industrial refrigeration handbook - capter 10 - vessels

367

CHAPTER10

VESSELS

10.1 VESSELS IN INDUSTRIALREFRIGERATION SYSTEMS

Vessels in industrial refrigeration systems serve either or both of the followingfunctions: (1) storage of liquid, and/or (2) separation of liquid from vapor. Themajor categories of vessels are:

• high-pressure receivers

• flash tank (or subcooler)/desuperheater

• low-pressure receiver for liquid recirculation

• surge drum on a flooded coil

• suction-line trap or accumulator

• thermosyphon receiver

Vessels are much more common in industrial refrigeration systems than inair-conditioning and there are several reasons for this difference:

Parallel refrigerant circuits. Air-conditioning systems usually are built witha single refrigerant circuit, while industrial systems incorporate parallelcompressors, condensers, and evaporators. With a system of multiplecomponents, liquid is likely to move from one condenser or evaporator to another.Also the liquid content in these components varies with time, so a vessel shouldbe available to provide a reservoir for these changes in liquid content.

Liquid recirculation and flooded coils. Liquid along with vapor leaves bothliquid overfeed and flooded coils, and this liquid must be separated for return

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Source: INDUSTRIAL REFRIGERATION HANDBOOK

368 INDUSTRIAL REFRIGERATION HANDBOOK

to the evaporator. The vapor that passes on to the compressor must be free ofliquid, so a vessel performs a process of separating the liquid from the vapor.

Defrosts. Industrial systems often refrigerate air at low temperatures whichbrings with it the need to periodically defrost evaporator coils. During hot-gasdefrosts, refrigerant liquid shifts locations.

Frequent expansions. When air-conditioning systems require additionalcapacity, it is usually provided by the installation of an additional single-circuitpackage. The expansion of industrial systems, on the other hand, is usuallyaccomplished by the installation of additional evaporators, compressors, andcondensers. Additional refrigerant inventory is required for such enlargements,and generously sized storage vessels facilitate such expansions.

10.2 LEVELS IN LIQUID RESERVOIRS

A fundamental principle in selecting the size of liquid vessels is to choosethem large enough that during operation they never become completely fullof liquid nor completely empty. As Fig. 10.1 shows, there must always besome vapor space above the highest liquid level to be experienced. A vesselcompletely filled with liquid may inadvertendly be valved off, and should thetemperature of the liquid increase, which would increase the liquid volume,pressures could develop so enormous that the vessel could rupture. Also, theliquid should not be permitted to completely drain from the vessel, becausethis would result in carrying vapor along with the liquid to the next component.Vapor bubbles adversely affect the performance of control valves and liquidpumps, for example.

FIGURE 10.1Useful storage volumes in (a) a vertical vessel and (b) in a horizontal vessel.



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10.3 VOLUME OF LIQUID IN APARTIALLY FILLED HORIZONTALVESSEL

Several occasions arise where the volume of liquid in a partially-filled vesselmust be calculated. One need will be encountered in this chapter in the sizingof low-pressure receivers. Another is when the inventory of refrigerant must becomputed, perhaps for regulatory purposes. The volume in a vertical vessel canbe computed easily by multiplying the cross-sectional area by the height of thethe liquid. For a horizontal vessel it is more complex. In the vessel of Fig. 10.2which has a radius r, the liquid level is at the plane indicated by AC. A formulafor the volume above the liquid level in Fig. 10.2 is:

where � is in radians=(� in degrees)÷57.3Usually the term sought is the volume of liquid or perhaps the fraction of

the vessel volume that is occupied by liquid, Frvol. It would be convenient tohave an expression for Frvol as a function of the fraction of liquid height, Frht.Frvol is the total volume less the vapor volume divided by the total volume,

From the triangle in Fig. 10.2b,

(10.1)

(10.2)

(10.3)

FIGURE 10.2Volume of liquid in a partially filled horizontal vessel.



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and since

Equation 10.4 for y/r may be substituted into Eq. 10.3 to find �, which canthen be substituted into Eq. 10.2 to calculate Frvol.

Example 10.1. For a horizontal cylindrical vessel 1.2 m (3.94 ft) in diameter and 3.5m (11.5 ft) long that is 2/3 full of liquid,

(a) what fraction of the vessel does the liquid occupy, and(b) what is the surface area of liquid?

