necessary for storage studies. For example, at the Western Regional Research Laboratory such a unit has been used for this purpose,


and counterflow, parallel-flow, and various combinations of two-stage drying conditions simulated.

The design and construction of a compartment tunnel requires the greatest of care to avoid operating difficulties. Elaborate provisions must be made for straightening out and equalizing the air flow across the trayed material and through the trucks. If air is forced to make sharp turns, it tends to hug the outside of the curve. The ordinary arrangement of guide vanes or “splitters” will not control this tendency sufficiently when the high velocity air stream is turned through 180”. The use of a system of perforated plates, the small holes acting as orifices or nozzles, has met with some success when properly designed and installed. How- ever, this requires a substantial increase in the power necessary for air movement. Short-circuiting of air from one compartment to another, without going through a truck, is another difficulty, unless provisions are made for a permanent and reasonably tight seal between the trucks and tunnel walls.

The general drying characteristics of the compartment type tunnel equipped with auxiliary heaters may be briefly characterized as follows: Each time that the air passes through a heater, both the wet-bulb and dry- bulb temperatures increase. If general movement of the air is toward the loading end of the tunnel, the wet-bulb temperature will progressively rise, unless additional fresh air is introduced a t each of the reheating stages. In this regard, the compartment unit differs from the simple counterflow tunnel, for in the latter, the wet-bulb air temperature remains substantially constant throughout the tunnel.

The commercial fruit and vegetable dehydration industry has used comparatively few compartment type tunnel driers. Preference has been shown to the counterflow and two-stage tunnels because of their relative simplicity and comparative freedom from the difficulties inherent in the compartment type.

4. Other Tunnel Arrangements

Guillou and Moses (1943) developed a modified form of cross-flow fruit dehydrator for farm use, and presented plans, construction details, and operating instructions. This is a modified, simple form of the compartment type drier and has been used in a number of California or- chards for dehydration of prunes.

There are other possibilities of tunnel arrangements. Only two will be mentioned briefly, and neither of these apparently has progressed beyond the pilot-plant study stage.

The closed-cycle system dehydrator does not exhaust to the atmosphere. It operates by partially dehumidifying the exhaust air from the


drier and this air is returned for reuse. Proposals have been made to use the system for the dehydration of onions and garlic in order to overcome the normally obnoxious exhaust from such dehydrators. The plan has also been used as a part of a pilot plant system for dehydrating food commodities in an atmosphere of oxygen-free gas.

A combination blancher-dehydrator has received some study. Essen- tially the arrangement consists of an isolated compartment a t the wet- end of a tunnel drier, with air temperature and humidity in the compartment under independent control. Humidity and temperature are controlled, so that the raw, wet commodity can be rapidly elevated to temperatures in the range of 180 to 210" F. with little drying and no condensation taking place. The wet product is held at the high temperature for a short period, as determined by the time-temperature requirement to inactivate its enzyme system completely, and thereby securing a full blanch. The hot product then immediately enters the tunnel drier section. Initial drying of the hot product is extremely fast, and its temperature rapidly drops to the vicinity of the ambient wet-bulb air temperature. Further dehydration proceeds in the usual manner. One of the system's advantages is the minimized loss of nutrient material from the product. During the conventional blanching procedure, there is a loss of such material due to leaching. This is obvious if water blanching is used, but it is also true to somewhat a lesser extent, if steam is used as the heat transfer medium. In the latter case, steam condenses on the product as i t is heated, and the hot condensate has a tendency to leach out soluble material rapidly from the product. Theoretically, it is possible to adjust the wet-bulb and dry-bulb temperature of the circulating air in the blanching compartment, so that the product temperature can be rapidly elevated without either condensation or evaporation of moisture taking place. However, a t the high temperatures involved, there is little margin between the conditions required for condensation and those for extremely rapid drying and scorching. Informal reports and observations made by the authors indicate that control is difficult. The method offers potential advantages, and perhaps some future investigator will develop a practical procedure.

111. MECHANICAL ELEMENTS OF TUNNEL CONSTRUCTIOX

1. Fans and Blowers

The introduction of forced air movement is probably the most important single contribution in the development of the modern dehydrator. Prior to the use of fans, air movement in dehydrators was entirely de- pendent upon natural circulation of a rising current of warm drying


air. Drying was slow and drier performance was poor, with the net result that a large number of driers was required for a given plant capacity. The modern dehydrator is a high-performance unit primarily because power- driven fans make possible t,he movement of very large quantities of drying air.

The performance of a drier is affected in two different ways by the velocity of the air moving through it. First, because an increase in air velocity past a moist body increases the rates of heat transfer and mass transfer, the rate of drying of the moist material increases. Second, and of greater practical importance, the mass of air moving through the drier is proportional to the velocity of the air, and the evaporative capacity of the drier is proportional to this mass velocity.

The effect of air velocity on drying rate is complex. The rate of evaporation from a free water surface is known to be proportional to the 0.8 power of the air velocity across the surface, but Guillou (1942) found that the drying rate of prunes increases only as the 0.2 power of the air velocity. Brown and Kilpatrick (1943) showed that the effect of air velocity on the drying rate of vegetables gradually decreases as the moisture content falls; below about 15 to 20% moisture content the drying rate is substantially independent of air velocity. High air velocity is effective in accelerating evaporation near the wet end of a vegetable dehydrator, but not near the dry end.

Increasing the mass flow-rate of air through the tunnel increases the evaporative capacity of the tunnel, essentially by supplying additional heat which is available to produce more evaporation. As is shown in a later section of this article (equation 2, p. 349) the temperature of the air falls and its humidity rises as it passes over the moist material in the drier, but the extent of these changes is inversely proportional to the mass- velocity of the air. At a very high rate of air flow a good drying potential can be maintained even near the cool end of a very long tunnel.

Air velocities in commercial fruit or vegetable tunnel dehydrators range from about 300 to over 1000 ft. per min., based on the entire cross- section of the tunnel, empty of trucks and trays; actual lineal velocity across the material in the trays will be from 50% to 100% greater. This is a range established by practical experience. To the writers’ knowledge, the only effort that has been made t o arrive a t an economic optimum air movement through application of information about the drying characteristics of a specific product was the design of Guillou and Moses (1943) of a farm fruit dehydrator. In this case the drying characteristics of prunes and the importance of keeping capital costs and operating costs low led to choice of a relatively low air velocity. In vegetable dehydrators, on the other hand, the air velocity in the empty cross-


section usually is as much as 600 t o 800 f t . per min. For tunnels of dimensions which are widely used (about 6.5 ft. wide by 7 f t . high) the air-handling requirement of the fan or blower ranges from about 10,000 cu. f t . per min. to about 40,000 cu. ft. per min.; if a single fan supplies air to two tunnels, these figures are, of course, doubled.

In considering fan performance it is customary to standardize the conditions to air weighing 0.075 lb. per cu. ft., corresponding to dry air at a temperature of 70“ F. and a barometric pressure of very nearly 29.90 inches. The performance of a fan whose rating is known in terms of this “standard air” can be readily computed for other temperatures and pressures by means of the well-known fan laws given in engineering handbooks.

Increasing the air flow in a dehydrator by increasing the speed of rotation of the fan is subject to a very drastic law of diminishing returns, because the power absorbed by a fan of given size varies as the cube of its rotational speed. For example, under otherwise identical conditions the time required for drying potato half-dice to 16.7% moisture content in a parallel-flow tunnel can be reduced from 3% hr. to 3 hr. by increasing the air velocity from 400 f t . per min. to 600 f t . per min. If the same size fan is used to obtain the higher air velocity in the same size dehydrator, the power consumed by the fan will increase 3.38 times. Whether the increase in power cost will offset the decreases in other costs occasioned by the 17 % increase in output can be determined only by an analysis of all the other cost items.

The resistance to air flow, or static pressure drop in tunnel driers ranges from a minimum of about 56 inch water gauge to a maximum of about 145 inches water gauge (standard air conditions), the magnitude of the resistance depending upon the length of the drier, the number of trucks in the drier, the air velocity, and the air-flow path. In some dehydrators, especially those of earlier vintage, the air-flow resistance is lower than might be expected because much of the air flows around instead of through the space between the trays. This happens when there is excessive clearance between the trucks in the drier and the walls, floor, and ceiling of the dehydrator. The air, seeking the path of least resistance, tends to bypass the drying trays by traveling down the clearance paths. In spite of a high rate of air circulation drying is slow because the air bypassing the trays has little influence on the drying.

The size and type of fan used in a tunnel dehydrator depend upon a number of factors, the air-handling requirement, the air-flow resistance, the permissible noise level in the plant, the space available for mounting the fan, the need for a fan with nonoverloading characteristics, and last but not least, the relative importance of minimum equipment cost as compared with minimum operating cost.


Space and cost play more important roles in the selection of a fan for use in a tunnel dehydrator than are generally recognized. For a given type of fan, there is usually more than one fan size that can be used for a given air volume and air-flow resistance. Of the several sizes involved, one size will be more efficient than the others, and therefore would be the size selected unless space requirements or the importance of keeping equipment cost a t a minimum dictate the use of a smaller, less efficient fan. Selecting a fan on a compromise basis is not at all uncommon, especially when large size fans are involved.

The need for a fan with nonoverloading characteristics depends a great deal upon the manner in which the tunnel is operated. If the drier is

k FIG. 8. Siniple propeller-type fan.

operated so that the fan is at times discharging against considerably less than normal resistance pressure, then a fan with nonoverloadiiig characteristics is needed to prevent the fan motor from being temporarily overloaded. On the other hand, if the fan is always discharging against a fixed resistance, then the nonoverloading characteristic is not essential. Nonoverloading type fans are used in most tunnel dehydrators because the flow resistance is considerably less than normal when the drier is only partly loaded, or when the end doors are open.

Three different types of fans or blowers are commonly used in food dehydrators, namely propeller, axial flow, and centrifugal fans.

I n general, simple propeller fans of the type shown in Fig. 8 are seldom found in tunnel dehydrators because they are used only in applications involving very low air-flow resistance, usually under $4 inch water gauge static pressure. To improve their ability to discharge against pressure, propeller-type fans are equipped with a special ring housing (see Figure 9*). When so equipped, they are used in tunnel dehydrators with moderate

* Mention of a specific manufacturer in the caption in Fig. 9, and at other places in the text, does not imply that the equipment shown or mentioned is recommended by the U. S. Department of Agriculture over similar equipment of other manufacture not mentioned or shown.


air-handling requirement and nominal flow resistance (e.g. 20,000 cu. ft. per min. against a resistance pressure of 1% inches water gauge). Oper- ating efficiency is improved if the fan discharge is equipped with an expanding conical duct connection to convert the velocity energy of the

FIG. 9. Propeller-type fan with special ring housing (courtesy of Hartzell Propeller Fan Co. ) .

fan to pressure energy. Propeller fans equipped with the special ring housing are usually nonoverloading.

