Figure 5.10 shows a system sometimes encountered when a large feed tank is used to balance load and demand at some point in the process. The tank is connected in series with the main process flow. Most of the flow from Process Unit No. 1 goes directly to Process Unit No. 2. The level in Process Unit No. 1 must be controlled; at the same time, it is desired to fix the flow rate into Process Unit No. 2.
It is usually undesirable to put everything through the feed tank, since this typically operates at a lower temperature than Process Unit No. 1. Cooling all the process stream and then feeding it to Process Unit No. 2 normally increases energy consumption in process Unit No. 2 significandy.
The control system shown in Figure 5.10A is a simple technique for minimizing energy consumption. The two valves on the makeup and purge lines from and to the feed tank are split ranged so that both valves cannot be open at the same time. This guarantees that a minimum amount of material is going to or coming from the tank.
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