Solution. (a) Fraction of vessel occupied by liquid. The vessel is 2/3 full of liquid, sofrom Eq. 10.4

From Eq. 10.3, � can be computed:

and finally, from Eq. 10.2

The volume of liquid in the vessel is

Volume of liquid=(�r2L) Frvol=�(0.62) (3.5) (0.709)=2.81 m3=99.1 ft3

(b) The liquid surface area is important in designing vessels that separate liquid andvapor,

To by pass the calculation chore, Table 10.1 lists values of Frvol as afunction of Frht.

10.4 LIQUID/VAPORSEPARATION—VERTICAL VESSEL

Along with providing storage for liquid, a chief function of most vessels used inindustrial refrigeration is to separate liquid from vapor to assure that the vaporreaching the compressor is free of significant amounts of liquid. The vessel shouldbe sized for whichever of the two functions, storage or separation, controls. Theprocess of separation that occurs in vertical vessels is different than that inhorizontal vessels, so design of the two orientations will be addressed separately.

(10.4)



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While it is usual to refer to liquid separation as though it is a process of completeremoval of liquid, such is not the case. In the vessel there is a spectrum of dropletsizes, and the separation techniques are successsful in removing only the largestdroplets. The small droplets are carried out, but some are evaporated in thesuction line and others vanish immediately on entering the suction of thecompressor. The droplet size is a defining characteristic in the separation principlesthat will be explained, which immediately raises the question of what is thelargest droplet size that should be permitted to escape. This question cannot beanswered by analytical means alone and must be decided on the basis of fieldexperience. Thus, if values used for the separation criteria in the design of thevessel result in liquid carryover problems, the criteria must be tightened.

Gravity is the fundamental force used for separating liquid from vapor andretaining the liquid in the vessel. When a drop of liquid falls freely in a motionless

TABLE 10.1Liquid volume fractions (Frvol) in horizontal vessels as a function of the liquidheight fraction, Frht.



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vapor, its maximum velocity occurs when the force of gravity just equals thedrag force, so no net force is available for acceleration. The upward velocity inthe vertical separator of Fig. 10.3 must be low enough that all but the small-diameter drops settle. It is reasonable to expect that separation will not beperfect, but if the only drops carried out are the small ones, the total mass ofliquid carried out will be small and the small drops can be vaporized moreeasily than the large ones.

When a drop of liquid falls freely in a motionless vapor, its maximum velocityis called the terminal velocity. In the separating vessel if the vapor moves upwardat the terminal velocity, a drop of the critical diameter remains suspended, thesmall drops are carried out, and the large ones settle. When a falling drop orparticle reaches its terminal velocity, the gravitational force equals the dragforce. The gravitational force is:

(10.6)

where Fgrav = gravitational force, newtons (lb)�f = density of liquid, kg/m3 (lb/ft3)�v = density of vapor, kg/m3 (lb/ft3)d = diameter of drop, m (ft)g = gravitational acceleration=9.81 m/s2 (32.2 ft/s2)

FIGURE 10.3Liquid separation in a vertical vessel.

(10.5)



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The drag force is:

where Fdrag = drag force, newtons (lb) CD = drag coefficient, dimensionless V = relative velocity of vapor and liquid drop, m/s (ft/s) g = gravitational acceleration=9.81 m/s2 (32.2 ft/s2)

The expression for the terminal velocity Vt is found by equating thegravitational force of Eq. 10.5 or Eq. 10.6 to the drag force of Eq. 10.7 or Eq. 10.8,

The drag coefficient CD is a function of the Reynolds number Re

where µv is the viscosity of the vapor. Several specific values of CD are 30, 4, 1.1,and 0.5 at Reynolds numbers of 1, 10, 100 and 1000, respectively.

For a number of years the ASHRAE Handbook1 has recommended separationvelocities, and these velocities are shown for ammonia and R-22 in Table 10.2.The table shows another dimension, which is the separation distance betweenthe liquid level and the vapor outlet. As expected, if there is only a shortseparating distance, the permitted velocity is lower.

Reverse engineering can be applied to the separating velocities of Table 10.2in order to determine critical drop diameters that correspond to therecommendations. Figure 10.4 shows the result of that process with separating

TABLE 10.2Maximum separation velocities in m/s (fpm) for ammonia and R-22 under steady-flowconditions1.