The chief advantages of using propeller-type fans are simplicity of installation due to the compactness inherent in a piece of equipment in which the air enters and leaves in the same direction, and comparatively

WHEEL OR ,/ UPEL LER

AIR FLOW -

FIG. 10. Cut-away VANES

view of vaneaxial fan (courtesy of Hartzell Propeller Fan C O . ) .

low equipment or first cost. The principal disadvantage is the high operating noise level, an important factor where driers are located in populated areas.

Axial flow fans can be divided into two general classifications, tubeaxial and vaneaxial (see Fig. 10). Both types resemble a propeller fan in that a rotating impeller moves air through the fan, with the air entering and leaving the fan in the same direction.


A tubeaxial fan consists essentially of an impeller or wheel with airfoil blades, rotating within a cylinder. Tubeaxial fans are used to move air over a wide range of volumes a t medium pressures. In tunnel driers, for example, tubeaxial fans are used to move from 30,000 to 75,000 cu. ft. of air per min. against a resistance pressure of from 1 to 1% inches water gauge.

Both tubeaxial and propeller-type fans discharge air traveling with a rotating or screw motion. When the rotating air stream enters the stack of drying trays in the drier, some of the tray surfaces will be exposed to an air stream approaching from the top while other tray surfaces will be exposed to an air stream approaching from the bottom, below the wood slats. The result is that drying will not be uniform because of differences in the air velocity over the trays. To correct this difficulty, air straight- eners of the egg-crate type are often used in tunnel driers equipped with tubeaxial or propeller-type fans.

A vaneaxial fan is essentially a tubeaxial fan with air-guide vanes located either before or after the impeller. The guide vanes improve the performance of the axial flow fan, especially when discharging against pressure. When used in tunnel dehydrators, for example, vaneaxial fans are generally somewhat more efficient than tubeaxial fans of equivalent size and rating. The vanes also straighten the air leaving the fan, elimi- nating the rotating or screw motion characteristic of the air stream leaving a tubeaxial or propeller-type fan. Vaneaxial fans are capable of delivering against higher pressures than tubeaxial fans, a factor important in drier applications only if the air-flow resistance in the drier is abnormally high.

Both tubeaxial and vaneaxial fans are available with nonoverloading characteristics. Like propeller-type fans, tubeaxial fans and vaneaxial fans are more efficient when the fan discharge is equipped with an expanding conical duct connection to convert the velocity energy of the fan t o pressure energy.

A centrifugal fan consists essentially of a fan rotor or wheel rotating within a scroll shaped housing. Centrifugal fans are capable of moving air over a wide range of volumes and pressures, and are commonly used in tunnel dehydrators of all types and sizes. When equipped with back- wardly inclined wheel blades, they are nonoverloading. Because the air enters from the side, centrifugal fans must be installed with ample air-flow clearance on one or both sides of the fan, depending upon whether the fan is single or double entry. This requirement, combined with the fact that centrifugal fans are, in general, relatively large in size, results in very large space requirements within the drier to accommodate the fan. This is especially true in high-performance driers using centrifugal fans


of the most efficient size. To conserve space in such cases, fan size is often compromised by using a smaller fan of lower efficiency.

Axial flow fans are competitive with centrifugal fans in most tunnel dehydrator applications. When compared with centrifugal fans, axial flow fans are more compact, easier, and therefore less expensive to install and, in general, lower in first cost. On the other hand, they are much noisier in operation and are somewhat less efficient if the fan is selected for maximum efficiency, independent of space requirements.

2. Heating Systems

Heating systems used in tunnel dehydrators are of two basic types, direct combustion heating and indirect heating. In a direct combustion heating system the gaseous products of combustion are mixed and circulated with the drying air and hence come in direct contact with the product in the drier. An open flame in the main air stream of the dehydrator is an example of this type of heating system. I n an indirect heating system, the products of combustion are not circulated with the drying air. Heating surfaces are used to transfer the heat from the primary source to the drying air. A dehydrator using steam-air heating coils is an example of this type of heating system.

Direct combustion heaters are widely used in tunnel dehydrators. Because there are no transmission losses, heat efficiency is at a maximum. The fuel used is usually either natural or manufactured gas, fuel oil, or bottled gas such as butane. A gaseous fuel is usually preferred to fuel oil because of the simplicity of the control equipment, the ease of handling, and the fact that the products of combustion are unlikely to affect the quality of the dried fruit or vegetable.

Gas burners are almost always of the “premixed” type, installed directly in the drier air stream with the flame shielded from the cooling effect of the surrounding air currents by a simple unlined sheet metal combustion chamber. Although not essential when using gaseous fuels, refractory-lined combustion chambers are sometimes used with gas burners to insure complete and therefore more efficient burning of the fuel.

Oil burners are of many types-rotary, atomizing, centrifugal, pressure, etc. A basic rule in connection with the use of oil is that combustion must occur in a relatively high temperature zone. If the flame is chilled so that some of the oil particles are cooled below their ignition point, smoke and soot will be formed which will contaminate the product in the dehydrator.

A common way of burning fuel oil in food dehydrators is in a refractory-lined steel sheet combustion chamber such as that shown in Fig. 11. The combustion chamber is divided into two zones by a refractory


checker brick partition, the partition serving to confine the radiant heat t o the primary zone. I n operation, this primary zone becomes incandescent so that combustion of the fuel occurs a t incandescent temperatures. The checker screen also serves as a baffle t o prevent the escape of unburned oil droplets since impingement of the droplets on the incandescent screen results in surface combustion of the fuel.

The secondary zone is an added precaution against smoking due to incomplete combustion. Large unburned particles of oil that escape from the primary zone will burn a t an accelerated rate when they come into contact with the high-velocity, high-temperature gases flowing through the checker wall restrictions. The particles are then given additional time in the secondary zone to burn completely before coming in contact with

/CYLINDRICAL STEEL SHELL\

BURNER

PRIMARY ZONE

\FIREBRICK LINING) [FIREBRICK CHECKER WALL’

FIG. 11. Refractory-lined combustion chamber.

the drying air. Without the secondary zone, combustion may not be complete enough t o eliminate smoking.

Considerable care must be exercised in the selection of a fuel oil for use in a direct combustion food drier. In most cases, oils with high sulfur content cannot be used satisfactorily because the product absorbs an excessive amount of the sulfur dioxide liberated during combustion.

Indirect heating systems for dehydrators usually involve steam-to-air heaters although combustion gas-to-air heaters are used, particularly in apple dehydrators. The principal advantage of using an indirect heating system is that there is no possibility of contaminating the material being dried with the products of combustion. The principal disadvantages are the additional equipment required and the lower heat economy.

3. Instrumentation

The dry-bulb temperature of the air entering the drying tunnel is automatically controlled in virtually all tunnel dehydrators. I n a very few plants the wet-bulb temperature of the entering air is also automatically controlled.

Dry-bulb temperature is controlled by regulating the flow of heat into the drier, ordinarily by means of a valve in the fuel or steam supply


line. Wet-bulb temperature is controlled by regulating the position of the recirculating air damper.

The instruments used to control dry-bulb temperature in tunnel dehydrators are of two basic types, on-off and modulating or proportional control. The latter is by far the most common. Wet-bulb temperature can be controlled satisfactorily only with modulating or proportional type instruments.

On-off instruments range in complexity from simple thermostatic switches to industrial type controllers that indicate and, if so desired, also record the temperature. On-off control of dry-bulb temperature is practical only if the air-heating system in the drier involves a large amount of thermal capacitance or heat inertia. In a tunnel dehydrator this usually means the use of a combustion chamber large enough to serve as a heat reservoir, storing heat while the burners are on and releasing the stored heat to the air when the burners are off. With an on-off control, air temperature will fluctuate to some extent, the ampli- tude and frequency of fluctuation depending upon the instrument, its adjustment, the size of the thermal capacitance, and the size of the heating load.

On-off control without thermal capacitance is unsatisfactory because air temperature will fluctuate excessively. Fluctuations can be minimized by by-passing the control valve with a manually operated valve adjusted to maintain, without help from the controller, an air temperature slightly lower than the correct temperature, and depending upon the controller to supply only the additional heat required to bring the air to the proper temperature. The practice is not a good one, however, because the air temperature can rise to damaging if not dangerous levels if the heating load is reduced much below normal, for example, by a slackening in the rate of supply of wet material to the dehydrator.

The simplest on-off temperature control system would consist of a thermostatic switch opening and closing an electric solenoid valve in the fuel supply line. A more elaborate system would consist of a pneumatic or air-operated controller opening and closing an air-operated control valve.

Modulating or proportional control is best exemplified by an ordinary float valve, wherein the valve opening is a function of the liquid level, the lower the level the greater the valve opening. In a modulating or proportional temperature control system, valve opening or damper position is a function of temperature, either dry- or wet-bulb. The control valve or damper is normally neither fully open nor fully closed but is modulating a t some intermediate position. As a result, the controlled temperature does not cycle between limits as i t does with the on-off control, but, remains steady once the system is stabilized.


Modulating control is applicable to all types of heating systems commonly used in tunnel dehydrators. Proportional controllers are either electrically or air operated or self-acting. Fig. 12 shows a simple potentiometer type used in conjunction with an electrically energized control valve. A self-acting controller is shown in Fig. 13.

In most plants the dry-bulb temperature is recorded continuously, either by use of a recording type controller or an independent recording thermometer. Ordinary mercury thermometers are frequently used to indicate both wet- and dry-bulb temperatures.

adjustment spring

y) Be'lows assembly

FIG. 12. Simple electrically-operated (potentiometer type) proportional temperature controller (courtesy of Minneapolis-Honeywell Regulator Co.).

4. Materials of Construction

For obvious reasons, modern tunnel dehydrators heated by direct combustion are almost invariably built of fire-proof material such as hollow concrete block, hollow tile, sheet metal, or asbestos-cement sheet- ing. Most of the tunnels built on the Wrest Coast in recent years have been of hollow concrete or pumice block construction. Some of the early direct-fired driers still in use are built of wood, but they are fast disappear- ing. As might be expected, the use of wood and other flammable materials of construction increases the cost of insuring the structure against loss due to fire.


FIG. 13. Companies).

Self-acting temperature controller (courtesy of Taylor Instr

Indirect-fired dehydrators are less vulnerable to damage by fire therefore, can be built from a wider variety of materials. Usually, ever, the materials of construction used are the same as those used in direct-fired counterpart.

Compartment type tunnel dehydrators are frequently of panel struction, with wood or metal structure frame, and wood, asbestos-ce

'ument

! and,

their how-

. con- ,merit


board, or sheet-metal panels. The panels are usually insulated, with the thermal insulation applied between the two faces of the panels.

