HARARE INSTITUTE OF TECHNOLOGY

NAME MAWONEKE KURAI GARETH

REG NUMBER H150218W

DERPARTMENT FOOD PROCESSING TECHNOLOGY

COURSE FOOD ENGINEERING

COURSE CODE SFP 114

ASIGNMENT No 2

LECTURER MR GWALA

DUE DATE 05 OCTOBER 2015a. Differentiate between : Contact type and non-contact heat exchangers

Co-current and counter-current flow in heat exchangersb. Write short notes on : Plate heat exchangers

Scrapped surface heat exchangersDouble pipe heat exchangersMultiple heat exchangersTubular heat exchangers

HEAT EXCHANGERS

A heat exchanger is a device in which heat is transferred deliberately from one fluid stream to another, either the fluids may be liquid or a gas. In other words, heat exchanger is a device built for efficient heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact In Food Processing the purpose is to heat or cool a liquid food in bulk. In pasteurization and the bulk sterilization of liquids the food is heated to a specific temperature and therefore the rate of heat transfer must be controlled carefully. The heating fluid may be steam or hot water. Alternatively, the purpose may be to exchange heat between two or more food streams, one of which is to be heated and the other to be cooled. Each may require further heating or cooling with steam or chilled, in order to reach the desired temperature but the overall energy input can be reduced by using what would be otherwise waste heat. They are found in many types which include tubular heat exchangers, double pipe heat exchanger, plate heat exchanger, and multiple heat exchanger. The mechanical design of a heat exchanger depends on the operating pressure and temperature. (Smith, 2011)

Contact and Non-contact Heat Exchangers

Contact Heat Exchanger Non-Contact Heat Exchanger

Heat is directly transferred between hot and cold fluids

There is no direct contact between the hot and cold fluids

There is no separating wall between the hot and cold fluids

There is a separating wall between the hot and cold fluids

Heat transfer takes place non-continuous in the form of drops, films, and sprays

Heat transfer takes place in continuously through a dividing wall

It is mostly used for those gases and liquids that are insoluble in nature

Used for those gases and liquids that are soluble in nature

It mostly uses drops and steam for heat transfer

There is no use of drops and stream for heat transfer

For example, steam heat exchanger, steam influence heat exchanger

For example, tubular heat exchanger, double pipe heat exchanger, plate heat exchanger

Co-current and counter-current flow in heat exchangers

There are several types of flows that occur in heat exchangers, but all of them can be classified

into two major categories which are co-current flow and countercurrent flow. In co-current flow,

both the hot and cold streams enter the heat exchanger at one point and leave at the same

opposite point, whereas in countercurrent flow the hot and cold streams enter at opposite ends of

the heat exchanger and also leave at opposite ends of the heat exchanger, as shown in Fig 1.1

below. In countercurrent flow, the temperature change (ΔT) can be larger at either end, whereas

in co-current flow it is always largest at the entry point of the hot and cold stream, as shown in

Fig 1.2 below.

Fig 1.1: Types of Flows in Heat Exchangers

Co-current flow

where mc – mass flow rate in cold stream

mh – mass flow in hot stream

Countercurrent flow

Fig 1.2: Qualitative sketches of Temperature in each flow

Co-current flow Countercurrent flow

Multiple Pass Heat Exchangers

When a heat exchanger's fluids pass each other more than once, a heat exchanger is called a

multi-pass heat exchanger. Commonly, the multi-pass heat exchanger reverses the flow in the

tubes by use of one or more sets of "U" bends in the tubes. The "U" bends allow the fluid to flow

back and forth across the length of the heat exchanger. A second method to achieve multiple

passes is to insert baffles on the shell side of the heat exchanger. These direct the shell side fluid

back and forth across the tubes to achieve the multi-pass effect. In order to shorten the overall

exchanger length, the tubes may be arranged within the shell so that half carry the fluid in one

direction and the same fluid then passes back down the length of the exchanger in the opposite

direction using the other half of the tube bundle. The exchanger would then be said to have a two

side-tube passes. Such arrangement has an advantage that the tube-side velocities are doubled,

for the same flow rate, thus increasing the heat transfer coefficient. The number of shell-side

passes can be increased by placing longitudinal baffles in the shell with a consequent increase in

the shell-side coefficient. The improved heat transfer characteristics for multiple-pass heat

exchangers is off-set, however, by the more complex and costly construction and the higher

pressure drops for each fluid.

Advantages of Multiple Pass Heat Exchangers

1. The pressure and pressure drops can be varied over a wide range.

2. Thermal stresses can be accommodated inexpensively.

3. There is substantial flexibility regarding materials of construction to accommodate

corrosion and other concerns. The shell and the tubes can be made of different materials.

4. Extended heat transfer surfaces can be used to enhance heat transfer.

5. Cleaning and repair are relatively straightforward, because the equipment can be

dismantled for this purpose.