(10.7)

(10.8)

(10.9)



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distances of 610 mm (24 in) chosen in Table 10.2. The slopes of the separatingvelocity curves are not identical using the two methods, but an ammonia dropdiameter of 0.38 mm (0.015 in) and an R-22 drop diameter of 0.20 mm (0.008in) yield magnitudes in the same order of magnitude. A consistent trend isthat the maximum velocity permitted decreases as the saturation temperaturesincrease. The reason is that as the saturation temperature increases, so doesthe density of vapor which, in accordance with Eq. 10.9, results in a reductionof the terminal velocity, Vt.

Examination of Fig. 10.4 raises the question of why the drop diameterselected for ammonia is so much larger than that chosen for R-22. The realcriterion is whether liquid carryover problems result when the drops ofdiameters smaller than a certain value escape the vessel. The ability of adrop to vaporize in the suction line and in the entrance sections of thecompressor is a deciding factor, so is ammonia more readily vaporized thanR-22? The liquid density of ammonia is half that of R-22, so half the mass ofliquid ammonia exists in a drop of a given size. The thermal conductivity ofliquid ammonia is five times that of liquid R-22, so heat can flow more easilyin ammonia. But on the other hand, the latent heat of ammonia is five timesthat of R-22, and all of this additional heat must pass through the convection

FIGURE 10.4Comparison of recommended separating velocities by two methods.



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coefficient at the surface of the drop, and this coefficient should be about thesame for ammonia and R-22.

The comparison of Fig. 10.4 raises a question whether the recommendedseparating velocities from ammonia are too high. There are occasional reportsfrom the field of liquid carryover of ammonia when the values from Table 10.2are used, and most designers use velocities approximately 0.5 to 0.8 m/s (100 to150 fpm). Providing the desired separating velocity is one influence on the vesselsize and the other, which will be addressed in Sec. 10.8, is to provide adequatevolume for storage of liquid. In most cases the size of low-pressure receiver thataccommodates the liquid storage will also result in low vertical velocities.

A question might be raised about the terminal velocity method in that itdoes not acknowledge separating distances as does Table 10.2. Certainly aminimum separating distance must be provided, and 450 to 600 mm (18 to 24in) is recommended. The feature of the separating distances emerges becausethe original technical paper2 on which the values of Table 10.2 are basedintroduced principles applicable to the design of fractionating columns3. Onefactor included is the surface tension of the liquid, which influences the abilityof vapor to tear itself away from liquid. This process occurs in the shell-side ofan evaporator and probably too when vapor is bubbled through liquid in adesuperheater. It is not known how much the surface tension influences theseparation of liquid and vapor returning from overfeed coils.

Lorentzen4 has influenced European practice, basically using the terminalvelocity technique and recommending separating velocities between 0.5 m/s and1 m/s (100 to 200 fpm). The higher of the recommended values corresponds to anammonia droplet diameter of 0.25 mm (0.010 in) at a temperature of -40°C (-40°F). At 0°C (32°F) with a drop diameter of 0.25 mm (0.010 in), the recommendedvelocity is 0.3 m/s (60 fpm). Wiencke5 supports the recommendations of Grassmannand Reinhart6 that drop diameters of 0.2 mm (0.008 in) for both ammonia andthe halocarbons should not be exceeded. Figure 10.5 shows separating velocitiesfor several drop sizes of ammonia and for R-22, R-134a, and R-507 with dropdiameters of 0.20 mm (0.008 in). Before using high separating velocities forammonia, the designer might want to be assured that unique features of thefacility will mitigate against liquid carryover problems.

10.5 LIQUID/VAPORSEPARATION—HORIZONTAL VESSEL

Both horizontal and vertical vessels are widely used, and it is often the physicalcharacteristics of the machine room that has a major influence on the choice oforientation. Many designers prefer a vertical vessel for a mechanically pumpedrecirculation system, because the net-positive-suction head is more easilyachieved. The vertical vessel usually requires less floor space than the horizontal,but does require greater vertical space in the machine room. For recirculationpackages that incorporate the vessel and the pumps on a skid, the vessels areusually oriented horizontally to meet head room limitations on shipping.