5. Trays and Trucks

Despite numerous attempts by designers and operators of tunnel dehydrators to find a better material of construction, drying trays used in most fruit and vegetable dehydration plants in this country are still made from wooden slats. The reasons are rather simple. A good drying tray

FIG. 14. Wooden drying trays (court,esy of Gentry Division, Consolidated Fo Corp.) .

as

must be easy to fabricate, inexpensive (in terms of its probable USE ul life), easily scraped clear of adhering dried material, and light but strong and rigid. Furthermore, the tray material must not contaminate the product. Few materials besides wood can be used to build trays which will possess a majority of the desired characteristics. All-metal trays, for example, are expensive, not easily fabricated, heavy in order to be strong and rigid, and if fabricated with wire-mesh drying surfaces, tend to develop a permanent sag with use. In some plants wooden trays enjoy an added advantage in that they can be readily fabricated by plant personnel during the off season and a t other times when the permanent members of the operating staff would otherwise be idle.

A typical dehydrator tray is shown in Fig. 14. Most trays are 3 ft . by


6 f t . , made from Ponderosa Pine, Douglas Fir, or a combination of both woods.

Wooden trays suffer from one serious drawback. Material drying on the trays tends to stick to the wood surface. During the de-traying operation some wooden splinters may pull loose and stick to the product as it is scraped off of the trays. Most of the splinters are removed during final inspection of the finished product, but unless this inspection is pains- taking, enough may remain in the dried material to pose a serious con- tamination problem. Elimination is especially difficult in the case of leafy

F ;ed

vegetables such as cabbage. Tooden slivers and pieces of dry produce with splinters adhering to them are usually removed from the dried product by hand, an expensive operation. To minimize product sticking and consequent pulling off of splinters, some operators oil or wax their wooden trays.

Leafy vegetables such as cabbage are blanched on the drying trays. Moisture absorbed by the trays during the blanching operation increases the drying load in the dehydrator. To minimize the amount of water absorbed, some plants use wooden frame trays with wire-mesh drying surfaces for the blanching-drying operation.

Drying trays are conveyed through the tunnels and to the tray loading and unloading stations on either of two types of vehicles-a flanged-wheel type that runs on steel rails (see Fig. 15), or a caster-wheel type that runs either on flat surfaces or in channel irons through the tunnels.


The rails for the flange-wheel trucks are laid flush with the concrete floor. To move trucks at right angles to their line of movement in the tunnels, turntables (see Fig. 16), or transfer cars and rails are used (see Fig. 17). Transfer rails are recessed so that the rails on the top of the transfer cars are flush with the tunnel rails. To change direction of movement, tray trucks are pushdd onto the transfer cars and moved at right angles to their previous direction of travel.

FIG. 16. Turntable for tray trucks (courtesy of Gentry Division, Consolidated Foods Corp.).

IV. TYPICAL COMMERCIAL TUNNEL DEHYDRATORS

1. Twin-Tunnel CounterJlow Dehydrator

Popularly known as a Puccinelli dehydrator (R. L. Puccinelli, promi- nent in the development of prune dehydration in California during the 1920's and still actively engaged in the business), the simple twin-tunnel counterflow drier is widely used for drying both fruits and vegetables, par-


ticularly on the West Coast. The basic elements of the drier are shown diagrammatically in Fig. 18, consisting essentially of a direct-fired combustion chamber and a blower, both located in an air passage between the two drying tunnels. Air enters the drier through openings surrounding the front of the combustion chamber, is heated to the proper drying temperature by the direct combustion heater, and is discharged by the blower into the two drying tunnels. Upon leaving the tunnels, part of the air is recirculated via the central air passage while the balance is exhausted to the atmosphere via overhead discharge ducts.

FIG. 17. Transfer car and tracks (courtesy of Gentry Division, Consolidated Fool ds Gorp.).

A variation of this twin-tunnel arrangement places the direct-fired heater and the blower in an air passage located above the two drying tunnels, which are arranged side by side.

Being custom-built in most cases, Puccinelli-type dehydrators vary somewhat in size. A typical unit would have drying tunnels about 6 f t . 4 in. wide by 7 f t . high (inside dimensions), with a central air passage of equivalent height, and a width of 9 f t . A drier accommodating in each drying tunnel 12 truckloads of trays measuring 3 f t . by 6 ft., would have an over-all length of approximately 50 f t . This allows about 7 f t . for the air stream to straighten out before entering the trays, after making the turn from the central air passage into the drying tunnels. About 5 f t . of space is left a t the opposite end of each tunnel for the air to enter the

/--

P 3

FIG

. 18.

Tw

in-tu

nnel

cou

nter

flow

deh

ydra

tor.


exhaust duct or t o be recirculated back into the central air passage. A shutter or sliding door is usually installed a t each dry-end air opening to shut off the flow of hot drying air into the tunnels during loading and unloading operations.

Total air movement and heating capacity vary from about 30,000 cu. ft. per min. and 3,000,000 B.t.u. per hr. for a fruit drier, t o 50,000 cu. f t . per min. and 5,000,000 B.t.u. per hr. for a vegetable drier.

A typical up-to-date Puccinelli-type dehydrator would have hollow concrete or pumice-block walls, prestressed hollow concrete block roof slabs, and wooden-frame, metal-clad, center opening end doors. The drier would be equipped with a natural gas burner complete with a simple unlined sheet-metal combustion chamber, a direct drive tubeaxial blower with the motor cooled by a suction duct, wooden trays measuring 3 ft,. by 6 ft., flanged-wheel tray trucks running on rails set flush with the concrete floor, and a modulating dry-bulb temperature recording-controller.

2. The Miller Tunnel Dehydrator

The Miller dehydrator (L. N. Miller Dehydrator Company, Eugene, Oregon) is widely used in the Pacific Northwest for drying fruits such as

FIG. 19. Miller tunnel dehydrator (elevation).

apples and prunes. The drier (see Fig. 19) is basically an indirect-fired, counterflow tunnel dehydrator, but of a special type. To create conditions believed to be desirable for fruit drying, the Miller dehydrator is equipped with shutters or louvers located above the tray trucks in the middle third of the drier. The shutters are adjustable, to vary the amount of air bypassing successive truckloads of product. The adjustable shutters make possible some measure of control of the humidity of the drying air in the various parts of the drying tunnel. By opening the shutters, for example, the relative humidity of the air a t the wet end of the drier can be increased. The resulting decrease in the rate of drying prevents the prune skins from


cracking due to “case-hardening,” with consequent loss of juice from the fruit.

Drying trays are usually of wood, measuring 3 ft. by 3 ft., and stacked 24 to a truck. Truck arrangement within the drier varies. Depending upon drier capacity, trucks are either 2 abreast or 3 abreast, with 10 to 12 trucks per row. A typical tunnel would, for example, hold 36 trucks, 3 wide by 12 long. The cross-sectional dimensions of such a tunnel would be 7 ft. 7 in. high by 9 ft. 8 in. wide. Gross air velocity in the empty tunnel, with the shutters closed, is usually not more than 500 ft. per min.

Trucks are of the caster-wheel type, running on steel tracks while inside the drier and directly on the concrete floor when outside. The drier is of panel construction, with the insulated, galvanized, sheet-metal-clad panels bolted to an angle iron frame.

The indirect air-heating system is commonly of the combustion gas- to-drying air type, consisting of a combustion chamber and flue pipes to transfer the heat from the combustion gases to the drying air. Steam- to-air heaters are used, but are less common.

3. The Carrier Compartment Drier

During World War 11, the Carrier Corporation of Syracuse, New York manufactured a vegetable drier which in many ways is typical of compartment-type tunnel dehydrators used commercially. The drier consists essentially of a steam-to-air preheater section, 6 drying compartments or sections each equipped with its own blower and steam-to-air heater, and an exhaust air fan section, arranged so that truckloads of trayed material are progressively pushed through the tunnel formed by the 6 compartments in series (see Fig. 20). Although the air flow across the trays is in a direction transverse to the direction of truck movement, the drier is essentially a counterflow unit, with the wet material entering the drier a t the end where the exhaust air is discharged from the tunnel. Fresh air enters a t the opposite end, through the steam-to-air preheater. From the preheater, the partially heated air enters the steam-to-air heater and blower units of the individual drying sections. Perforated baffles progressively decrease the amount of fresh air taken in a t each compartment (thereby progressively increasing the amount of air recirculated a t each drying section), starting a t the dry end of the dehydrator and ending a t the wet end. The mixture of fresh and recirculated air is reheated a t each stage of drying, the amount of reheating automatically controlled by a separate dry-bulb temperature controller a t each compartment. The amount of air discharged from the drier by the exhaust fan, and consequently the amount of fresh make-up air entering the drier, is controlled by the wet-bulb temperature of the exhaust air.


In operation, the product moving through the drier is subjected to a different temperature condition at each compartment, and the direction of air flow over the material is reversed at each succeeding drying section.

Although basically a counterflow unit, the Carrier dehydrator has some of the characteristics of a two-stage drier. Both air flow and drying temperature are higher at the 2 wet-end drying sections than they are a t the other 4 sections. This is made possible by using larger heater and blower units at the 2 wet-end compartments.

EXHAUST EXHAVST FAN UNIT

H

STEAM-AIR P ~ E H E ~ T E R S PLAN VIEW

PERFORATED PLATE BAFFLE

TYPICAL CROSS SECTION FIG. 20. Diagrammatic sketch of Carrier Compartment Drier.

Air velocities in the four dry-end and two wet-end sections are 700 and 1200 f t . per min., respectively, through the free area of the loaded trucks. When operated at maximum capacity under high ambient moisture conditions, fresh make-up air enters the drier a t a rate of approximately 20,000 cu. ft. per min. Steam consumption under these conditions is about 5500 Ib. per hr.

Trays are either of wood or metal, measuring 36 in. by 36 in., and are stacked forty to a truck.

The Carrier dehydrator has a nominal rating of 30 tons of wet vegetables per 24 hr., based on a tray loading of 1.2 lb. per sq. ft . and 2 hr. drying time to reduce the moisture content of the product t o a level suitable for bin finishing.


V. CRITERIA FOR SELECTION OF TUNNEL DEHYDRATORS Numerous factors govern the selection of the drying system, or type

of dehydrator, chosen for a particular plant. Economic factors, such as initial and installation costs and operating labor costs, although important, are not the only criteria for proper selection. Some of the other important considerations are listed below :

(1) Flexibility with respect to integrated plant operation. (2) Ability to handle a wide range of commodities, if required or if the

(3) Ability to dry the product to meet current specifications for

(4) Adaptability to meet future contract specification changes for the

(5) Floor-space requirements. (6) Capacity requirements, current and future; and the possible use

of other final driers, such as finishing bins. (7) Availability of critical materials of construction and precision

machine parts in case of emergencies. (8) Mechanical reliability and foolproofness to guard against com-

plete plant shut-down. Truck-and-tray type tunnel dehydrators are generally satisfactory for

drying most of the various fruit and vegetable commodities which are processed in piece form. This type of drier can be used to dehydrate a wide range of products and can be operated continuously or intermittently as desired. These factors add to the flexibility of plant operation.

The type of drier used influences, to some extent, the characteristics of the finished product. I n general, multistage driers permit the use of higher temperatures during the initial part of the drying cycle when the product’s surface is still moist, and consequently the product dries faster. This combination of higher temperatures and shorter drying time often produces a more porous and bulky material. The greater porosity of the finished product makes reconstitution faster and easier. However, the greater bulk may cause difficulty in meeting some contract specifications, i.e., getting the required weight in the containers.

