Control Loop Introduction
Understanding a Control Loop Heat Exchanger (Instantaneous Water Heater)
3 Respond
3-15 PSI Air
TA987 Air Filter/ Regulator
TA901 I/P Transducer Converts 4-20 mA (current) electrical signal to a 3-15 PSI (pressure) air signal
4 Affect
Air Signal Line
Control Valve
Air Inlet
3-15 PSI Air
4-20 mA
2 Compare
Steam Inlet
Compares output of temperature sensor to the temperature Set Point and outputs an appropriate 4-20 mA response to the I/P Transducer TR890 Controller
Controlled Steam Flow
HB Series Pneumatic Control Valve
4-20 mA
Heat Exchanger (Water Heater)
sensor out- put
1 Sense
Temperature Sensor (RTD or Thermocouple) Outputs a varying resist- ance (RTD) or voltage (T/C) to Controller Input
Steam Trap
Hot Water Outlet
Cold Water Inlet
Control Loop A control loop is a process management system designed to maintain a process variable at a desired set point. Each step in the loop works in conjunction with the others to manage the system. Once the set point has been estab- lished, the control loop operates using a four-step process.
Sense Measure the current condition of the process using a sensor, which can be a thermocouple or RTD transmitter.
1
Compare Evaluate the measurement of the current condition against the set point using an electronic PID controller. Respond Reacts to any error that may exist between the measured tem- perature value and the temperature set point by generating a corrective pneumatic signal. Affect Actuate the control valve that will produce a change in the process variable. The loop continually cycles through the steps, affecting the process variable (water temperature) in order to maintain the desired temperature set point.
2
3
4
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