TITLE.PM5

HEAT TRANSFER 815

dharm

\M-therm\Th15-2.pm5

Let us first determine the type of the flow

m = ρAu = 983.2 × π

4

× (0.02)^2 × u =

0.5

60

(kg/s)

or u =
0.5
60
×
4
π
×
1
983.2 (0.02)×^2
= 0.0269 m/s

Reynolds number, Re =

Du...

ν

= ×

×

0 02 0 0269− =

0 478 10

6 1125 5

Since Re < 2000, hence the flow is laminar.

With constant wall temperature having fully developed flow,

Nu =

hD

k = 3.65

or h =
3 65 3 65 0 659
002

k

D

= × = 120.26 W/m^2 °C

The rate of heat transfer, Q = As h (ts – t∞) = m cp (to – ti)

Here, t∞ =

20 50

2

+

= 35°C = tb

∴ (π × 0.02 × L) × 120.26 × (85 – 35) =^05

60

. × (4.178 × 10 (^3) )(50 – 20)
or 377.8 L = 1044.5
or L = 1044.5
377.8
= 2.76 m. (Ans.)

15.4. Heat Exchangers

15.4.1. Introduction

A ‘heat exchanger’ may be defined as an equipment which transfers the energy from a hot

fluid to a cold fluid, with maximum rate and minimum investment and running costs.

In heat exchangers the temperature of each fluid changes as it passes through the exchang-

ers, and hence the temperature of the dividing wall between the fluids also changes along the

length of the exchanger.

Examples of heat exchangers :

(i) Intercoolers and preheaters ; (ii) Condensers and boilers in steam plant ;

(iii) Condensers and evaporators in refrigeration units ;

(iv) Regenerators ; (v) Automobile radiators ;

(vi) Oil coolers of heat engine ; (vii) Milk chiller of a pasteurising plant ;

(viii) Several other industrial processes.

15.4.2. Types of heat exchangers

In order to meet the widely varying applications, several types of heat exchangers have been
developed which are classified on the basis of nature of heat exchange process, relative direction of
fluid motion, design and constructional features, and physical state of fluids.

1. Nature of heat exchange process
   Heat exchangers, on the basis of nature of heat exchange process, are classified as follows :
   (i) Direct contact (or open) heat exchangers.
   (ii) Indirect contact heat exchangers.
   (a) Regenerators. (b) Recuperators.