The counterflow tunnel is, perhaps, the most versatile of the truck- and-tunnel driers for the dehydration of fruits and vegetables. These units are relatively easy to operate and are of comparatively simple design. I n the fruit dehydration industry, the counterflow truck-and- tunnel drier is the type most widely used, and the design has been more or less standardized (Perry, 1947, and Perry and associates, 1946).

Tunnel-and-truck dehydrators with one, two and three stages may

current economic demand changes.

moisture content, product damage tolerances, etc.

product.


be found operating satisfactorily in the vegetable dehydration industry, and they have been built by many different people. As previously mentioned in section I1 (Classification of Tunnel Dehydrators), the design has not been universally standardized.

Broadly speaking, two-stage tunnel driers are preferable to single- stage units for vegetable dehydration ij" the commodity has a very high percentage of water that can be removed rapidly (such as cabbage), or, if the commodity cannot tolerate a high final drying temperature (onions, 135 to 140" F.). The choice between one-stage and two-stage tunnel driers becomes more or less an arbitrary decision for operators drying commodities which have a relatively low initial moisture content and which can tolerate a reasonably high final drying temperature, for example, carrots. For a given capacity, the single-stage drier would occupy substantially the same floor space as the two-stage unit, provided the stages in the latter were not physically separated.

Multiple-stage construction permits the use of drying conditions which change in a predetermined manner as the material progresses through the tunnel. This flexibility offers distinct advantages in operation. Moreover, the multistage unit, when compared to a single-sbage dehydrator, provides more rapid drying of the product and somewhat better heat economy. Against the advantages for the multistage unit, there should be weighed the higher capital cost, the increased labor cost (unless an additional investment is made for an automatic mechanism to handle the trucks between stages), and the increased complexity of operation. Some of the justification for two- or three-stage operation is. also being weakened by the increased reliance upon finishing bins to accom- plish the late stages of drying.

There are also certain general basic considerations which should govern the selection of the drying system. Driers installed in multiples are less likely to cause a complete plant shutdown due to mechanical failure. Drying systems of proved or unquestioned performance have advantages since there is a calculated risk for each installation and the individual must decide how much of a pioneer he can afford to be. Initial cost of the equipment is often not as important in determining production costs, as is the effectiveness of the equipment chosen. For example, assume that two drying systems are available, one of which involves a sizeable capital investment, but has been proved to be efficient and foolproof, and the other involves a modest investment, but is of doubtful efficiency and surety of operation. There is little question but that, generally, the more expensive unit should be given preference.

Comparison of the truck-and-tray tunnel dehydrator with the continuous belt-conveyer dehydrator for a specific drying operation should take into account a t least the following general considerations.


Assuming both units have the same drying capacity under the speci- fied conditions, the truck-and-tunnel arrangement has the following advantages :

(1) The installed cost of the system will probably be less. (During the war, the cost of a simple tunnel was considerably less.)

(2) Smaller quantities of critical materials are required for construction, and fewer precision-fabricated parts are needed (an important consideration in wartime).

(3) Operation can be either intermittent or continuous. (4) A wider range of commodities can be dried satisfactorily. (For

example, whole or halved fruit, such as prunes or peaches, are unsuited to conveyer-belt drying because of the long drying time involved. Shredded cabbage is likewise unsuited because of the tendency for the blanched material t o mat and not dry uniformly.)

On the other hand, the conveyor drier has certain advantages over the truck-and-tunnel dehydrator of' a similar drying capacity, for example: (a) Less floor space is needed. (b) The dryiiig time can be made shorter, and product quality may thereby be improved. (c) Less operating labor is required since the conveyer drier is fundamentally automatic in operation.

VI. BASIC THEORY OF TUNNEL DEHYDRATORS The quantitative theory of drying, applicable to the design and con-

trol of dehydrators for fruits and vegetables, is the work of many investigators, nearly all within the past 50 years. Advances in development of the theory, especially pertinent t o the subject of this chapter, were made by Grosvenor, 1908; Carrier, 1911, 1921 ; Hausbrand, 1912; Tiemann, 1917; Lewis, 1921; Cruess and Christie, 1921b; Sherwood, 1929-1932, 1936; Newman, 1931a,b; McCready and McCabe, 1933; Bateman et al., 1939; Hougen et al., 1940; Van Arsdel, 1942, 1947, 1951a; Marshall, 1942, 1923; Brown and Kilpatrick, 1923; Cruess and hlackinney, 1943: Perry, 1944; Perry et al., 1916; Ede and Hales, 1948; Marshall and Fried- man, 1950; Broughton and Mickley, 1953; and Hendel et al., 1954.

Several of the earlier discussions of tunnel design, in the absence of quantitative information about the effects of temperature, humidity, air velocity, and other factors on the drying rates of specific commodities, simply assumed that the time required to dry a commodity was well- established by practice and on that basis computed the requisite air flow and heat input by methods established in the field of heating and venti- lating engineering. This procedure of course forswore any adventure off well-trodden paths. Lewis (1921), followed by Sherwood and several of his other colleagues at the Massachusetts Institute of Technology and by a number of other chemical engineering investigators, noted that many wet materials exhibit two sharply distinct phases of drying be-


havior under constant drying conditions: an initial phase of constant rate of loss of water per unit surface exposed to the air, and a final phase during which the drying rate falls steadily toward zero. Generalized correlations of drying rate with vapor pressure relations and air velocity were derived, and these in turn were applied to the heat-balance and mass-balance equations characteristic of practical dehydrators. Good correspondence of prediction with experience was obtained in the drying of several industrial materials, and Perry (1944) and his co-workers used the same principles successfully in designing prune dehydrators. Van Arsdel (1942) and Brown and Kilpatrick (1943), concerned a t that time with the dehydration of vegetables to quite low levels of moisture content, found that drying rates determined experimentally could not be represented satisfactorily by any simple mathematical formula. They and their colleagues at the Western Regional Research Laboratory published a series of bulletins (AIC-3 1, I-VIII, 1943-1947) which summarized the drying behavior of a number of common vegetable materials in the form of nomographs readily applicable to dehydrator calculations. Broughton and Mickley (1953) have made the final step in the retreat from Lewis’ highly idealized picture of drying behavior by basing dehydrator design upon an actual analog” drying experiment in which the temperature and moisture-content history of the experimental material serves directly as the basis for the design. Their procedure obviates the difficulty, recognized but not fully overcome by earlier investigators, that the drying rate of a hydrophilic material at any instant depends to some extent upon the previous drying history of the sample (the internal distribution of moisture within the pieces is determined by that history). Some of the later AIC-31 nomographs contain a correction factor intended to deal approximately with this situation.

Van Arsdel (1942), noting that fruit and vegetable dehydration presented a special case in which the heat absorbed in evaporation of water far outweighed all other causes of heat usage, proposed the following theorem: In any section of a tunnel dehydrator where no reheating of the air takes place, the change in air temperature i s proportional to the change in moisture content of the material, if moisture content is expressed on the ,‘dry basis.” This is, expressed in differential form,

d t dw” dx dz - = b -

Under these special conditions the tunnel acts substantially like an adiabatic humidifier. It was already well-known that in such an adiabatic system the wet-bulb temperature of the air remains constant, and that, at

* Refer to list of symbols on p. 369.

TUNNEL DEHYDRATORS FOR FRUITS -4ND VEGETABLES 349

constant wet-bulb temperature, the fall in temperature of the air is very nearly a linear function of its rise in absolute humidity. A mass balance relating the loss of moisture by the material to the uptake of water vapor by the air then led directly to equation 1.

Wet-bulb temperature lines on a humidity chart using temperatures as abscissas and absolute humidities as ordinates are slightly curved and differ slightly in slope. The actual variations within the general range of interest in fruit and vegetable dehydration are as follows:

Wet-bulb temperature 90" F. : Fall in air temperature per 0.001 rise in absolute humidity:

Air temperature 120' F., 4.28" Air temperature 180" F., 4.35"

Wet-bulb temperature 120" F. : Fall in air temperature per 0.001 rise in absolute humidity:

Air temperature 140" F., 3.81" Air temperature 200" F., 3.96"

Van Arsdel (1942) suggested that for general exploratory computations a temperature change of 5" F. per 0.001 change in humidity be used; this would correspond to a lumped total of about 20% for all heat losses; a t the same time, however, he proposed that wet-bulb temperature be taken as constant in spite of heat losses. If the 5" F. figure is accepted, the coefficient b in equation 1 can be evaluated readily, and the equation becomes,

The plus sign will be used for a parallel-flow arrangement, the minus sign for counterflow.

Brown (1943) and Lazar (1944) examined this approximation criti- cally, and concluded that for the range of conditions encountered in parallel-flow and counterflow tunnel dehydrators for vegetables the errors to be expected from i t are smaller than the other inherent uncertainties of such systems. The wet-bulb temperature in practical tunnels will usually fall less than 1" F. between the hot end and the cool end of the tunnel. Perry (1947) computed the heat balance for a typical counterflow prune dehydrator, with a hot-end air temperature of 165" F . and wet-bulb temperature of 110" F. ; the wet-bulb temperature of the exhaust air was calculated to be 109.6" F. The British Ministry of Foods, in its bulletin ((Vegetable Dehydration" (1946) described the performance of two-stage tunnels in potato dehydration; in the first stage the fall in air temperature



averaged 4.6" F. for each increase of 0.001 in absolute humidity, and the fall in wet-bulb temperature through the tunnel was somewhat less than 0.5" F.

The following discussion of theoretical tunnel behavior is based on computations made by use of the approximate relationship of equation

I . Theoretical Tunnel Behavior

Figure 21 shows the computed course of moisture content of prunes being dehydrated in a counterflow tunnel and the change in air temperature as i t passes through the tunnel. The conditions assumed were those

(2).

70

I- z 0 (r: W

c 50

cn

60

a v, 3 I- 40 w 3 u

W 30 LT 3

20 0 2

- I70

10 130 0 6 12 18

TI ME, HOURS

drying prunes, computed from drying-rate expression derived by Guillou (1942). FIG. 21. Moisture content of fruit and temperature of air in counterflow tunnel

given by Perry et al. (1946), in Figs. 14 and 15, and the drying rate expression used in the computation was that published by Guillou (1942), which correlates the drying rate with air temperature, humidity, and velocity and with the size of the fruit. The computed curves agree well with the curves of Fig. 14 in the cited publication by Perry and his associates, which represent data secured from commercial counterflow tunnels.

Figure 22 is a similar diagram showing computed conditions in a counterflow tunnel dehydrating white potato half-dice (x in. x in. x x~ in. in the wet state). The conditions assumed for the example, which is taken from Van Arsdel's (1951b) Fig. 7, are substantially those followed in commercial dehydration. The drying rate expression used in the com-


putation is given by the group of nomographs in AIC-31-VII, Western Regional Research Laboratory (1945).