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The mechanism for separation of liquid from vapor in horizontal vessels issomewhat different than the process in a vertical vessel, but some of the sameprinciples apply. In the horizontal vessel of Fig. 10.6, the horizontally flowingvapor carries liquid drops while at the same time the drops have a verticalcomponent of velocity because of the gravitational force. If the drops are initiallyassumed to have no horizontal or vertical velocity when they enter the vessel,the drops accelerate horizontally due to drag force of the vapor on the dropswhile they accelerate vertically due to gravity. If a drop descends to the liquidsurface before being carried out, it will be captured.

A key term is thus the time T for the drop to fall a distance y and settle to theliquid level. This time must not exceed the time to traverse the separatinglength L in Fig. 10.7. Thus arises the concept of T as the minimum residencetime. Furthermore,

FIGURE 10.5Recommended maximum liquid/vapor separating velocities for various refrigerants andthe drop diameters on which the recommendations are based.

(10.10)



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where T = minimum residence time, s L = separating length, m (ft) V = horizontal vapor velocity, m/s (ft/s) A = flow area, m2 (ft2)

The liquid level carried in the horizontal vessel has a significant impact onthe ability to separate liquid from vapor, because as the level rises the flowarea decreases, which reduces the actual residence time. For this reason, manydesigners try to avoid operation with liquid levels much above the midheight ofthe vessel. The vessel should be designed, then, so that drops of larger than thecritical diameter fall a distance y, in Fig. 10.7, before the vapor with its velocityV travels the distance L. The time required for a drop to settle due to the force

FIGURE 10.7Horizontal velocity and separating length which combine to yield the concept of residencetime for a horizontal vessel.

FIGURE 10.6Separation of liquid from vapor in a horizontal vessel.



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of gravity for ammonia, R-22 and R-507 at saturation temperatures of -40°C(-40°F) and 0°C (32°F) are shown in Figs. 10.8, 10.9, and 10.10, respectively.

The calculations assume that the drop starts its descent with zero verticalvelocity, but when the liquid/vapor enters vertically from the top, as in Fig.10.7, there will be an initial velocity which makes the recommendations inFigs. 10.8 to 10.10 conservative. The lines in the figures are essentially straight,because the terminal velocity is reached very quickly—usually within 0.1 s.The values are of the same order of magnitude quoted by Richards,7 who suggestsfor ammonia 0.7 s at -1°C (30°F) and 0.5 s at -18°C (0°F). The critical dropdiameter for the three refrigerants has been chosen as 0.2 mm (0.008 in) andhad a drop diameter of 0.3 mm (0.012 in) been selected, the required residencetimes would have been approximately 30% to 40% shorter.

Once the residence time is determined, the area A in Eq. 10.10 can be computedfor the prevailing volume flow rate. Table 10.1 facilitates the area calculation.

10.6 HIGH-PRESSURE RECEIVER

The high-pressure receivers in small plants that operate on a seasonal scheduleare typically sized to contain all the refrigerant existing in the system. Duringpump-down operation, the king valve (Fig. 10.11) in the liquid line from the

FIGURE 10.8Residence times with ammonia for various values of y in horizonal vessels.



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receiver is closed and the refrigerant delivered by the compressor(s) condensesand drains into the receiver. When all liquid has been withdrawn from the lowside, the valves between the condenser(s) and receiver are closed to confine allliquid refrigerant in the receiver. In these plants, the receiver must accommodateall the liquid of the system in the storage volume between the high and lowliquid levels, as shown in Fig. 10.11. Some vapor volume is always requiredabove the highest liquid level, and a reserve of liquid should always prevail inthe receiver, even when the remainder of the system is fully supplied. Thepiping of the receiver illustrated in Fig. 10.11 is top inlet, and the other conceptin piping, as was shown in Fig. 7.27, is the bottom inlet.

Many large plants operate all year, and receivers for these plants are neverexpected to contain all the liquid in the system. Two of several bases used bydesigners to select the size of receivers are:

• storage volume to pump down the largest refrigerated room or unit servedby the plant

• store full refrigerant flow for a specified duration of time, 30 minutes, forexample.

The rationale behind the first basis is that any of the refrigerated spaces,including the largest, may need to be taken out of service. The second basis is

FIGURE 10.9Residence times with R-22 for various values of y in horizonal vessels.



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FIGURE 10.10Residence times with R-507 for various values of y in horizonal vessels.

FIGURE 10.11Liquid storage volume between the high and low levels in a high-pressure receiver.



