Sometimes, as shown in Figure 8.1, a column that is a very large energy user becomes the energy source for a number of loads, each of which acts as a condenser. Two methods have been used to allocate the energy to be recovered: (1) throttling the vapor-heating medium to each condenser, as shown in Figure 8.1, and (2) operating each condenser partly flooded by throttling the condensate. Some priority scheme must be established for startups and for any other occasion when vapor supply is temporarily short.
One method of handling the priority problems is to use overrides and to split-range the various valves involved, as shown in Figure 8.2. The scheme shown illustrates the use of pneumatic devices, but the concepts readily may be implemented with some digital or analog electronic controls. For the six loads in Figure 8.2, we employ six gain 6 relays. For load 1, which has the highest priority, the gain 6 relay is calibrated to have an output span of 3-15 psig for an input span of 3—5 psig. Load 2 has the next highest priority, so its gain 6 relay is calibrated for 5—7 psig input, 3—15 psig output. This continues until the load with lowest priority, load 6, has a gain 6 relay calibrated for 13-15 psig input, 3—15 psig output. At the design stage of such a system, considerable care must be exerted to obtain suitable value of controller gain and also proper valve sizing.
Process-to-process heat exchangers are commonly designed for very small temperature differences, say 8-10°C. If vapor throttling is used, it should be recognized that the vapor-supply valves will tend to have small pressure drops. Accordingly, it is advisable to have vapor flow control to each load, with a set
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