The pictures presented by Figs. 21 and 22 exhibit the characteristic behavior of counterflow tunnels: maintenance of a relatively high air temperature through much of the tunnel, somewhat slow initial drying of the wet material, and good drying conditions a t the dry end of the tunnel.

If the drying rate characteristics of a product have been determined, a single composite diagram can be used to summarize the behavior of that product in counterflow tunnels. Figure 23 is such a diagram for the drying of prunes, published by Perry et al. (1916), Fig. 15. Figure 24 is a similar

L

c

AIR TEMPERATURE 150

140

130

120

110

h IT W a E W

I 2 4 6 8

2 0 ' 0 0 E TIME, HOURS

FIG. 22. Moisture content of material and temperature of air in counterflow tunnel drying potato half-dice, computed from drying-rate nomographs of AIC-31-VII.

diagram for the counterflow drying of potato half-dice, published by Van Arsdel (1951b), Fig. 8. The general similarity of the two diagrams is striking, only the time scales being substantially different.

The characteristics of a parallel-flow tunnel are, of course, the reverse of those of a counterflow tunnel; a relatively low air temperature prevails throughout much of the tunnel, very rapid evaporation occurs a t the wet end, and drying conditions are poorest a t the dry end of the tunnel. Van Arsdel (1951b), Fig. 10, illustrates the contrast in an example reproduced in Fig. 25, showing the behavior of counterflow and parallel- Aow tunnels of the same length and same air flow, operated in such a way as to produce the same hourly output of dehydrated potato half-dice. Van Arsdel points out that whereas the most striking contrast is in the heat consumption of the two arrangements (almost 50% greater for the parallel-flow tunnel) there will also certainly be differences in the quality characteristics of the products turned out. Material dried in the parallel-


Net tunne l length, ft. FIG. 23. Relation between air velocity and drying time, with final exhaust tem-

perature also given, for counterflow prune dehydrators of various lengths. Initial air temperature 165" F., wet-bulb temperature not over 105" F. Initial prune moisture content 70%, final prune moisture content 16.7%. Prune size, dry count of 50 per pound (from Perry et al., 1946, Fig. 15).

4 5 6 7 8 9 10 I I 12 13 14 15 16 Number of active trucks in tunnel

FIG. 24. Relation between air velocity and drying time, with final exhaust temperature also given, for counterflow potato half-dice dehydrators of various lengths. Initial air temperature 150" F., wet-bulb temperature 85" F. Initial moisture content of material 76% final moisture content 6%. Tray-loading, 1.50 lb. per sq. ft., trucks contain 540 sq. ft. active tray surface (adapted from Van Arsdel, 1951b, Fig. 8).


flow tunnel will be the more bulky because shrinkage stresses will open up internal voids in the pieces. Some difference in the extent of "heat damage " suffered during dehydration may also be apparent, damage probably being slightly greater in the counterff ow-dried material.

If only the first stage in a multiple-stage dehydration system is being considered, there can be no doubt that the parallel-flow arrangement offers substantial advantages over counterflow. In such a system product

I90

180 W' 170

5 160

5 150

5 140

Q 130

5 120 I -

110

cn a m v,

I - + w z

I- cn 0 I

80

70

60

50

40

30

20

10

n " 0 I 2 3 4 5 6 7

T I ME, HOURS FIG. 25. Comparison of counterflow and parallel-flow drying; moisture content of

material and temperature of air in tunnels drying potato half-dice, under conditions chosen to make outputs equal. Twelve trucks in each tunnel, 540 sq. f t . per truck, loading 1.50 lb. per sq. f t . Air velocity 1000 f t . per min. between trays. In counterflow drying, initial air temperature 150" F., wet-bulb temperature 85" F. In parallel-flow drying, initial air temperature 185" F., wet-bulb temperature 90" F. (from Van Arsdel, 1951b, Fig. 10).

is discharged from the first-stage tunnel still somewhat moist; the relatively poor drying conditions a t the dry end do not matter so much. The evaporative capacity of the tunnel can be increased substantially by raising the temperature of the inlet air to a point far above that which would be safe for a counterflow tunnel. Van Arsdel (1951b) presents an example of first-stage driers for potato half-dice in which a parallel-flow tunnel, operated a t an inlet temperature of 200' F., turns out almost 50%


more material dried to 16.7% moisture than a counterflow tunnel of the same length, operated a t an inlet temperature of 145" F.

Two-stage tunnels now generally consist of a parallel-flow first stage and counterflow second stage, the arrangement first suggested by Eidt (1938) for the dehydration of apples. The course of moisture content and air temperature in such a two-stage tunnel, as used for the dehydration

220

200 0 -

.=I 160

5 140 n 2 120 W I- 100

2 180

80 80

70

60 - m 50

W E 30

I- s 10 0 I 0

Lg 40 -0 3 w

5: 20

0 1 2 3 4 5 6 7

TIME, HOURS FIG. 26. Moisture content of material and temperature of air in a

drying potato half-dice. Eight trucks (540 sq. f t . of tray surface in stage, 16 trucks in secondary. Tray loading, 1.50 Ib. per sq. ft. Air trays, 1000 ft. per min. (from Van Arsdel, 1951b, Fig. 15).

, two-stage tunnel each) in primary velocity between

of potato half-dice, are shown in Fig. 26, taken from an example computed by Van Arsdel (195lb), Fig. 15. In this case the secondary tunnel is twice as long as the primary. Van Arsdel showed that the combination, carrying altogether 24 truckloads of material, should produce about 7 % more product dried to 6% moisture than two 12-truck counterflow tunnels and use about 5 % less fuel. To offset these advantages, the capital cost of the two-stage dehydrator would be somewhat greater, especially if truck transfer between stages were handled automatically, and the control of the unit would be more complex and critical with respect to maintenance of product quality.


The ratio of lengths of the two stages of this type of dehydrator apparently can vary within rather wide limits without much effect on performance. Commercially they have ranged between equality in length and 2 to 1. Theoretically, for the example of Fig. 26, maximum output would have been realized if the total of 24 trucks were divided 7 to 1, that is, 3 trucks in the parallel-flow primary, 21 trucks in the counterflow secondary; however, the gain in output would have been negligible, and operation of the 3-truck primary would make extreme demands on control and scheduling of the system.

None of the theories referred to above is directly applicable to study of the operation of transverse-flow compartment tunnels. In commercial forms of these dehydrators there may be as many as 6 or 8 compartments in which the air temperature is individually controlled a t desired levels. A truckload of material standing a t any one of these positions is exposed to unvarying drying conditions until i t is shifted to the next position, where the direction of air flow is reversed and a new set of drying conditions is maintained. No satisfactory mathematical formulation has been proposed for this kind of system. Prediction of the performance of a transverse-flow dehydrator, with given values for the air flow and air temperature a t each compartment, could presumably be accomplished by means of an analog type of drying experiment somewhat similar t o that proposed by Broughton and Mickley (1953).

2. Optimum Tray Loading

Drying-rate experiments have invariably shown that the rate of drying of wet materials spread on trays decreases as the load of material on the trays increases, once the load exceeds appreciably that of a single layer of pieces on the tray. The drying time is therefore shortest for light tray loadings. But for a fixed area of tray surface in a dehydrator, the output of produce is proportional t o the tray loading. Net dehydrator capacity is the resultant of these two effects.

Quantitative estimations have been made of the effect of tray loading on dehydrator output, for 2 quite different vegetable products, potato half-dice and cabbage shreds. The drying-rate data are those of the ATC-31-VII and AIC-31-IV nomographs. Trucks in the tunnel postulated for these examples each contain 400 sq. f t . of useful tray surface. The mass air flow through the tunnel is 2000 lb. of dry air per min. and the air velocity between trays a t the wet end of the tunnel will be 900-1000 ft. per minute. For cabbage, a hot-end temperature of 140" F. and a wet-bulb temperature of 90" F. are assumed in counterflow drying, 180" F. and 95" F. in parallel-flow drying; for potatoes, a hot-end temperature of 150' F. and a wet-bulb temperature of 90" F . in counterflow drying,


20 - PARALLEL-

CABBAGE SHREDS

0 0.5 I .o 1.5 2.0 TRAY LOADING, L BS./ S 0. FT.

FIG. 27. Effect of tray-loading upon tunnel capacity in dehydrating cabbage shreds; counterflow tunnel, 9 trucks, drying to 4.75 % moisture; parallel-flow tunnel, 12 trucks, drying only to 9.1 % moisture.

35 t PARALLEL- - I

POTATO HAL F-DICE

0 " 0 I 2 3

T RAY L 0 AD 1 N G, L BS. / S 0. F T FIG. 28. Effect of tray-loading upon tunnel capacity in dehydrating potato half-

dice; counterflow tunnel, 10 trucks, drying to 6.55 % moisture; parallel-flow tunnel, 12 trucks, drying only to 16.7% moisture.


200' F. and 100" F. in parallel-flow drying. Final moisture content, for counterflow operation, is 4.75% (wet basis) for cabbage, 6.55% for potatoes; for parallel-flow operation (used as the first stage in multiple- stage dehydration), 9.1 % for cabbage, 16.7% for potatoes.

Figures 27 and 28 show estimated tunnel capacity for dehydration of cabbage shreds and potato half-dice (tons of prepared material per 24-hr. day) as a function of tray loading, pounds of prepared material per square foot. Tunnel capacity reaches substantially a maximum at a loading of about 1.5 lb. per sq. ft. for the cabbage shreds, something over 3 lb. per sq. ft. for the potato half-dice. The figures for drying time marked on the curves indicate that, if the tunnel is being operated near its maximum loading, a material shortening of drying time may be accomplished with only minor sacrifice of capacity by lightening the tray loading. If heat damage is being experienced, this may be an important remedial measure.

3. Optimum Recirculation of Air

The reasons for, and advantages and disadvantages of recirculation of air in a tunnel dehydrator have been discussed by Van ArsdeI (195la, pp. 80-84). Recirculation raises the wet-bulb temperature of the air, returns some heat t o the system and thereby saves fuel, but a t the same time reduces the drying rate of most materials. Purely from a drying cost standpoint, the saving in fuel cost must be balanced against the decrease in production and the attendant increases in other unit costs. Ramage and Rasmussen (1943) noted that there should be some proportion of recirculation that would give the minimum drying cost per pound of product, and they computed this optimum for one simple set of conditions. So many combinations of the numerous variables are possible that no single general principle has been established. The following example illustrates the procedure and typifies the kind of results that map be computed

A counterflow tunnel long enough to hold'a maximum of eleven %foot trucks (400 sq. ft. of useful tray surface on each truck) is to be used to dehydrate potato half-dice to 6.55% moisture (wet basis) or cabbage shreds to 4.75% moisture. Trays will be loaded with 1.40 lb. of prepared cabbage or 2.50 lb. of prepared potato per sq. f t . The fan supplying the air flow through a pair of these tunnels is of the "limit-load" type, double width, with a standard air rating of 54,000 cu. ft. per min. against 1.5 in. static pressure, when operated at 423 r.p.m. and absorbing approximately 17 h.p. The hot-end temperature for the cabbage dehydration will be 140" F., for the potato dehydration 150" F. The power absorbed by the fan a t a temperature of 140-150" F. is approximately 15 h.p. The outside fresh air temperature is 60" F., absolute hu-midity is 0.0100 lb. of water


vapor per lb. dry air. The tunnel will be operated with a minimum of a 15"-wet-bulb depression a t the cool end, and, if an increase in recirculation would result in a lower wet-bulb depression than that, the load on the tunnel will be reduced by decreasing the number of trucks in it. The cost of heat is taken as 35# per million B.t.u. transferred to the air stream, the cost of power 26 per kw.-hr., operating labor cost for each side of the double tunnel $1.45 per hr. regardless of output within the range considered in the example, and all other costs (plant overhead and fixed charges) $1.35 per hr.

n

t

OTHER 7 POWER

c

t LABOR

" 00 90 100 0 Q20 0.40 0.60 0.00 1.00

WE T-BULB PROPORTlON CF AIR REClRCULATED FIG. 29. Effect of air recirculation upon drying costs: shredded cabbage, counter-

flow tunnel.

