CHEMICAL ENGINEERING

HEAT TRANSFER 145

Basing this value on the outside diameter of the pipe:

hioD 

97 ð 0. 038 / 0. 048 D 76 .8W/m^2 K

Outside:
From Table 9.17,hofor condensing steam will be taken as 10,000 W/m^2 K.

Wall and scale:
The scale resistance based ondD 0 .038 m is 0.0009 m^2 K/W or:

k/xD 

1 / 0. 0009 D 1111 .1W/m^2 K.

Basing this on the tube o.d.,k/xD 

1111. 1 ð 0. 038 / 0. 048 D 879 .6W/m^2 K.

1 /UD 1 /hioC 1 /hoCx/k (equation 9.201)

D 0. 0130 C 0. 0001 C 0. 00114

or: UD 70 .2W/m^2 K

Area

ADQ/UmD 

85. 33 ð 103 /

 70. 2 ð 52 D 23 .4m^2

Area per unit length of pipeD
$ð 0. 048 ð 1. 0 D 0 .151 m^2 /m

and length of tube bundleD
23. 4 / 0. 151 D 154 .9m

A very large tube length is required because of the very low inside film coefficient and
several passes or indeed a multistage unit would be specified. A better approach would be
to increase the tube side velocity by decreasing the number of tubes in each pass, though
any pressure drop limitations would have to be taken into account. The use of a smaller
tube diameter might also be considered.

PROBLEM 9.15

It is proposed to construct a heat exchanger to condense 7.5 kg/s ofn-hexane at a pressure
of 150 kN/m^2 , involving a heat load of 4.5 MW. The hexane is to reach the condenser
from the top of a fractionating column at its condensing temperature of 356 K. From
experience it is anticipated that the overall heat transfer coefficient will be 450 W/m^2 K.
Cooling water is available at 289 K. Outline the proposals that you would make for the
type and size of the exchanger, and explain the details of the mechanical construction
that you consider require special attention.

Solution

A shell-and-tube unit is suitable with hexane on the shell side. For a heat load of
4 .5MWD 4. 5 ð 103 kW, the outlet water temperature is:

4. 5 ð 103 Dmð 4. 18 

T 289