McQuay IM 743 User Manual

Page 56 OM 780-2
The CSM automatically resets the System Setpoint. You can ignore the remaining reset variables.
Note: To use the External reset method, an external analog signal (0–10 Vdc, or 0–20 mA) must be connected to UI-10 of
the CSM’s onboard I/O panel, see the Field Wiring section of IM 781.
Constant Return Chilled Water Temperature Control
The constant return reset method uses a proportional-integral (PI) control loop to generate a System Setpoint that keeps the
return chilled water temperature at the Constant Return Setpoint. It is different from the other three reset methods because it
does not use a mathematical function to reset the System Setpoint.
Constant return temperature control is typically used only in systems that have constant chilled water flow. This is true
because return water temperature is a good indicator of cooling load only when the flow is constant. If your system has
three-way valves at the loads or a supply-to-return loop bypass valve, it probably has constant flow.
When the return temperature is above the Constant Return Setpoint, the control loop lowers the System Setpoint. When the
return temperature is below the Constant Return Setpoint, the control loop raises the System Setpoint. The System Setpoint
is limited to a range defined by the Minimum System Setpoint and Maximum System Setpoint.
The PI control loop has four adjustable variables that are dedicated to return chilled water temperature control: (1)
Constant Return Deadband, (2) Constant Return Prop Band, (3) Constant Return Sample Time, (4) Constant Return
Integral Time. For many applications, the default values for these variables provide good control. The Constant Return
Temp PI Function time plot is provided on the Chilled Water Supply Temp screen to assist in tuning the PI loop.
To set up Constant Return reset
1. Set the Chilled Water Temperature Reset Type variable to “Constant Return”.
2. Set the following variables as required:
• Minimum System Setpoint
• Maximum System Setpoint
• Constant Return Setpoint
• Constant Return Deadband
• Constant Return Propband
• Constant Return Sample Time
• Constant Return Integral Time
The CSM automatically resets the System Setpoint. You can ignore the remaining reset variables.
Note: To use the Constant Return reset method, a return chilled water temperature sensor must be connected to UI-2 of the
CSM’s onboard I/O panel. For more information, see the Field Wiring section of IM 781.
Cooling Tower Control
The CSM can maintain a common entering or leaving condenser water temperature by controlling up to 12 cooling tower
stages, tower fan VFD’s and a tower bypass valve. To view the current values of the cooling tower operation go to the Clg
Tower Status screen. To configure the cooling tower operation, go to the Clg Tower Control screen to change the values
described in Table 14.
Table 14. Clg Tower Control (Main > Configuration > Clg Tower Control)
Cooling Tower Stages
Name Description
Tower Control Flag This input allows CSM’s control of the cooling tower to be turned on and off. If this flag = No, the tower outputs will not be
enabled by the CSM. Default = No
Control
Temperature
Source
This input defines whether the CSM will maintain the Entering or Leaving condenser water temperature at the setpoint. If
constant approach reset is used, the Control Temperature Source variable must be set to Entering Cond Water (the water
temperature leaving the cooling tower). Range = Entering Cond Water, Leaving Cond Water. Default = Entering Cond Water

McQuay IM 743 Specifications

General

Brand	McQuay
Model	IM 743
Category	Controller
Language	English