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predicated on being able to interrupt the liquid supply to the plant for a shortperiod of time, meanwhile continuing compressor operation. Standard shelldiameters in North America are 0.51 m (20 in); 0.56 m (22 in); 0.61 m (24 in);0.76 m (30 in);…1.22 m (0.48 in); 1.37 m (54 in), etc. The cost of vessels larger indiameter than 1.52 m (60 in) increases abruptly.

10.7 FLASH-TANK/DESUPERHEATERS

The flash-tank/desuperheater is the vessel in a two-stage system operating atthe intermediate pressure that provides removal of flash gas in direct cooling ofliquid (Fig. 3.2) or subcooling liquid with a heat exchanger (Fig. 3.5). The otherpurpose of the vessel is to desuperheat discharge vapor from the low-stagecompressor by bubbling it through the liquid in the vessel (Fig. 3.9). Section10.1 indicated that vessels in industrial refrigeration systems have either orboth the function of liquid storage and/or separation of liquid from vapor. Usuallythe flash-tank/desuperheater must be sized only for separation of liquid andvapor. The exception to that limited role is when other streams in addition tothe liquid supply and discharge vapor from the low-stage compressor flow intothe vessel. If there are intermediate-temperature evaporators discharging intothe vessel, for example, surge volume should be provided to allow for the rushof liquid coming from the evaporators during defrost.

Desuperheating of vapor, which is a unique function of this vessel, is notrequired in other vessels of the system. One method of desuperheating is tobubble the vapor through the liquid, a process that is effective, but requires

FIGURE 10.12A flash-tank/desuperheater with (a) desuperheating by bubbling vapor through liquidand (b) desuperheating by spraying liquid into vapor.



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two simultaneous functions that are somewhat conflicting. One process isagitation for good desuperheating and the other is an orderly flow pattern forgood liquid/vapor separation. Some vessel designers choose a configuration likethat shown in Fig. 10.12a which injects the vapor below a perforated plate tospread out the vapor bubbles, thus providing more intimate contact of vaporand liquid and avoiding a geyser which would make the liquid and vaporseparation more difficult. Still another feature used by some designers is theinstallation of a plate against which the vapor is directed. The purpose of thisplate is to prevent the vapor bubbles from blowing to the liquid outlet andbeing drawn out of the vessel. This precaution would be especially appropriateif the vessel is part of a liquid recirculation systems where vapor bubbles in theliquid could vapor- bind the pump.

The other concept of desuperheating, as shown in Fig. 10.12b, is to sprayliquid into the incoming vapor. The diagram shows a control valve regulatingthe liquid flow, but, as was discussed in Chap. 9, the bypass from a centrifugalpump may serve as the liquid source.

10.8 LOW-PRESSURE RECEIVER

This vessel is one that performs both the role of liquid/vapor separation andliquid storage. The dashed lines in Fig. 10.13 show five distinct liquid levels ofinterest. The level controller, whether it is a capacitance level sensor as shown inFig. 10.13, or a float switch, regulates the solenoid valve in the liquid line. If thelevel in the vessel drops below the control point, the solenoid valve in the liquidsupply line opens. When the level in the vessel reaches the control point, thesolenoid valve closes. Volumes above and below this controlled level accommodatesurge volume and provide ballast volume. The surge volume serves the purposeof accommodating liquid that might be forced out of evaporators during defrost.Another source of liquid surge in some plants is from the liquid/vapor line whenpitched downward to the low-pressure receiver. Should the electric power in theplant be interrupted, the liquid continues to drain from the liquid/vapor line.

The controlled level is not the lowest operating level, because some liquidsupply should be available between the controlled level and level that actuatesthe low-level alarm. The reason for needing this ballast volume is that duringstartup or resumption of operation of one or more evaporators, the pump maywithdraw refrigerant from the vessel at a greater rate than is supplied at thatmoment by the combination of the controlled liquid supply and from the returnfrom the liquid/vapor line. If the vessel empties to the low-level alarm status,operation continues but an operator or the security system is informed of thefact. On a further drop in level the low-level cutout is reached, which is usuallyset to stop pump operation.

There are also two designated control levels above the controlled level: thehigh-level alarm and the high-level cutout. The alarm simply notifies the operatoror the security system to summon an on-site investigation. Should the liquidlevel reach the point of the high-level cutout, the compressors are shut down for



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their protection. Figure 10.13 shows for controllers a popular combination ofthe capacitance level controller and float switches. Several electrical currentvalues can be picked off the 4–20 mA output of the capacitance level sensorthat correspond to the controlled level, the high-level alarm, and the low-levelalarm. To provide an independent backup, the ultimate safety provisions of thehigh-and low-level cutouts are actuated by separate float switches.

