Ask Greg McMillan

Greg's Response:

Flows much lower than design causes a high process gain and high process deadtime that can lead to temperature oscillations and possible instability. Low velocities can cause a dramatic increase in fouling of heat transfer surfaces. Flows much higher than design causes a low process gain (poor sensitivity) that can lead to wandering of the temperature and possible loss of temperature control.

When peak error or initial transient must be minimized, feedforward control should be used. A rather simple energy balance that equates heat lost from the hot side to heat gain by the cold side yields a solution. Normal operating values are used for those inputs not measured that are relatively constant. For example, if the main upset is feed flow, a measurement of this flow is required, but assumed operating conditions can be used for the inlet temperatures that are not measured. It is critical that the controlled temperature be the set point rather than the measurement to avoid positive feedback. The feedforward signal is added to the output of the feedback controller. A bias of 50% is also subtracted so that the temperature controller can make a negative correction as large as the positive correction to the feedforward signal. The manipulated flow is best achieved by means of a flow controller and a cascade of exchanger temperature to coolant or steam flow. If the temperature controller output goes directly to a control valve, signal characterization of the installed valve characteristic should be used to convert from desired flow to required valve position. The signal divider for compensation of process gain nonlinearities should be applied to the controller output before the summer.

If the exchanger outlet temperature can be adjusted, a valve position controller can be used to slowly change the temperature set point to optimize the coolant valve position that minimizes the fouling of heat transfer surfaces by prevention of low throttle positions but also maintains the process gain and reduces utility usage by the prevention of high throttle positions.

Simulations that include operating conditions and equipment design with Digital Twin can help find and confirm the best control strategies.

For much more knowledge, see the ISA book Advanced Temperature Measurement and Control, Second Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).