CHEMICAL ENGINEERING

HEAT TRANSFER 129

xx^0.^25 x^0.^25 sh

(m) (m) W/m^2 K

00 1 0 1

0.1 0.562 1.778 1. 58 ð 10 ^4951

0.2 0.669 1.495 1. 89 ð 10 ^4800

0.4 0.795 1.258 2. 24 ð 10 ^4673

0.6 0.880 1.136 2. 48 ð 10 ^4608

0.8 0.946 1.057 2. 67 ð 10 ^4566

1.0 1.000 1.000 2. 82 ð 10 ^4535

1.2 1.047 0.956 2. 95 ð 10 ^4512

1.4 1.088 0.919 3. 07 ð 10 ^4492

1.6 1.125 0.889 3. 17 ð 10 ^4476

1.8 1.158 0.863 3. 27 ð 10 ^4462

2.0 1.189 0.841 3. 35 ð 10 ^4450

PROBLEM 9.4

It is desired to warm 0.9 kg/s of air from 283 to 366 K by passing it through the pipes
of a bank consisting of 20 rows with 20 pipes in each row. The arrangement is in-line
with centre to centre spacing, in both directions, equal to twice the pipe diameter. Flue
gas, entering at 700 K and leaving at 366 K, with a free flow mass velocity of 10 kg/m^2 s,
is passed across the outside of the pipes. Neglecting gas radiation, how long should the
pipes be?
For simplicity, outer and inner pipe diameters may be taken as 12 mm. Values ofk
and, which may be used for both air and flue gases, are given below. The specific heat
capacity of air and flue gases is 1.0 kJ/kg K.

Temperature Thermal conductivity Viscosity

(K) k(W/m K) (mN s/m^2 )

250 0.022 0.0165

500 0.044 0.0276

800 0.055 0.0367

Solution

Heat load,QD 0. 9 ð 1. 0
366  283 D 74 .7kW

Temperature driving force, 1 D
700  366 D334 deg K,

 2 D 

366  283 D83 deg K

and in equation 9.9,mD
334  83 /ln
334 / 83 D180 deg K