The computations involve estimation of drying time for selected values of the wet-bulb temperature, using the AIC-31 nomographs, and then deriving the corresponding values of tunnel output, proportion of air recirculated, and necessary heat input. The approximations described by Van Arsdel (1951a) were used. The results of the computations are shown graphically in Figs. 29 and 30.

It is evident from these curves that, for the combination of conditions chosen, the minimum total cost for drying both vegetables would be realized by employing little or no recirculation of the air; the saving in cost of heat through recirculation would be more than offset by the increases in other costs.

Quite a different result would have appeared if a material like prunes were being dehydrated. According to Guillou (1942) and Perry (1944), the drying rate of prunes is substantially independent of the relative humidity of the drying air unless the relative humidity exceeds about 35 %. That being the case, raising the wet-bulb temperature by increasing


the proportion of recirculation within reasonable limits will decrease the cost of heat materially without a t the same time increasing other costs.

8 TRUCKS

TOTAL / 8 I TOTAL

z w o 0 80 100 120 0 0.20 0.40 0.60 a00 1.00

POWER

WET- BULB TE MP E RAT U RE

PROPORTION OF AIR REClRCULATED

FIG. 30. Effect of air recirculation upon drying costs: potato half-dice, counterflow tunnel.

No data are available to indicate whether other fruits, such as grapes and sliced apples, will behave more like the cut vegetables than like prunes.

4. Product Temperature in the Dehydrator

Perry (1944) and Perry et al. (1946) have published measurements of the internal temperature of prunes during dehydration. Figure 31, taken

I-

100 } ~

0 6 I2 18 24

TIME, HOURS FIG. 31. Air temperature and fruit temperature in a typical counterflow prune

tunnel (from Perry et al., 1946, Fig. 14).

from the second of these references, illustrates the typical course of fruit temperature during counterflow dehydration, with the air temperature


also shown for comparison. There appears to be no tendency for the fruit temperature to hold for a period of time a t or near 105" F., the wet-bulb temperature of the air.

No similar measurements of material temperature of vegetable pieces undergoing dehydration have been published, in spite of the obvious importance of the time-temperature relationship to the quality of the dry product. The following procedure leads t o a rough approximation to the material temperature under typical tunnel conditions.

There is some evidence that during early stages of drying the shrinkage in volume of vegetable pieces very nearly equals the volume of water lost by evaporation, but in later stages the volume shrinkage is less, and no substantial further decrease in volume occurs as the pieces dry below about 15 to 20% moisture. If the density of the dry substance is 1.25 g. per ml., the piece area during early stages should change as follows:

(3)

Study of drying rates for potatoes shows that this relationship holds down to about w = 1.50 lb. of moisture per lb. dry solids (60% moisture) for potato pieces. A graph of area versus moisture content can be extrapolated with only moderate curvature to an expected final dry area of 43 % of the original area.

If it is now assumed that the drying rate in very early stages maintains the material a t the wet-bulb temperature of the air by convective heat transfer (radiative and conductive transfer being neglected), a convective heat transfer coefficient can be computed from the following relationship:

Lo dw w,, + 1 d6

-H, ~. - - (4)

The transfer coefficient k is expressed here in terms of unit area of tray surface. Now if i t be further assumed that this transfer coefficient will remain unchanged throughout the entire tunnel, allowance being made for the area shrinkage of the material, the piece temperature will be given approximately by the following equation :

HL, dw* i%a(w, -+ 1) '2Z T = t f (5 )

Material temperatures computed in this way, and the corresponding



220

2oo-l

180

w 160- [L 3

160 L

I \

SECONDARY \ PRIMARY

7 STAGE STAGE - \

\ \

', ' AIR TEMPERATURE

g 120 , 4 E

WET-BULB TEMPERATURE I


can easily lead to the serious quality defect variously known as “heat damage,” ‘‘ caramelization,” L‘scorching,’l or simply “browning.” A product which has been damaged in this way by the drying step may be unacceptable as a food product because of off-color and off-flavor.

The phenomenon, by whatever name called, is the result of a highly complex system of reactions in and among the natural constituents of the fruit or vegetable. Its chemistry is beyond the scope of this article. The two points of importance here are these: (a) the reactions leading to (‘browning” or ‘(scorching” have a very large positive temperature coefficient; (b) the reaction rate increases as the concentration of the components increases (i.e., as the moisture content of the product is reduced), but water itself must be involved in the reactions in some way, because when the moisture content is reduced far enough the rate of browning becomes very low. The result is that, in some intermediate range of moisture content, the rate of browning a t any given temperature goes through a maximum. Studies of the kinetics of this system have been reported by Hendel et al. (1954). For white potato the maximum browning rate occurs in the range of 15 to 20% moisture content. When sufficient quantitative information becomes available, it may for the first time be possible to devise dehydrator-operating procedures for an optimum combination of high output and minimum heat damage.

5. Departures from Theory

The foregoing discussion of tunnel theory ignores several complica- tions of actual dehydrator operation. One of these is inherent in the nature of a tunnel-and-truck drier, whereas the others may be classed as in- evibable imperfections of design and operation.

The simple theory that has been presented assumes that the tunnel is loaded continuously and operates in the steady state so that the material moisture content and the air temperature and humidity a t any given point along the length of the tunnel will remain unvarying. The fact that truckloads of material are introduced a t finite intervals of time and are advanced through the tunnel in discrete steps, introduces a complication for which no mathematical description has been proposed. The temperature to which a product is exposed in its progress through the tunnel changes discontinuously, not along a smooth curve. This so-called “saw- tooth effect” has been discussed by Van Arsdel (1951a). Practical experience suggests that its effect cannot be very great, except possibly in short parallel-flow tunnels.

The imperfections of construction and operation, on the other hand, sometimes may have very serious effects. The commonest causes of trouble are poor air distribution, temperature stratification in the air,

TUNXEL DEHYDRATORS FOR FRUITS AND YEGETM3LES 363

uneven loading of trays, and poorly designed or sagging trays. Even with good design and careful maintenance and operation, these troubles will always be present to some extent. Since they obviously cannot be em- bodied in the theory in any quantitative way, the dehydrator operator must learn by experience how much of a safety factor he must apply to the predictions of dehydrator theory.

VII. OPERATING PROCEDURES FOR TUNNEL DEHYDRATORS

Methods for dehydrating fruits and vegetables have been and are constantly being improved, for competition is keen and i t is necessary to increase product quality and general efficiency. Although the dehydration ar t is well established, the drying characteristics of most vegetables and fruits are known only in rather general terms. Conditions and operation procedures differ from plant t o plant, and general recommendations may not apply to a particular dehydrator. Therefore, only certain items of general interest will be brought t o the reader's attention, and details of operating a tunnel drier will be described only briefly. From the discussion which follows, it will become apparent that, t o achieve best results, each operator must take into consideration the physical setup and conditions present in his plant and use his initiative to secure the best combinations for high product quality, capacity, and over-all efficiency.

As illustrated by the series of curves (Figs. 23 and 24), and as previously mentioned, if the tunnel length, air velocity, and initial temperature are fixed, then the characteristics of the material being dried will determine the drying time and final air temperature. The original authors have pointed out that even though the curves apply quantitatively only to a particular set of conditions, the character of the curves is such that broad generalities can be deduced. Some of these generalized facts are pertinent to the operation and proper use of tunnel driers and merit discussion. If the commodity being dehydrated is a slow-drying material, the temperature drop per foot of tunnel length will be small and long tunnels can be used. If the tunnel is relatively short, the temperature drop through i t will be small, and unless air recirculation is used, the heat efficiency will be low. On the other hand, if the commodity is a fast- drying material, the temperature drop per foot of tunnel length will be large. In this case, if a long tunnel is used, it should not be completely filled with trucks, otherwise drying conditions will be unsatisfactory a t the cool humid end. However, if the number of trucks in the tunnel is increased, the tunnel output capacity will be increased (assuming there is no change in the air velocity), but a t the possible expense of injury to the product. The latter is particularly true, since both the drying time and


possibility of other unfavorable drying conditions will be increased. Therefore, it is apparent there is an optimum balance for each different operating condition. The series of curves referred to previously, also illustrate another fact. If the number of trucks is increased, and a t the same time the air velocity is increased just enough to keep the wet-bulb depression unchanged a t the wet end of the tunnel, two desirable things occur. The product drying time becomes shorter (a tendency that favors product quality), and the output of the tunnel increases. Thus the operator must use good judgment and balance the gains in increased capacity and better product against the substantial increase in the power required to run the blower. It should be remembered that power consumption increases approximately eight-fold if the air velocity is doubled, even though the number of trucks remains the same (see pp. 326-332, Fans and Blowers). Power consumption will be even greater if the number of trucks is increased simultaneously.

Although several tunnels in a dehydration plant may be of identical design and be drying the same commodity to a given moisture content, the drying time will inevitably differ somewhat from tunnel to tunnel. Also there may be even a day to day difference in the drying time for a given tunnel. Small variations in drying time can be caused by slight differences in the amount of recirculated air, effectiveness of air distribution in the tunnel, hot-air temperature, uniformity of tray loading, atmospheric conditions at the fresh air intake, and last, but not the least important, are the variations in the nature of the prepared commodity. The effect of these variables has been discussed in TJnited States Department of Agri- culture Miscellaneous Publication No. 540 (1944), and by Van Arsdel (1951a).

Although the drying times will differ as indicated, there will be a mean time interval for each truck handled. The preparation line must be geared to that rate, and even then there will occasionally be either a shortage or an excess of loaded trucks at the dehydrators. Several tunnels may be needing trucks at the same time, and a little later other loaded trucks may begin to accumulate because no tunnel is ready to accommodate them. Regardless of this situation, a crew a t the dry end of the tunnel must be ready to remove the trucks of dried product whenever the cars are scheduled to be pulled. It is necessary to keep the dry-end foreman in- formed as to the exact time each wet truck load is placed in a tunnel, so that he can add the probable cycle times and schedule the pull times. This calls for some kind of message or signal system plus a running log sheet a t the dry end of the tunnel on which the probable pulling times can be entered. If the preparation line must be slowed down for a number of hours, the drier foreman needs advance notice, for he may have to change


drier conditions to compensate for partly empty tunnels. A more complex scheduling system is needed in a plant which operates two-stage tunnel dehydrators, than in one which uses only counterflow units.