Computing the ballast volume. The ballast volume is provided to permit thepumps to draw liquid from the low-pressure receiver for a short interval tobring the liquid content of evaporators up to the steady-state amount followinga shutdown. Typically, a five-minute time period is assumed adequate for thispurpose, so the ballast volume is the design pump flow rate in volume flow perminute multiplied by 5.

Computing the surge volume. Two major contributors to momentary excessflow into the low-pressure receiver are flooding of abnormal rates of liquid out

FIGURE 10.13Liquid levels maintained in a low-pressure receiver.



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of the evaporator due to defrost or to sudden increases of refrigeration load,and the liquid in the liquid/vapor return line that drains back to the low-pressurereceiver in the event of pump or power failures. During a defrost it is assumedthat the entering defrost gas pushes all liquid in the coil out to the return line.The fraction of liquid in the coil during operation depends on whether the coil istop or bottom fed. For a top-fed coil the percentage of liquid is often assumed tobe about 30%. For bottom-fed coils the percentage is sometimes chosen as highas 80%. Another approach to the estimate5 that incorporates the circulationratio n but does not distinguish between top and bottom feed is

Equation 10.11 gives with a circulation ratio of 3, for example, the percentageof volume occupied by liquid to be 33%.

The premise that the liquid in a coil is pushed out during a hot-gas defrostand sent to the low-pressure receiver is now being questioned by some engineers.In times past, it was certainly true that the defrost gas was sent to the coilimmediately after interrupting the supply of liquid to the coil, and the liquidwould flow rapidly through the pressure-regulating valve or through a floatdrainer. As was emphasized in the treatment of hot-gas defrost in Chapter 6, thecoil should first be emptied of liquid by closing the liquid supply solenoid butcontinuing refrigeration until all or most of the liquid in the coil has been drawnoff by the compressor. The liquid that was contained in the coil thus never leavesas liquid. Only experience will determine whether the modern recommendeddefrost sequence will permit reduction in the required surge volume.

To estimate the fraction of liquid FL in the liquid/vapor return line, Sec.9.12 presented relations that should bracket the correct value. If the liquid isassumed to move at the same velocity as the vapor, thus with no slip,

On the other hand, the other extreme described in Sec. 9.12 is if the liquidflows along the bottom of a horizontal pipe, dragged by the faster-moving vapor.The expression is:

Equation 10.12 underestimates the fraction of liquid occupying the pipe,because slip will always occur. Equation 10.13 overestimates the value, if thevapor velocity is high enough to generate mist flow rather than stratified flow,and also if the pipe is sloped in the direction of flow.

(10.11)

(10.12)

(10.13)



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10.9 SURGE DRUM FOR A FLOODEDEVAPORATOR

Flooded evaporators, first described in Sec. 6.7, are equipped with a surge drumat a level slightly above the evaporator coils. Either a horizontal or verticalsurge drum is possible, as shown in Fig. 10.14, with the choice usually dependentupon the space restrictions. The surge drum serves the purpose of liquid storageand liquid/vapor separation. A need for surge volume arises when the evaporatoris subjected to a sudden heavy heat load, in which case the rate of boiling abruptlyincreases and the vapor thus developed pushes liquid out of the evaporator. Inthe case of air coils during hot-gas defrost the surge drum provides storageduring the transient conditions.

The principles for disengaging vapor from liquid are the same ones applicableto low-pressure receivers, as discussed in Secs. 10.4 and 10.5. Usually themanufacturer of the coil provides the surge drum as part of the package andthereby makes the decision on the dimensions of the surge drum. As a suggestionof the order of magnitude of the volume of the surge drum, one recommendation8

for finned evaporator coils is that horizontal surge drums have a free volume asgreat as the internal volume of the evaporator coils. The free volume is definedas the vapor space from the controlled level to the level at which liquid wouldcarry out of the vessel.

One manufacturer9 offers. guidelines for dimensions of horizontal surgedrums, such as shown in Fig. 10.14a, as expressed in Table 10.3. For verticalsurge drums, as shown in Fig. 10.14b, the permitted refrigeration capacity ofthe coil is about twice that shown in Table 10.3 for a given vessel size andevaporating temperature.

