Controls and Sensors Care, Location, Zones

Temperature control can be no better than the sensors upon which it relies. Although operators and engineers are inclined to trust the measurement of temperature to those who specialize in that field, the operating engineers must be aware that they cannot expect greater accuracy from a control than is put into it by the sensors. (This applies to pressure and other sensors as well.). While T-sensors are usually very good at replicating, they need to be calibrated. And it is the duty of everyone involved around a furnace to be alert to conditions that may cause sensors to deteriorate. 

Cold junction temperatures should be uniform for all sensors. Check regularly for causes of either hot or cold junction degradation. Avoid exposure to high temperature, oxygen, moisture (condensation), or corrosive atmospheres or liquids. 

Remember that many control measurements are implied or indirect or have a time delay, and need study to improve operations. 

Control of input, flow, or pressure is generally more gradual and more precise with variable frequency drives (VFD see glossary) on pumps, blowers, and fans than with control motors and valves, or (worse yet) with dampers. If many zones are supplied from one blower, VFD is not practical therefore, careful linearization of both actuators and valves is necessary. 

Moisture control in drying processes has conventionally been done inferentially by humidity sensors in the discharge air stream, but moisture content sensors at the discharge end of the dryer are preferred. Both amount to feedback control, which responds more slowly than feedforward control. For thick load pieces, the mass transfer time to their surfaces may dictate use of feedforward control by locating sensors within the loads (usually difficult) or earlier in the traverse time within continuous dryers. In view of the dead time of some moisture sensors, locating the control moisture sensor(s) at or nearer the entrance will help improve production, product quality, and energy conservation. 

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