Operation of a tunnel dehydrator under normal conditions presents few difficulties. At the beginning and end of a processing run, certain modifications of tunnel drying conditions are usually necessary to prevent product injury during the starting-up and shutting-down periods. There are various procedures for starting and shutting down tunnel dehydrators, as discussed by Perry and associates (1946), and as explained in the United States Department of Agriculture Miscellaneous Publication No. 540 (1944). Exact procedures will, of course, vary from plant t o plant. Essentially, the methods consist of readjusting either the amount of recirculation air, the air temperature, or rate of tunnel loading, or adjusting and balancing combinations of these factors which affect the drying conditions within the tunnel. The procedures are a necessary operating requirement, not only to minimize excessive heat damage to the product, but also to avoid other undesirable drying conditions, which may in turn, cause other types of quality deterioration before the product is dried.

As an example of the latter condition, let us assume that several trucks or cars of wet material are placed in a counterflow tunnel on start-up. The evaporation rate from the first truck would be great, and the drop in temperature of the air passing through that car would therefore be high. The air moving through the last truck would be nearly saturated. The product in that car would be heated up rapidly to the wet-bulb temperature of the air, and little evaporation would take place until the first cars of material had lost a considerable part of their moisture content. Such drying conditions would be very undesirable, and probably adversely effect the product quality. For example, under some conditions, there might be an excessive loss of ascorbic acid, or rapid growth of micro- organisms and attendant spoilage. On the other hand, as the leading truckload of product progressed through the tunnel, i t would be subjected to an abnormally hot, untempered blast of air during its entire time in the tunnel. If the product happened to be heat sensitive, then heat damage might result. Furthermore, each succeeding truck would be subjected to nonuniform drying conditions as the tunnel was progressively filled.

In order to avoid the difficulties mentioned, one of the start-up procedures for counterflow tunnels uses a modified hot-end air temperature schedule, and loaded trucks are pushed into the tunnel a t the normal or regular scheduled rate. Another method fixes the hot-end air a t the normal operating temperature, and a t first the loaded trucks are rolled into the tunnel a t a faster rate than the normal schedule. The time interval between introduction of cars is progressively increased until normal

366 P. W. KILPATRICK, E . LOWE, *4ND W. B. VAN ARSDEL

operating conditions are reached. If the latter method is used, scheduling can become rather complex if there are numerous tunnels.

Regardless of the starting method used, as the pull-time approaches for the first car, the dried material is usually examined and the drying schedule modified if necessary. Experience shows that a trained operator can estimate the moisture content of the warm, nearly dry material rather closely by “feel.” One of the authors, during the course of research work on vegetable dehydration, developed this knack and could readily estimate moisture contents of dehydrated products, such as cabbage, carrots, white potatoes, sweet potatoes, onions, spinach, corn, peas, and green beans. Normally, the uncertainty of the estimates, as shown by subsequent assays, was about 1 % moisture content.

Terminating a run in a counterflow dehydrator is relatively simple. As the evaporative load in the tunnel decreases, there will be a decline in the wet-bulb temperature of the air if recirculation is being used. Ordinarily this condition will not result in product injury, but may carry the material to a lower moisture content than desired. To prevent this, some operators readjust the recirculation damper, and by doing so also save some fuel. Sometimes the tunnel temperature is lowered slightly, or the last trucks are removed ahead of schedule.

In the first stage tunnel (or parallel-flow section) of a two-stage dehydrator, drying conditions are rather critical during the start-up and shutdown periods. During normal operation, the controlled air temperature a t the hot end is usually above 180” F., and, in some cases, may be set in the region of 250” F. Most of the original moisture is removed from the product while it is in the first-stage drier. The material is usually dried to a moisture content of about 50% in that section of the dehydrator. This corresponds to a moisture removal of roughly 75 t o 90% for most commodities. However, the evaporative load in the tunnel is obviously light during the start-up period. To retard excessive drying of the product’s outer surface and prevent the consequent possibility of case hardening, heat damage, or even scorching, it is necessary to keep the wet-bulb temperature of the air a t a reasonably high level. Therefore, during the starting-up period, it is necessary to readjust the recirculation damper carefully, so as to maintain normal operating wet-bulb conditions in the parallel-flow tunnel section. Readjustment of the damper may be required after each truck enters that tunnel section, and until the section is full and normal operating conditions are reached. Likewise, when operations are being terminated, good judgment must be used in shutting down the first stage. Near the end of a run, as filled trucks are replaced by empties, the evaporative load decreases, and consequently, since recirculation is normally used, the wet-bulb temperature of the air will


begin to drop. Instrumentation usually controls the dry-bulb temperature of the air. Unless the instrument is manually reset, the air will remain at its constant high temperature level. If changes are not made, the last filled truck passing through the first stage will be subjected to a constant high dry-bulb temperature and a continually decreasing wet-bulb temperature. Unless controlled, there will be a cumulative effect that will produce higher and higher drying rates within each car of the last tunnel load. This may result in case hardening, discoloration, scorching, or other product injury. Corrective measures are relatively simple ; for example, the hot-end, dry-bulb temperature can be progressively decreased step- wise, as empty trucks replace full ones. Theoretically, each temperature drop should be equal t o the average temperature drop across a truck. The latter can be estimated from the known operating data for that particular first-stage tunnel (normal hot-end temperature minus normal cold-end temperature divided by the number of trucks in that tunnel section). In a similar manner, the wet-bulb can be kept a t a relatively constant level by adjusting the recirculation damper.

VIII. RECENT TRENDS IN TUNNEL DEHYDRATION OF FRUITS AND VEGETABLES

The vegetable dehydration industry has gradually come to rely more and more on the use of bin-type finishing driers. Their operation, general use, and design are discussed in United States Department of Agriculture Miscellaneous Publication No. 540 (1944), and Bureau Publication AIC-15 (1943, Revised 1944). The advantages obtained through the use of finishing bin driers are numerous. The bins provide a low-cost method for removing the moisture from the product during the slow-drying stage near the end of the dehydrating process, and they permit close control of the product’s moisture content during the final stage of drying. The bins also provide an economical means for holding the product and equalizing its moisture content before the material is packaged. Dried apples and prunes are commonly stored for some time in bins after drying for this same purpose. In addition, a properly designed bin finishing system can materially increase tunnel capacity and improve operation flexibility. In some plants the bins are made portable for convenience. additional flexibility of operation, and also, they reduce conveyer equipment requirements. Sometimes dehumidified air is supplied to the bins to expedite the drying process. The theory of through-flow drying in the low-moisture region, as applied to bin finishing driers, has been discussed by Van Arsdel (1953).

In order to make the dehydration operation less critical with respect t o possible heat damage or other injury to the product, morKattention is


being directed to the quality and preparation of the raw material. For example, consideration is now being given to the control of the sugar content in potatoes. Campbell and Kilpatrick (1945) demonstrated that sensitivity to heat damage during dehydration is a function of the reducing sugar content in the potatoes. It has been shown that blanching is necessary during the preparation of most vegetables that are to be dehydrated. The two primary reasons for this requirement are: (I) to prevent or check the development of undesirable colors, flavors, and loss of vitamins which occur during dehydration and subsequent storage, and (2) to obtain a finished product which will rehydrate readily, cook rapidly, and yield a cooked product of desirable texture and flavor qualities. There are no data available which directly correlate the degree of enzyme in- activation and quality retention of dehydrated products during storage. However, most investigators agree that blanching to a degree sufficient to inactivate the peroxidase enzyme systems present in the various commodities is sufficient for quality retention. Blanching much beyond this point is undesirable due to the loss of soluble nutrients as a result of leaching caused by extended exposure of the product to the blanching medium. In order to prevent undesirable changes during dehydration and storage, there is a recent trend to treat the raw prepared material with additives. Sulfur dioxide has, of course, been used for this purpose for many years. Some of the new additives may have certain advantages over sulfur dioxide. As examples of the new additives, starch has been used to coat prepared carrots. Compared with sulfur dioxide, the starch gives carrots equal or better protection from loss of carotene and color during subsequent storage, according to Masure and associates (1950). Treatment of potatoes with calcium chloride to control sloughing and reduce heat damage during dehydration was suggested by Campbell and Kilpatrick (1945). Experimental confirmation of the favorable results of this treatment has recently been presented by Simon et al. (1954). The same authors (1953) proposed the use of thinner pieces, treated with calcium chloride to control sloughing, in order to speed up the drying and thus reduce heat damage. In effect, most of these procedures increase the drying capacity of the dehydrators by permitting the use of higher drying temperatures.

The mounting cost of labor has caused a trend toward the use of labor-saving devices in the operation of dehydrators. Semi-automatic and automatic tray loaders, stackers, and unloaders have been recently developed for use in conjunction with tunnel drier operations in the dehydration industry, and they have been used with apparent success. Automatic truck transfer equipment is being used to handle cars between staged tunnels. Reducing the complexity of operation tends to alleviate


the labor problem. Therefore, there is a tendency to favor the simple single-stage tunnels, rather than the more complex multistage units. Some vegetable dehydration plants even prefer to operate their tunnels without recirculation of air, sacrificing fuel economy in order to simplify the job for the tunnel crew. As pointed out in another section of this chapter (pp. 357-359), the drying characteristics of cut vegetables are such that this practice probably entails little or no increase in total drying cost.

The vast majority of the fruit and vegetable dehydration plants in the United States use truck-and-tunnel driers in preference to other type dehydrators. A small number of conveyer driers is in use, and the popu- larity of this type of dehydrator appears to be increasing. However, for many years to come, the tunnel drier will continue to serve as the depend- able workhorse of the fruit and vegetable dehydration industry. The authors believe it will continue to occupy a foremost place in the further development of the industry.

IX. LIST OF SYMBOLS USED

A-Evaporating area of a piece, square feet. A a-Area as a proportion of the original area, = - A 0

83XL, G(w, + I ) . -Coefficient in tunnel equation, =

G--Mass air flow, pounds dry air per minute. 11-Latent heat of evaporation, B.t.u. per pound. /<-Heat transfer coefficient,,B.t.u. per hour per degree F. per square

L-Tray loading, pounds per square foot. ,\'--Total tray area in the tunnel, square feet. ?'-Material temperature, degrees F. t --Air temperature, degrees F. w-Moisture content of the material, pounds water per pound dry

%-Distance along tunnel, feet. 0-Time, hours. Subscripts : o-Initial conditions.

foot of tray surface.

solids.

-At wet-bulb temperature.

REFERENCES Bateman, E., Hohf, J. P., and Stamm, A. J. 1939. Unidirectional drying of wood.