FIGURE 10.14Surge drums for flooded coils, (a) oriented horizontally, and (b) oriented vertically.



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10.10 SUCTION-LINEACCUMULATOR

The vessel shown in Fig. 10.15 should normally not be necessary, and in newlydesigned and constructed systems it usually is not found. In a two-stage liquidrecirculation system the low-pressure receiver should be designed to preventliquid from reaching the low-stage compressors, and the flash-tank/intercoolershould prevent liquid from reaching the high-stage compressors.

The operators of many plants that employ a suction-line accumulator,however, are comforted by having one. In plants that through the years havebeen expanded, modified, and restructured there is sometimes liquid carryoverand the suction-line accumulator is the final protector of the compressor fromslugging with liquid. Some of the reasons that liquid carryover occurs are becauseof improper design of some other separating vessel, or occasional liquid carryoverfrom a leaking expansion valve on a direct-expansion coil or the imposition of asudden heat load on a flooded coil. The installation of a suction-line accumulator

FIGURE 10.15A suction-line accumulator with the option of passing warm liquid through the accumu-lator to vaporize any trapped liquid from the suction line.

TABLE 10.3Appropriate refrigerating capacities of flooded coils corresponding to several surge drum sizes.



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in a plant that is carefully designed could permit smaller surge drums on allthe flooded coils and the occasional carryover of liquid could be accommodatedby the suction-line accumulator.

Figure 10.15 shows the option of directing warm liquid from the high-pressurereceiver to a coil in the bottom of the accumulator. When liquid does collect inthe accumulator the vaporization by heat from the machine room is a slowprocess, and the supply of heat from the warm high-pressure liquid acceleratesvaporization and recovers a small amount of refrigeration as well. The warmliquid coil in the accumulator will have the undesirable effect of super-heatingthe suction vapor when no liquid is present in the accumulator. Some plantsequip the accumulator with a transfer system employing mechanical or gaspumping, which is initiated by a rise in the liquid level in the accumulator.These arrangements deliver the liquid to the high-pressure receiver.

10.11 THERMOSYPHON RECEIVER

A small but important vessel is the thermosyphon receiver which is an integralpart of that type of oil cooling concept for screw compressors. The procedure forselecting the size of the thermosyphon receiver is explained in Sec. 5.14 andthe key requirement is that it provides a reserve for five minutes of flow to theoil cooler if the supply of liquid from the condenser is interrupted. Separationof liquid and vapor indeed also takes place in the thermosyphon receiver, butthe requirements are not stringent. Some liquid mist could pass through thevent line to the condenser inlet without an adverse effect, but it should beemphasized that the return of liquid and vapor from the oil cooler should not bedelivered to the condenser inlet, because such a large fraction of liquid woulddegrade the condenser performance.

Some designers and contractors combine the thermosyphon receiver and thesystem receiver as shown in Fig. 10.16. Rather than transferring the liquidfrom a separate thermosyphon receiver to a separate system condenser, theliquid simply spills over from the oil cooler reserve into the system receiver.When multiple condensers feed the receivers, the individual condensate drainlines are trapped as was described in Chapter 7.

10.12 OIL POTS

A small vessel frequently installed beneath the low-pressure receiver and otherlarger vessels is an oil pot whose purpose is to accumulate oil to facilitate periodicdraining. Figure 10.17 shows oil pots with their typical connections, includingthe line from the bottom of the low-pressure receiver, the drain line, and theequalizer line, which is connected to the vapor section in the upper portion ofthe low-pressure receiver. Without the equalizer line it may be difficult to achievedrainage into the oil pot because of the vapor pressure that could build up. Inaddition, since the oil pot is usually classified as a vessel, it must be protectedby relief valves.



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If both the inlet line and the equalizer line are flush with the top of the oil pot,as shown in Fig. 10.17a, it will be possible to completely fill the vessel with liquid,which could be dangerous should all the valves in the connecting lines beinadvertently closed. To avoid the possibility of complete liquid filling of the vessel,the equalizer line can be extended into the oil pot a short distance. When the levelof oil (and refrigerant) rises in the oil pot to the bottom of the equalizer line, liquidis forced up the equalizer line but the vapor space at the top of the vessel is preserved.

10.13 SEPARATION ENHANCERS

Certain refinements of the basic inlets and outlets to the vessel are usually used tofacilitate the separation of liquid and vapor. Any of the entrance lines that arecarrying liquid are usually turned downward. The vapor line leaving for thecompressor usually has an upward turn, as shown in Fig. 10.18a, which requires

FIGURE 10.16Combination of a thermosyphon receiver within the system receiver.



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any liquid drops to travel upward and make a 180° turn to inhibit capture by thesuction line. This suction line should be sloped back to the vessel, so a liquid trapdevelops which can be prevented by drilling a weep hole at the elbow of the pipe.

Another approach to improved liquid separation is through coalescing thesmall drops into larger ones by passing the liquid/vapor mixture througheliminator baffles or a mesh, as is shown in Fig. 10.18b. These devices arecommonly used for vessels in chemical plants10,11 and surely permit the choice

FIGURE 10.18(a) Separation enhancers including (a) directing inlet flows downard and drawing vaporfrom the top, and (b) installation of a metal mesh for mist elimination.

FIGURE 10.17Oil pots with (a) the equalizer line flush with the top of the oil pot, and (b) extended intothe oil pot to provide a vapor space at the top.



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of a smallersize vessel. They have the disadvantage of introducing pressuredrop which in the view of many refrigeration system designers is a penalty thatcan be avoided by a larger-size vessel with no obstructions.

10.14 VESSEL SIZING INPERSPECTIVE

The thrust of this chapter has been to compute the minimum sizes of vessels.In this task the simple rule is, “Never scrimp on the size of the vessel.” Certainlythere is a price to pay for the installation of a generously sized vessel—theincreased cost of materials and installation. Furthermore, additional space willbe occupied and larger vessels may entail a larger refrigerant charge. Thepayback for this extra cost is improved separation of liquid and vapor and theability to accommodate surges in the shifts of liquid between components, bothof which provide additional protection for the compressor.

All those involved in the planning of a system may be convinced that thecapacity specified for the plant is final and will suffice for eternity. Five yearslater, however, another refrigerated space or process may need to be added,and if the vessels were generously sized at the time of the original installation,they may be able to accommodate a moderate-sized expansion without the needfor replacement.

REFERENCES

1. ASHRAE Handbook, Refrigeration Systems and Applications, American Societyof Heating, Refrigerating and Air-conditioning Engineers, Atlanta, Georgia, 1994.

2. Miller, D., “Recent Methods for Sizing Liquid Overfeed Piping and SuctionAccumulator Receivers,” Proceedings of the XIII International Congress ofRefrigeration, vol. 2, International Institute of Refrigeration, Paris, 1971.

3. Brown, G.G., Unit Operations, Chapter 24, “Vapor-Liquid Transfer Operations—Design and Control of Fractionating Columns,” John Wiley & Sons, 1951.

4. Lorentzen, G., “On the Dimensioning of Liquid Separators for RefrigerationSystems,” Kaeltetechnik-Klimatisierung, 18(3): 89–97, 1966.

5. Wiencke, B., “Richtlinien fuer die Dimensionierung von Schwerkraftfluessigkeitsab-scheidern in Kaelteanlagen,: Die Kaelte und Klimatechnik, 9:496–508, 1993.

6. Grassmann, P. and A. Reinhart, “Zur Ermittling der Sinkgeschwindigkeiten vonTropfen und der Steiggeschwindigkeiten von Blasen,” Chem. Ing. Techn., pp 348–349,1961.

7. Richards, W.V., “Old Habits in Ammonia Vessel Specification,” Air Conditioning,Heating and Refrigerating News, p 28, May 13, 1985.

8. Stoecker, W.F., “How to Design and Operate Flooded Evaporators for Cooling Airand Liquids,” Heating/Piping/Air Conditioning, 32(12): 144–168, December 1960.

9. “Pressure Vessel Sizing,: Automatic Refrigerant Control, Section 1, Niagara BlowerCompany, Buffalo, New York, 1980.

10. Gerunda, A. “How to Size Liquid-Vapor Separators,” Chemical Engineering, 88(9):81–84, May 4, 1981.

11. Wu, F.H., “Drum Separator Design—A New Approach,” Chemical Engineering,91(7): 74–80, April 2, 1984.



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industrial refrigeration handbook - capter 10 - vessels

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