CHEMICAL ENGINEERING

146 CHEMICAL ENGINEERING VOLUME 1 SOLUTIONS

In order to avoid severe scaling in such a case, the maximum allowable water temper-
ature is 320 K and hence 310 K will be chosen as a suitable value forT.

Thus: 4. 5 ð 103 D 4. 18 m

 310  289 andmD 51 .3 kg/s

The next stage is to estimate the heat transfer area required.

QDUAm (equation 9.1)

where the heat loadQD 4. 5 ð 106 W

UD450 W/m^2 K

 1 D 

356  289 D67 deg K, 2 D 

356  310 D46 deg K

and from equation 9.9,mD
67  46 /ln
67 / 46 D 55 .8deg K
No correction factor is necessary as the shell side fluid temperature is constant.

∴ AD
4. 5 ð 106 /
450 ð 55. 8 D 179 .2m^2

A reasonable tube size must now be selected, say 25.4 mm, 14 BWG.
The outside surface area is therefore
$ð 0. 0254 ð 1. 0 D 0 .0798 m^2 /m and hence
the total length of tubing requiredD
179. 2 / 0. 0798 D2246 m.
A standard tube length is now selected, say 4.87 m and hence the total number of tubes
requiredD
2246 / 4. 87 D460.
It now remains to decide the number of tubes per pass, and this is obtained from a
consideration of the water velocity. For shell and tube units,uD 1. 0  1 .5m/s and a
value of 1.25 m/s will be selected.
The water flow, 51.3 kg/sD
51. 3 / 1000 D 0 .0513 m^3 /s.
The tube i.d. is 21.2 mm and hence the cross-sectional area for flow/tubeD

$/ 4
0. 02122 D 0 .000353 m^2.
Area required to give a velocity of 1.25 m/sD
0. 0513 / 1. 25 D 0 .0410 m^2 and hence
number of tubes/passD
0. 0410 / 0. 000353 D116 and number of passesD 460 / 116 ³4.
As the shell side fluid is clean, triangular pitch might be suitable and 460ð25 mm
o.d. tubes on 32 mm triangular pitch with 4 tube side passes can be accommodated in a
0.838 m i.d. shell and still allow room for impingement plates.
The proposed unit will therefore consist of:
460 , 25 .4 mm o.d. tubes ð14 BWG, 4.87 m long arranged in 4 tube side passes on

32 mm triangular pitch in a 0.838 m i.d. shell.

The general mechanical details of the unit are described in Section 9.9.1 of Volume 1
and points of detail are:

(i) impingement baffles should be fitted under each inlet nozzle;

(ii) segmental baffles are not usually fitted to a condenser of this type;

(iii) the unit should be installed on saddles at say 5°to the horizontal to facilitate

drainage of the condensate.