CHEMICAL ENGINEERING

HEAT TRANSFER 191

The outlet temperatureT^0 is then given by:

∫T 0

290

dT



320 T

D 0. 0000319 h

∫ 5

0

dL

or: ln
320 T^0 Dln 30 0. 0001595 hD 3. 401  0. 0001595 h

At a mean temperature of say 300 K, in equation 9.221:

hD 4280

0. 00488 ð 300  11. 20.^8 / 0. 0250.^2 D4805 W/m^2 K

Thus: ln
320 T^0 D 3. 401 
0. 0001595 ð 4805

and: T^0 D 306 .06 K]

PROBLEM 9.59

Heat is transferred from one fluid stream to a second fluid across a heat transfer surface. If
the film coefficients for the two fluids are, respectively, 1.0 and 1.5kW/m^2 K, the metal
is 6 mm thick (thermal conductivity 20 W/m K) and the scale coefficient is equivalent to
850 W/m^2 K, what is the overall heat transfer coefficient?

Solution

From equation 9.201:

1 /UD 1 /hoCxw/kwCRC 1 /hi

D 

1 / 1000 C 

10. 006 / 20 C 

1 / 850 C 

1 / 1500

D 

0. 001 C 0. 00030 C 0. 00118 C 0. 00067 D 0 .00315 m^2 K/W

∴ UD 317 .5W/m^2 Kor 0.318 kW/m^2 K

PROBLEM 9.60

A pipe of outer diameter 50 mm carries hot fluid at 1100 K. It is covered with a 50 mm
layer of insulation of thermal conductivity 0.17 W/m K. Would it be feasible to use
magnesia insulation, which will not stand temperatures above 615 K and has a thermal
conductivity of 0.09 W/m K for an additional layer thick enough to reduce the outer
surface temperature to 370 K in surroundings at 280 K? Take the surface coefficient of
transfer by radiation and convection as 10 W/m^2 K.

Solution

The solution is presented as Problem 9.8.