This method of controlling pressure, although once popular, has fallen into some disfavor in recent years. This is particularly true for once-through coolant. Since its flow rate cannot be allowed to go too low—which would lead to fouling as well as excessive exit coolant temperatures, which, in turn, contribute to corrosion—it permits only limited control of pressure. Tempered coolant, which avoids these problems, is a better choice for column pressure control via coolant flow manipulation.
The signal flow diagram for pressure control via coolant flow manipulation is given in Figure 17.15. Once-through coolant and steam flow or flow-ratio control are assumed. The left-hand side of this diagram comes from Figure 17.3. Note that wc is the rate of condensation, Ibm/sec.
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