Page 254 - Chemical engineering design

230

(a) CHEMICAL ENGINEERING
(b)

(c) (d)

Figure 5.17. (a) Pressure control by direct venting (b) Venting of non-condensables after a condenser
(c) Condenser pressure control by controlling coolant flow (d) Pressure control of a condenser by varying
the heat-transfer area, area dependent on liquid level

5.8.4. Heat exchangers
Figure 5.19a (see p. 232) shows the simplest arrangement, the temperature being con-
trolled by varying the flow of the cooling or heating medium.
If the exchange is between two process streams whose flows are fixed, by-pass control
will have to be used, as shown in Figure 5.19b (see p. 232).

Condenser control
Temperature control is unlikely to be effective for condensers, unless the liquid stream is
sub-cooled. Pressure control is often used, as shown in Figure 5.17d, or control can be
based on the outlet coolant temperature.

Reboiler and vaporiser control
As with condensers, temperature control is not effective, as the saturated vapour temper-
ature is constant at constant pressure. Level control is often used for vaporisers; the
controller controlling the steam supply to the heating surface, with the liquid feed to the
vaporiser on flow control, as shown in Figure 5.20 (see p. 232). An increase in the feed
results in an automatic increase in steam to the vaporiser to vaporise the increased flow
and maintain the level constant.