From the process viewpoint, the two parameters that should be regulated are the concentration and flow rate of the bottoms product. If the composition of the feed stream is constant, good control of the feed rate and the evaporation rate will give the desired concentrated product at the proper production rate (see Fig. 1). Of course, the method of control can depend upon the evaporator type and method of operation. When evaporation rate is to be maintained at a constant rate, a steam flow controller is generally used. Steam flow control usually is accomplished by throttling the steam which results in a loss of temperature difference. Steam may, therefore, be uncontrolled to achieve maximum capacity. Steam pressure controllers may be used to protect the equipment or to assure substantially constant temperatures in the front end of a multistage evaporation system. Constant temperatures in the later effects of the evaporator can be controlled with a pressure controller on the last effect.
A control system consists of three parts: a measurement; a control algorithm; and a process actuator. The process actuator (often a control valve) is always a direct user of energy; the measurement may take energy from the process (as in the case of a head-type flow meter); and the control calculation never requires a significant energy supply. However, the correct control calculation is essential for energy-efficient operation of any process.
The well-engineered control system depends on the ability to directly measure the parameter that is to be controlled, or to measure another parameter from which the controlled variable can be inferred. In every case, a measurement of the controlled variable is preferred. A survey of the measurements in a major production unit gave the following distribution of process instrumentation^241
Flow rates are the largest single group of process measurements used for control, and flow is the only process variable for which significant energy may be required by the measuring device. Most flows are measured by orifice meters which are heat-type devices that extract head loss from the pumping
Type of Measurement
Temperature and analytical
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