Apparatus for controlling a process variable of a flowing medium

Details Apparatus for controlling a process variable of a flowing medium Apparatus for controlling a process variable of a flowing medium

1984-12-13

1987-10-27

Davis, Jr., Albert W.

Heat exchange

Movable heating or cooling surface

Rotary drum

165 35, 165 40, 364137, 236 24, G05D 2300

Patent

active

047023066

ABSTRACT:
The plant has a controlled section through which water flows, for example, for cooling or heating purposes. The operation of the controlled section is controlled by obtaining temperature measurements of the flow via a pair of temperature sensors and emitting a control signal in response to a difference to a valve in a connecting line which is in parallel with the heat exchanger. The same signal is also directed to a comparator in a control circuit within which a second process variable, i.e. flow quantity, is measured. A comparison between these latter signals is made and a resulting difference signal is used by a controller to adjust a valve in the feed line to the heat exchanger to adjust the flow of medium. Should the temperature of the flow downstream of the junction point of the connecting line and heat exchanger increase, the valve in the connecting line is moved in closing direction to increase the flow to the heat exchanger and at the same time, the valve in the feed line is further opened to additionally increase the flow to the heat exchanger. Shortly thereafter, this increase in flow causes the valve in the feed line to be moved back in closing direction.

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Anderson, N. A., Instrumentation for Process Measurement and Control, Chilton, Penna., 3rd Edition, 1980, pp. 352-357.
Wills, D. M., "Cascade Control, When, Why and How" System Design, Sept. 1961, pp. 37-39.

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