CHEMICAL ENGINEERING

HEAT TRANSFER 175

and hence the heat loadD 30 ð 4. 18
300  290 D1254 kW
At atmospheric pressure, benzene condenses at 353 K and hence:

 1 D 

353  290 D63 deg K, 2 D 

353  300 D53 deg K

and from equation 9.9:

mD 

63  53 /ln 

63 / 53 D 57 .9degK

No correction factor is required, because of isothermal conductions on the shell side.
For condensing benzene,howill be taken as 1750 W/m^2 K. From Table 9.18:hiD
850 W/m^2 K or, based on the outside diameter,hioD
850 ð 22 / 25 D748 W/m^2 K.
Neglecting scale and wall resistances:

1 /UD 

1 / 1750 C 

1 / 748 D 0 .00191 m^2 K/W

and: UD524 W/m^2 Kor0.524 kW/m^2 K

Therefore, from equation 9.1:AD 1254 /

 0. 524 ð 57. 9 D 41 .3m^2.

Outside area of 0.025 m tubingD

$ð 0. 025 ð 1. 0 D 0 .0785 m^2 /m

and total length of tubing requiredD
41. 3 / 0. 0785 D526 m.

PROBLEM 9.44

In a contact sulphuric acid plant, the gases leaving the first convertor are to be cooled
from 845 to 675 K by means of the air required for the combustion of the sulphur. The
air enters the heat exchanger at 495 K. If the flow of each of the streams is 2 m^3 /sat
NTP, suggest a suitable design for a shell-and-tube type of heat exchanger employing
tubes of 25 mm internal diameter.

(a) Assume parallel co-current flow of the gas streams.

(b) Assume parallel countercurrent flow.

(c) Assume that the heat exchanger is fitted with baffles giving cross-flow outside the

tubes.

Solution

Heat load

At a mean temperature of 288 K, the density of airD
29 / 22. 4
273 / 288 D 1 .227 kg/m^3 ,
where 29 kg/kmol is taken as the mean molecular mass of air.

∴ mass flow of airD
2. 0 ð 1. 227 D 2 .455 kg/s.

If, as a first approximation, the thermal capacities of the two streams can be assumed
equal for equal flowrates, then the outlet air temperatureD 495 C
845  675 D665 K