CHEMICAL ENGINEERING

178 CHEMICAL ENGINEERING VOLUME 1 SOLUTIONS

0

330

Mean heat transfer coefficient (

h W/m

2 K)

250

200

150

1

Pipe length (l m)

2345678910

Figure 9d.

which would imply that the flow is fully developed at this point. For further discussion
on this point reference should be made to the turbulent flow of gases in Section 9.4.3.

PROBLEM 9.47

A liquid is boiled at a temperature of 360 K using steam fed at 380 K to a coil heater.
Initially the heat transfer surfaces are clean and an evaporation rate of 0.08 kg/s is obtained
from each square metre of heating surface. After a period, a layer of scale of resistance
0 .0003 m^2 K/W, is deposited by the boiling liquid on the heat transfer surface. On the
assumption that the coefficient on the steam side remains unaltered and that the coefficient
for the boiling liquid is proportional to its temperature difference raised to the power of
2.5, calculate the new rate of boiling.

Solution

When the surface is clean, taking the wall and the inside resistances as negligible, the
surface temperature will be 380 K.

Thus: QDh 0 A
TsT

whereQDGL,Gkg/s is the rate of evaporation of fluid of latent heatLJ/kg,AD1m^2 ,
andTsandTare the surface and fluid temperature respectively.

∴ 0. 08 LDhoð 1. 0
380  360 orhoD 0. 004 L

ho/

TsT^2.^5

or: hoDk^0
380  3602.^5 D 1. 79 ð 103 k^0

∴ k^0 D 0. 004 L/

1. 79 ð 103 D 2. 236 ð 10 ^6 L