1 - >fi(s) dQv/dPa has lead characteristics and a static gain of less than unity, permitting higher loop gain, faster flow-control response, and much higher flow controller gain. To avoid problems with flow controller tuning, the system should always be operated in one flow regime or the other. In most cases steam flow is noncritical.
Since we have called for averaging level control, the natural frequency of the level control loop will be much lower than that of the flow control loop. Note that:
This leads us to the final signal flow diagram of Figure 15.11.
Base Level Control by Direct Manipulation of Steam Valve
From Figure 15.9 and by assuming A(s) = 0, we can prepare the signal flow diagram of Figure 15.12. Note that if steam flow is critical, dQv/dPa = 0.
It has already been indicated that for averaging level control cascaded to steam flow control, one may substitute l/K„f for the flow control loop. Use of the mathematical models discussed here on commercial reboilers indicates that typical time constants range from a fraction of a second to 5—10 seconds. Practically speaking y(s), <t>(s), and A(i) reduce to constants. As we will see in Chapters 16 and 17, it usually will be possible to use much simpler reboiler models than that discussed in this section.
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