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Ventilation systems are designed to supply a specific volumetric airflow to a system. As zone dampers and diffusers downstream of the main air supply increase and decrease their demand, the supplied air pressure in the main duct will increase and decrease.
The increase in pressure can have a negative effect on the system by increasing noise and restricting the flow of the air through the HVAC unit. Having too low of a pressure will also decrease system efficiency.
In order to control the static pressure of a ventilation system, which has a variable air resistance, bypass dampers are often used. The bypass damper will open and close to keep up with air flow demand.
As the static pressure in the duct increases, the bypass damper will open further to re-circulate air from the ducting back through the air handler to decrease the pressure. This method keeps the air within the system to help with efficiency.
D = Damper
DF = Diffuser
SPS = Static Pressure Sensor
PID control is the best method to control the position of a damper based on pressure feedback.
The error between the setpoint and the feedback from an analog device can be fed into a PID function. The output of the PID can then directly control an analog output to a damper position actuator.
PID control is the best method to control the position of a damper based on pressure feedback.
The error between the setpoint and the feedback from an analog device can be fed into a PID function. The output of the PID can then directly control an analog output to a damper position actuator.
The easyE4 nano programmable logic controller’s PID control capability is easy to set up using easySoft. The programmer can select which gains they want to enable by setting EP, EI, or ED and simply has to set I1 to be the setpoint and I2 to be the feedback from the pressure sensor.
Additionally, the proportional gain KP is configured directly on the faceplate. This value will have to be determined empirically based on the desired response.
The easyE4 calculates the integral and derivative gain slightly differently. The integral gain, KI=KP*TC/TN. Where KP is the proportional gain, TC is the scan time of the function block, and TN is the integration time.
Likewise, the derivative gain KD=KP*TV/TC, where KP and TC are the same parameters and TV is the differential time. Like the proportional gain, the integral and derivative gains need to be determined empirically. However, for damper position control, it is recommended that the derivative term should not be used.
The output of the PID, QV then directly sets the output to the system, which in this case is the damper position reference. As the error increases and decreases, the position of the damper will intelligently adjust to constrict and increase the airflow to the controlled area.
Damper position control can be used to address a variety of control applications. These include:
