heat exchangers in brief

8/13/2019 Heat Exchangers in brief

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HEAT EXCHANGERS


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In a heat exchanger, heat energy is transferred

from one body or fluid stream to another.

In the design of heat exchange equipment, heat

transfer equations are applied to calculate this

transfer of energy so as to carry it out efficientlyand under controlled conditions.

The equipment goes under many names, such as

boilers, pasteurizers, jacketed pans, freezers, airheaters, cookers, ovens and so on


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Continuous-flow Heat Exchangers

The materials that are exchanging heat are fluids,flowing continuously through the equipment and

acquiring or giving up heat in passing.

One of the fluids is usually passed through pipes ortubes, and the other fluid stream is passed round

or across these.

At any point in the equipment, the localtemperature differences and the heat transfer

coefficients control the rate of heat exchange.


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The fluids can flow in the same direction through the

equipment, this is called parallel flow ;

they can flow in opposite directions, called counter

flow ;

they can flow at right angles to each other, called cross

flow .

Various combinations of these directions of flow can

occur in different parts of the exchanger. Most actual

heat exchangers of this type have a mixed flow pattern.

Examples of these exchangers are illustrated in the

Figure


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Applying the basic overall heat-transfer equation for the heat

exchangerq = UA T


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Scraped Surface Heat Exchangers One type of heat exchanger, that finds

considerable use in products of higher viscosity,

consists of a jacketed cylinder with an internal

cylinder concentric to the first and fitted with

scraper blades, as illustrated in Fig.

The blades rotate, causing the fluid to flow through

the annular space between the cylinders with the

outer heat transfer surface constantly scraped.


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Plate Heat Exchangers

A popular heat exchanger for fluids of low viscosity,

such as milk, is the plate heat exchanger, whereheating and cooling fluids flow through alternate

tortuous passages between vertical plates as

illustrated in Fig.

The plates are clamped together, separated by

spacing gaskets, and the heating and cooling fluids

are arranged so that they flow between alternate

plates .


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Suitable gaskets and channels control

the flow and allow parallel or countercurrent flow in any desired number of

passes.

A substantial advantage of this type of

heat exchanger is that it offers a large

transfer surface that is readily accessible

for cleaning.

The banks of plates are arranged so that


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heat exchangers in brief

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