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Air coil freeze stat. A freeze stat is a temperature sensing device for HVAC that monitors a heat exchanger to prevent its coils from freezing.Freeze stats can be used on both refrigerant-to-air, and refrigerant-to-liquid type heat exchangers and serve different purposes with similar goals for each.
Normally, a thermostat is used to energise when it measures temperature falling below a set temperature value - usually between 3 °C and 5 °C and often referred to as the 'setpoint'. The thermostat will de-energise the trace heating when it measures temperature rising past another set temperature value - usually 2 °C higher than the setpoint ...
Typically the central system would be switched on when the temperature drops below the setpoint on the thermostat, and switched off when it rises above, with a few degrees of hysteresis to prevent excessive switching. Bi-metallic sensing is now being superseded by electronic sensors. A principal use of the bi-metallic thermostat today is in ...
The DCS sends the setpoint required by the process to the controller which instructs a valve to operate so that the process reaches and stays at the desired setpoint. (see 4–20 mA schematic for example). Large oil refineries and chemical plants have several thousand I/O points and employ very large DCS.
A setpoint can be any physical quantity or parameter that a control system seeks to regulate, such as temperature, pressure, flow rate, position, speed, or any other measurable attribute. In the context of PID controller , the setpoint represents the reference or goal for the controlled process variable.
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Proportional control, in engineering and process control, is a type of linear feedback control system in which a correction is applied to the controlled variable, and the size of the correction is proportional to the difference between the desired value (setpoint, SP) and the measured value (process variable, PV).
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