TITLE.PM5

dharm

\M-therm\Th15-3.pm5

818 ENGINEERING THERMODYNAMICS

temperature difference between the two fluids remains more or less nearly constant. This type of

heat exchanger, due to counter flow, gives maximum rate of heat transfer for a given surface area.

Hence such heat exchangers are most favoured for heating and cooling of fluids.

tc 1

tc 2

Cold

Hot th 2

(a)

Cold

th 1

t

L

th 1

tc 1

th 2

tc 2

Hot fluid

Cold fluid

(b)

Fig. 15.32. Counter-flow heat exchanger.
(iii) Cross-flow heat exchangers. In cross-flow heat exchangers, the two fluids (hot and
cold) cross one another in space, usually at right angles. Fig. 15.32 shows a schematic diagram of
common arrangements of cross-flow heat exchangers.

Cold fluid (out)

(a)

Hot

fluid

(out)

Hot

fluid

(in)

(Unmixed

stream)

Cold fluid (in)

Tubes (Mixed stream)

Cold fluid (out)

(b)

Hot

fluid

(out)

Hot

fluid

(in)

(Unmixed

stream)

Cold fluid (in)

(Unmixed stream) Baffles

Fig. 15.33. Cross-flow heat exchangers.

l Refer Fig. 15.33 (a) : Hot fluid flows in the separate tubes and there is no mixing of the

fluid streams. The cold fluid is perfectly mixed as it flows through the exchanger. The

temperature of this mixed fluid will be uniform across any section and will vary only in

the direction of flow.

Examples : The cooling unit of refrigeration system etc.

l Refer Fig. 15.33 (b) : In this case each of the fluids follows a prescribed path and is

unmixed as it flows through heat exchanger. Hence the temperature of the fluid leaving

the heater section is not uniform.

Examples : Automobile radiator etc.

l In yet another arrangement, both the fluids are mixed while they travel through the

exchanger ; consequently the temperature of both the fluids is uniform across the section

and varies only in the direction in which flow takes place.