Pi Control Loops; Discharge Air Temperature Control - McQuay MicroTech II Operation & Maintenance Manual

PI Control Loops

The MicroTech II UVC uses PI-loop control for heating, cooling and
ventilation processes within the unit ventilator. As many as six PI
algorithms may be used for software model 04, dependent upon the unit
Table 15. PI Loop List
PI Loops
PI-1 SpaceTemperature
Primary Cooling
PI-2
(Economizer)
PI-3 Secondary Cooling
PI-4 Primary Heating
PI-5 CO (optional)
2
PI-6 Low Limit
Figure 17. PI Loop Graphic for CO

DISCHARGE AIR TEMPERATURE CONTROL

The UVC uses two "cascaded" 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 Setpoint Output, a second PI-loop (Primary Cooling,
Figure 18. Cascading PI Loop Graphic 1 (Primary Heat)
Figure 19. Cascading PI Loop Graphic 2 (Primary Cool – Economizer)
OM 750
PI Loop
Setpoint
Type
Effective Heating or Cooling
Temperature Setpoint
Calculated Discharge Air
Cascaded Temperature Setpoint Output
Calculated Discharge Air
Temperature Setpoint Output
Calculated Discharge Air
Temperature Setpoint Output
Single Effective CO Setpoint
2
Calculated Discharge Air
Single
Temperature Setpoint Output
2
ventilator configuration. The UVC uses "single" and "cascading" PI
loops where needed.
Feedback
(Controlled Variable)
Space Temperature
Discharge Air Temperature
Discharge Air Temperature
Discharge Air Temperature
Space CO
2
Discharge Air Temperature
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. The second PI-loop used is dependent
upon unit mode (i.e. Heat or Cool).
Output
Calculated Discharge Air Temperature
Setpoint Output
Position the OA Damper
Operate the Compressor
Operate Electric Heat
Position the OA Damper
Position the OA Damper
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