Broughton, D. C., and Mickley, H. S. 1953. Design of full-scale continuous-tunnel Ind. Eng. Chem. 31, 1150-1154.

drier. Chem. Eng. Progr. 49, 319-324.

370 P. W. KILPATRICK, E. LOIVE, AND W. B . VAN ARSDEL

Brown, A. H. 1943. Heat balance in tunnels. Unpublished report. U. S. Dept. Agr., Bureau Agr. Ind. Chem., Western Regional Research Lab., Albany, Calif.

Brown, A. H., and Kilpatrick, P. W. 1943. Drying characteristics of vegetables-riced potatoes. Trans. Am. SOC. Mech. Engrs. 66, 837-842.

Campbell, H., and Kilpatrick, P. W. 1945. Effect of storage temperatures on sensitivity of White Rose potatoes to processing heat. Fruit Products J . 26(4), 106-108, 120-12 1.

Carrier, W. H. 1911. Rational psychrometric formulae. Trans. Am. SOC. Mech. Engrs. 33, 1005-1053.

Carrier, W. H. 1921. The theory of atmospheric evaporation-with special reference

Chapman, F. C. 1922a. U. S. Patent No. 1,404,369. Chapman, F. C. 192213. U. S. Patent No. 1,422,416. Christie, A. W. 1926. The dehydration of prunes. Univ. Calif. Agr. Expt. Sta. Bull .

No. 404, 10-19, 19-21, 21-23, 23-25. 25-26, 26-32. Revised by P. F. Xichols, Dec. 1929.

Cruess, W. V. 1919. Evaporators for prune drying. Univ. Calif. Agr. Expt. Sta. Circ. No. 213.

Cruess, W. V. 1935. “Commercial Fruit and Vegetable Products,” 2nd ed., pp. 462- 501, 502-513, 463-479, 479-490. McGraw-Hill, New York.

Cruess, W. V., and Christie, A. W. 1921a. Dehydration of fruits (A Progress report). Univ. Calij. Agr. Expt. Sta. Bull. No. 330.

Cruess, W. V., and Christie, A. W. 1921b. Some factors of dehydrator efficiency. Univ. Calif. Agr. Expt. Sta. Bull. No. 337.

Cruess, W. V., and Mackinney, G. 1943. The dehydration of vegetables. Cniv. Calif . Agr. Expt. Sta. Bull. No. 680, 9-19, 19-22, 41-48.

Ede, A. J., and Hales, H. C. 1948. The physics of drying in heated air, with particular reference to fruits and vegetables. Dept. Sci. Ind . Research Food Incest. Board (Brit.) Special Repts. No. 63.

Eidt, C. C. 1935. Principles and methods involved in dehydration of apples. Can. Dept. Agr. Publ. No. 626, Tech. Bull. No. 18.

Great Britain, Ministry of Foods. 1946. Vegetable dehydration. (Scientific and Technical Series.)

Grosvenor, W. M. 1908. Calculations for dryer design. Trans. Am. Inst. Chem. Engrs.

Guillou, R. 1942. Developments in fruit dehydrator design. Agr. Eng. 23, 313-316. Guillou, R., and Moses, B. D. 1943. Farm fruit dehydrator. University of California

Agricultural Engineering and Agricultural Extension, Farm Building Plan C-214: 1-22. (Mimeo.)

Hausbrand, E. 1912. “Drying by Means of Air and Steam.” Scott, Greenwood & Sons, London,

Hendel, C. E., Silveira, V. G., and Harrington, W. 0. 1954. Non-enzymatic browning of white potato. Presented June 1954, Institute of Food Technologists, Los Angeles, Calif.

Hougen, 0. A., McCauley, H. J., and Marshall, W. R., Jr. 1940. Limitations of diffusion equations in drying. Trans. Am. Inst. Chem. Engrs. 36, 183-209.

Lazar, M. E. 1944. Deviations from adiabatieity in tunnel dehydrators. Unpublished report. U. S. Dept. Agr., Bureau Agr. and Ind. Chem., Western Regional Re- search Lab., Albany, Calif.

Lewis, W. K. 1921. The rate of drying of solid materials. Ind. Eng. Chem. 13,427-432.

to compartment dryers. Ind. Eng. Chem. 13, 432-438.

1, 154-202.

TUNNEL DEHYDRATORS FOR FRUITS AND VEGETABLES 37 1

McCready, D. W., and McCabe, W. L. 1933. The adiabatic air drying of hygroscopic

Marshall, W. R., Jr. 1942. The drying of food. Heating, Piping Air Conditioning 14,

Marshall, W. R., Jr. 1943. The drying of food. Heating, Piping Air Conditioning 16,

Marshall, W. R., Jr., and Friedman, S. J. 1950. In “Chemical Engineers Handbook” (Perry, ed.), 3rd ed., pp. 800-884. McGraw-Hill, New York.

Masure, M. P., Bohart, G. S., Eastmond, E. J., and Boggs, M. M. 1950. Value of starch coating in the preservation of quality of dehydrated carrots. Food Technol. 14, 9G97.

Newman, -4. B. 1931a. The drying of porous solids: Diffusion and surface emission equations. Trans. Am. Inst. Chem. Engrs. 27, 203-220.

Newman, A. B. 1931b. The drying of porous solids: Diffusion calculations. Trans. Am. Inst. Chem. Engrs. 27, 310-333.

New Zealand Department of Science and Industrial Research 1944. Vegetable dehydration. (Mimeo.)

Pearson, J. W. 1923. U. S. Patent No. 1,461,224. Perry, R. L. 1944. Heat and vapor transfer in the dehydration of prunes. Trans. Am.

SOC. Mech. Engrs. 66, 447-456. Perry, R. L. 1947. Designing a counterflow tray drier from laboratory data. Unpub-

lished report, Apr. 15, 1947. Univ. Calif. Agr. Expt. Sta., Davis, Calif. Perry, It. L., Mrak, E. M., Phaff, H. J., Marsh, G. L., and Fisher, C. D. 1946. Fruit

dehydration. I. Principles and equipment. Dec. 1946. Univ. Calif. Agr. Ezpt. Sta. Bull. No. 698.

Prescott, S. C., and Proctor, B. E. 1937. “Food Technology,” 1st ed., pp. 488-508. McGraw-Hill, New York.

Puccinelli, R. L. 1923. U. S. Patent No. 1,464,338. Raniagr, IT-. D., and Rasmussen, C. L. 1943. This is what i t costs to dehydrate

vegetables. Food Inds. 16(7), 64-71, 137-138; (8), 66-67, 118-119; (9), 75-77, 126. Rasmussen, C. L., and Shaw, W. L. 1953. Preliminary planning for vegetable dehydra-

tion. Bureau Publication AIC 356, U. S. Dept. Agr., Bureau Agr. and Ind. Chem., Western Reg. Research Lab., Albany, Calif., June 1953.

solids. Trans. Am. Inst. Chem. Engrs. 29, 131-160.

527-531, 588-591, 671, 673, 724-728.

10-12, 567-572.

Itees, C. 1922. U. S. Patent No. 1,413,135. Itidley, G. B. 1921. Tunnel dryers. Ind. Eng. Chem. 13, 453-460. Sherwood, T. K. 1929. The drying of solids, I, 11. Ind. Eng. Chem. 21,12-16,97&-980. Sherwood, T. K. 1930. The drying of solids, 111. Mechanism of the drying of pulp and

paper. Ind. Eng. Chem. 22, 132-136. Sherwood, T. K. 1931. Application of theoretical diffusion equations to the drying of

solids. Trans. Am. Inst. Chem. Engrs. 27, 190-202. Sherwood, T. K. 1932. The drying of solids: Application of the diffusion equations.

Ind. Eng. Chem. 24, 307-310. Sherwood, T. K. 1936. The air drying of solids. Trans. Am. Inst. Chem. Engrs. 32,

15&168. Simon, M., Wagner, J. R., Silveira, V. G., and Hendel, C. E. 1953. Influence of piece

size on production and quality of dehydrated Irish potatoes. Food Technol. 7,

Simon, M., Wagner, J. R., Silveira, V. G., and Hendel, C. E. 1954. Calcium chloride as a non-enzymatic browning retardent for dehydrated white potatoes. Presented June 1954, Institute of Food Technologists, Los Angeles, Calif.

423-428.


Tiemann, H. D. 1917. The theory of drying and its application to the new humidity- regulated and recirculating dry kiln. U. S. Dept. Agr. Bull. No. 609.

Thorp, F. H. 1905. “Outlines of Industrial Chemistry,” 2nd ed., pp. 545-546. Mac- millan, New York.

United States Western Regional Research Laboratory. 1943 (Revised 1944). Informa- tion sheet on bin-type finishing driers in vegetable dehydration. U. S. Bureau of Agricultural and Industrial Chemistry, Albany, Calif. AIC-15.

United States Bureau of Agricultural and Industrial Chemistry. 1944. Vegetable and Fruit Dehydration-A Manual for Plant Operators. U. S. Dept. Agr. M i x . Publ.

United States Western Regional Research Laboratory. 1943-1947. The application of drying-rate nomographs to the estimation of tunnel dehydrator drying capacity. U. S, Bureau Agr. Ind. Chem., Albany, Calif., AIC-31. I. Riced white potatoes (Rev. June 1947). 11. Blanched sweet corn (Nov. 1943). 111. White potato strips, vertical airflow (Jan. 1944). IV. Shredded cabbage (Feb. 1944). V. Onion slices (Apr. 1944). VI. Sweetpotato strips (Sept. 1944). VII. White potato half-cubes (Mar, 1945). VIII. Carrot pieces (May 1947).

Van Arsdel, W. B. 1942. Tunnel dehydrators and their use in vegetable dehydration. Food Inds. 14(10), 43-46, 106; (ll), 47-50, 103; (12), 47-50, 108-109.

Van Arsdel, W. B. 1947. Approximate diffusion calculations for the falling-rate phase of drying. C h e m Eng. Progr. 43, 13-24: (also U. S. Bureau Agr. and Ind. Chem., AIC-152, issued a t Albany, Calif.).

Van Arsdel, W. B. 1951a. Principles of the drying process, with special reference to vegetable dehydration. U. S. Bureau Agr. and Ind. Chem., Albany, Calif.,

Van Arsdel, W. B. 1951b. Tunnel-and-truck dehydrators, as used for dehydrating vegetables. U. S. Bureau Agr. and Ind. Chem., Albany, Calif., AIC-308.

V-an Arsdel, W. B. 1953. Simultaneous heat and mass transfer in nonisothermal systems: Through-flow drying in the low-moisture range. Presented December 1953, American Institute of Chemical Engineers, St. Louis, Mo.

NO. 640, 46-74, 77-88, 96-104, 114-115.

AIC-300.

Wiegand, E. H. 1923. Recirculation driers. Oregon Agr. Expt . Sta. Circ. No. 40. Yule, W. T. 1845. English patent, Jan. 28, 1845. Abstract in J . Franklin Znst. Ser.

111, 11, 179-180 (1846).

[advances in food research] advances in food research volume 6 volume 6 || tunnel dehydrators for...

Documents