Diagram: Multiple-pass Heat Exchangers

b)

Extractedfrom:https://www.google.co.zw/url?

sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAYQjB1qFQoT

CNzViKTznMgCFUSPPgodL78OmA&url=http%3A%2F%2Fwww.real-world-physics-

problems.com%2Fheat-exchanger.html&psig=AFQjCNEmFqyRyBhJbuvt63nZ_G5-

h8qsVg&ust=1443637302414029

Plate Heat Exchangers

Plate heat exchangers were originally developed for the pasteurization of milk, plate heat

exchangers are now used for a vast variety of heating, cooling and evaporation applications in

the food industry. They consist of a stack of corrugated thin metal plates, pressed together so as

to form two continuous flow channels for the fluids exchanging heat. Gaskets are placed between

the plates to prevent leakage. This type of heat exchanger was developed for the dairy industry.

It consists of a series of plates clamped together on a frame. Channels are formed between each

plate. The product and heat transfer medium flow through alternate channels. Because of the

narrow channel between the plates, the fluid flows at high velocities and in a thin layer resulting

in very high heat transfer rates per unit heat transfer surface area. The plate heat exchanger is

mostly used for heating fluids to temperatures below the boiling point of water at atmospheric

pressure. However there are units designed for high temperature service are commercially

available. Plate heat exchangers are now used in virtually any application where tubular heat

exchangers were previously commonly used. Newer designs have strength to withstand moderate

pressure or vacuum. A major limitation is the inability to handle viscous liquids.

Advantages of Plate Heat Exchangers

1. Flexibility: the capacity can be increased or decreased by adding or removing plates

2. Sanitation: by opening the stack, both sides of the entire exchange area are made

accessible for cleaning and inspection

3. High heat transfer coefficient, due to increased turbulence in the narrow flow channel

4. Compactness: high exchange surface to volume ratio.

5. Their capacity can easily be increased by adding more plates to the frame.

6. With plate heat exchangers, we can heat or cool product to within 1°C of the adjacent

media temperature, with less capital investment than other noncontact-type heat exchangers.

7. Plate heat exchangers offer opportunities for energy conservation by regeneration. A liquid

food is heated to pasteurization or other desired temperature in the heating section; the heated

fluid then surrenders part of its heat to the incoming raw fluid in the regeneration section. The

cold stream is heated to a temperature where it requires little additional energy to bring it up to

the desired temperature. For regeneration, additional plates are required; however, the additional

capital cost may be recovered quickly by lowered operating costs.

Disadvantages of Plate Heat Exchangers

1. On the other hand, the narrow size of the flow channels results in high pressure drop and limits

its use to low viscosity fluids not containing large suspended particles. The need for gaskets is

also a disadvantage.

2.Plate exchangers are limited when high pressures, high temperatures, or aggressive fluids are

present.

3.Because of this problem these type of heat exchangers have only been used in small, low

pressure applications such as on oil coolers for engines.

Diagrams: Plate Heat Exchanger

a)

Plate heat exchanger. (Courtesy of Alfa-Laval)

Scrapped Surface Heat Exchangers

In conventional types of tubular heat exchangers, heat transfer to a fluid stream is affected by

hydraulic drag and heat resistance due to film build up or fouling on the tube wall. This

heat resistance can be minimized if the inside surface of the tube wall is scrapped by some

mechanical means. The scrapping action allows rapid heat transfer to a relative small product

volume. The food contact areas of a scrapped surface cylinder are fabricated from stainless steel,

pure nickel, hard chromium plated nickel, or other corrosion-resistant material. The inside rotor

contains blades that are covered with plastic laminate, or molded plastic. The rotor speed varies

between 150 and 500 rotations per minute. Although high speed allows better heat better heat

transfer, it may affect the quality of the processed product by possible maceration. Thus, rotor

speed must be carefully selected and the annular space between the motor and the cylinder for

the product. The cylinder containing the product and the rotor is enclosed in an outside in an

outside jacket. The commonly used media include steam, hot water, brine, or a refrigerant.

Typical temperatures used for processing foods in scrapped surface heat exchangers range from -

35oC to 190oC. The blading action accomplished in the scrapped-surface heat exchanger is often

desirable to enhance the uniformity of product flavor, color, aroma, and textural characteristics.

In Food Processing Industries, the applications of scrapped-surface heat exchangers include

heating, pasteurizing, sterilizing, whipping, gelling, emulsifying, plasticizing, and crystallizing.

Liquids with a wider range of viscosities that can be pumped are processed in these heat

exchangers, these include fruit juices, citrus concentrate, peanut butter, baked beans, tomato

paste, and pie fillings.

Advantages of Scraped Heat Exchanger

1.

Diagram: Scrapped Surface Heat Exchanger

Extracted from: Berk. (2009).Food Process Engineering and Technology.

