CHEMICAL ENGINEERING

HEAT TRANSFER 169

Outside:hoD 1 .70 kW/m^2 K.

Wall: TakingksteelD45 W/m K,x/kD 

0. 003 / 45 D 0 .00007 m^2 K/W or

0 .07 m^2 K/kW.

Thus neglecting any scale resistance: 1/UD 

1 / 0. 748  

1 / 1. 70 C 0. 07

D 1 .995 m^2 K/kW

and: UD 0 .501 kW/m^2 K

Area

In equation 9.1:ADQ/UmD 118. 75 /
0. 0501 ð 18. 2 D 13 .02 m^2.

Surface area of a 0.025 m o.d. tubeD
$ð 0. 025 ð 1. 0 D 0 .0785 m^2 /m and hence total
length of tubing requiredD
1302 / 0. 0785 D 165 .8m

PROBLEM 9.38

Calculate the rate of loss of heat from a 6 m long horizontal steam pipe of 50 mm
internal diameter and 60 mm external diameter when carrying steam at 800 kN/m^2 .The
temperature of the surroundings is 290 K.
What would be the cost of steam saved by coating the pipe with a 50 mm thickness of
85% magnesia lagging of thermal conductivity 0.07 W/m K, if steam costs £0. 5 /100 kg?
The emissivity of both the surface of the bare pipe and the lagging may be taken as 0.85,
and the coefficienthfor the heat loss by natural convection is given by:

hD 1. 65 

T^0.^25 W/m^2 K

whereTis the temperature difference in deg K. The Stefan-Boltzmann constant is
5. 67 ð 10 ^8 W/m^2 K^4.

Solution

Forthebarepipe

Steam is saturated at 800 kN/m^2 and 443 K.
Neglecting the inside resistance and that of the wall, it may be assumed that the surface
temperature of the pipe is 443 K.
Forradiationfrom the pipe, the surface areaD
$ð 0. 060 ð 6. 0 D 1 .131 m^2 and in
equation 9.119:

qrD 

5. 67 ð 10 ^8 ð 0. 85 ð 1. 131

4434  2904 D1714 W.