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McQuay OM 751 29
Description of Operation
Proportional Integral (PI) Control Loops
The MicroTech II UVC uses PI-loop control for heating, cooling and ventilation processes
within the unit ventilator. Numerous PI algorithms can be used depending upon the unit
ventilator configuration. The UVC uses “single” and “cascading” PI loops where needed.
Table 16: PI loop list
Figure 17: PI loop graphic for CO
2
Discharge Air Temperature Control
The UVC uses two “cascading” PI loops to aid in providing very stable space temperature
control. The Space Temperature PI-loop is used to calculate the Discharge Air Temperature
Setpoint Output required to meet the Effective Temperature Setpoint Output. A second PI-loop
(Primary Cooling, Secondary Cooling, or Primary Heating) is then activated to control the
heating or cooling device required to achieve the calculated Discharge Air Temperature
Setpoint Output.
Figure 18: Cascading PI loop graphic 1 (primary heat)
Figure 19: Cascading PI loop graphic 2 (primary cool—economizer)
PI loops PI loop type Set point
Feedback
(controlled
variable)
Output
PI-1 Space Temperature
Cascading
Effective Heating or Cooling
Temperature Setpoint
Space Temperature
Calculated Discharge Air
Temperature Setpoint Output
PI-2 Primary Cooling (Economizer)
Calculated Discharge Air Temperature
Setpoint Output
Discharge Air
Temperature
Position the OA Damper
PI-3 Secondary Cooling
Calculated Discharge Air Temperature
Setpoint Output
Discharge Air
Temperature
Operate the Compressor
PI-4 Primary Heating
Calculated Discharge Air Temperature
Setpoint Output
Discharge Air
Temperature
Position the Wet Heat Valve or
F&BP Damper
PI-5
CO
2
(optional) Single Effective CO
2
Setpoint Space
CO
2
Position the OA Damper
PI-6 Low Limit Single
Calculated Discharge Air Temperature
Setpoint
Discharge Air
Temperature
Position the OA Damper
+
-
Space CO
2
sensor
Effective
CO
2
set point
PI - 5
Position the
OA damper