Double Pipe Heat Exchangers

It is the simplest type of exchanger. This heat exchanger consists of one pipe inside another that

is it consists of two concentric tubes with a fluid passing along the center tube and the second

fluid flow in the annular space created between the tubes. The walls of the inner pipe forms the

heat transfer surface. This type of heat exchanger is usually built and installed in the field. A

major disadvantage is the relatively large space it occupies for the quantity of heat exchanged,

compared to other types of heat exchangers. Such exchangers are limited to a relative small heat

transfer area resulting in a long piece which must be doubled back on itself to fit conveniently

into a process line. Although the capital costs are low. The disadvantages of double pipe heat

exchangers are such that they operate at relatively low pressures, such that they are rarely used

except in the form of scrapped surface heat surface exchanger.

Advantages of Double Pipe Heat Exchangers

1. Double pipe heat exchanger consists of two concentric pipes are hot fluid, cold fluid.

2. Economically adaptable to service differentials. Ideal for wide temperature ranges and

differentials.

3. Provides shorter deliveries than shell and tube due to standardization of design and

construction.

4. Operates in true counter current flow permitting extreme temperature cross.

Uses of Double Pipe

1. Pasteurization.

2. Digester heating.

3. Heat recovery.

4. Pre-heating.

5. Effluent cooling.

Diagram: Double Pipe Heat Exchanger

Extractedfrom:https://www.google.co.zw/url?

sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=&url=http%3A

%2F%2Fwww.cheresources.com%2Finvision%2Ftopic%2F21954-double-pipe-and-shell-

and-tube-heat-exchangers

%2F&psig=AFQjCNEmFqyRyBhJbuvt63nZ_G5h8qsVg&ust=1443637302414029

Tubular Heat Exchangers

The simplest noncontact-type heat exchanger is a double-pipe heat exchanger, consisting of a

pipe located concentrically inside another pipe. The two fluid streams flow in the annular space

and in the inner pipe, respectively. The streams may flow in the same direction (parallel flow) or

in the opposite direction (counter flow). A slight variation of a double-pipe heat exchanger is a

triple-tube heat exchanger. In this type of heat exchanger, product flows in the inner annular

space, whereas the heating/cooling medium flows in the inner tube and outer annular space. The

innermost tube may contain specially designed obstructions to create turbulence and better heat

transfer. Some specific industrial applications of triple-tube heat exchangers include heating

single-strength orange juice from 4 to 93°C and then cooling to 4°C; cooling cottage cheese

wash water from 46 to 18°C with chilled water; and cooling ice cream mix from 12 to 0.5°C with

ammonia. Another common type of heat exchanger used in the food industry is a shell-and-tube

heat exchanger for such applications as heating liquid foods in evaporation systems. One of the

fluid streams flows inside the tube while the other fluid stream is pumped over the tubes through

the shell. By maintaining the fluid stream in the shell side to flow over the tubes, rather than

parallel to the tubes, we can achieve higher rates of heat transfer. Baffles located in the shell side

allow the cross-flow pattern. One or more tube passes can be accomplished, depending on the

design. The shell-and-tube heat exchangers are one shell pass with two tube passes, and two shell

passes with four tube passes.

Diagram: Tubular Heat Exchanger

Extracted from: Berk. (2009).Food Process Engineering and Technology.

b)

Tubular heat exchanger assembly in aseptic processing plant. (Courtesy of Rossi & Catelli)

Extracted from: Berk. (2009).Food Process Engineering and Technology.

References

Berk, Z. (2009). Food Process Engineering & Technology (1st Edition). Elsevier Inc.

https://www.google.co.zw/url?

sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=&url=http%3A%2F

%2Fwww.cheresources.com%2Finvision%2Ftopic%2F21954-double-pipe-and-shell-and-tube-

heat-exchangers

%2F&psig=AFQjCNEmFqyRyBhJbuvt63nZ_G5h8qsVg&ust=1443637302414029

https:/ www.google.co.zw/url?

sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAYQjB1qFQoTCNzV

iKTznMgCFUSPPgodL78OmA&url=http%3A%2F%2Fwww.real-world-physics-problems.com

%2Fheat-exchanger.html&psig=AFQjCNEmFqyRyBhJbuvt63nZ_G5-

h8qsVg&ust=1443637302414029

Singh, P.R., Heldman, D. R. (2009). Introduction to Food Engineering (4th Edition). Elsevier Inc.

Smith, P. G. (2011). Introduction to Food Process Engineering. Springer Science & Business

Media

Subramanian, R, S. Thermal Analysis of a Steady State Heat Exchanger. Department of

Chemical and Bimolecular Engineering Clarkson University Journal.

Toledo, R. T. (2007). Fundamentals of Food Process Engineering. (3rd Edition). Springer Science

Business Media, LLC. New York.

Wilhelm, L. R., Suter, D. A., Brusewitz, G. H. (2005). Food & Process Engineering Technology.

(Revised Edition)