OM 780-2 Page 57
Number of Tower
Stages
This input sets the highest tower stage that the CSM will enable. It is typically set to the highest Stage # row with a Tower
Output set to On in the Tower Stage Table. Range = 1 to 12. Default = 6
Tower Stage
Differential
The cooling tower will not be staged down until the Control Temperature is less that the current Stage Setpoint minus this Tower
Stage Differential. Range = 0 – 9.5°F (0 – 5.2°C). Default = 3°F (1.7°C)
Tower Stage-Up
Delay Time
After a tower stage-up occurs, the next stage-up cannot occur until after this period of time expires. Range = 1 to 60 minutes.
Default = 2 minutes
Tower Stage-Down
Delay Time
After a tower stage-down occurs, the next stage-down cannot occur until after this period of time expires. Range = 1 to 60
minutes. Default = 5 minutes
Stage 1 to 12
Setpoints
Enter the temperature setpoints (up to 12) which correspond to the Stage # in the Tower Stage Table. When the control
temperature is higher than the setpoint the CSM will stage-up to enable more cooling tower capacity. These inputs also define
the differentials that will be maintained between each stage if the Stage 1 Setpoint is being reset. Range = 40-99.5°F (4.4 to
37.4°C). Defaults; Stage 1 Setpoint =74°F (23.3°C), 2=76°F (24.4°C), 3 through 12=78°F (25.5°C)
Tower VFD
Control Flag
This input allows CSM’s control of the cooling tower VFD to be turned on and off. If this flag = No, the CSM’s tower VFD
analog outputs will not modulate to maintain the control temperature. Default = No
Tower VFD
Deadband
This input sets a Deadband around the Stage 1 Setpoint if tower VFD control is used. No tower VFD control action is taken
when the current Control Temperature is within this Deadband around the Stage 1 Setpoint. Range = 0.5 – 10°F (0.2 – 5.6°C).
Default = 1.5°F (0.8°C)
Tower VFD
Propband
This value sets the “proportional band” used in the PID control function that modulates the tower VFD speed. In general,
increasing this value has a slowing effect and decreasing this value has a speeding effect on the control of the VFD speed.
Range = 1 – 60°F (0.5 – 33.3°C). Default = ±30°F (±16.7°C)
Tower VFD
Sample Time
This value sets the “sampling period” used in the PID control function that modulates the tower VFD speed. In general,
increasing this value has a slowing effect and decreasing this value has a speeding effect on the control of the VFD speed.
Range = 1 – 3600 seconds. Default = 15 seconds
Tower VFD
Integral Time
This value varies the “integral time” used in the PID control function that modulates the tower VFD speed. In general,
increasing this value has a slowing effect and decreasing this value has a speeding effect on the control of the VFD speed.
Range = none. Default = 120 seconds
Tower VFD PI
Function Graph
A graph of the tower Control Temperature vs. Time is shown at the bottom of the page when the Tower VFD Control Flag = Yes
to assist in tuning the Tower VFD PI loop.
Tower VFD
Minimum Speed
This input sets the minimum percentage from the CSM’s tower VFD analog output. Range = 25 – 99 %. Default = 45%
Fan Speed of the
First Non-VFD Fan
When VFD’s are used on some (but not all) cooling tower fans and the non-VFD fans have multi-speed control, set this input to
the lowest multiple fan speed. Then place all VFD controlled fans in Stage 1, and place the lowest fan speed in Stage 2. Stage 2
will only be enabled after the VFD controlled fans reach the Fan Speed of the First Non-VFD Fan. Range = 33 – 100 %.
Default = 100%
Constant Approach
Reset Flag
This input allows Stage 1 Setpoint reset based on the outside air wet-bulb temperature to be turned on and off. If this flag = Yes,
the CSM’s cooling tower stage setpoints will change to maintain a constant temperature differential between the common
entering condenser water and the ambient wet-bulb. Relative humidity and outside air temp sensors are required. Default = Off
Constant Approach
Temperature
Differential
This input sets the constant temperature differential that will be added to the wet-bulb temperature to reset the CSM’s cooling
tower Stage 1 Setpoint (if Constant Approach Reset Flag = Yes). Note that all of the other Stage Setpoints (2 through 12) are
also reset to maintain their original user defined differentials from the Stage 1 Setpoint. Range = 5-25 °F (2.8 - 13.9°C). Default
= 7°F (3.9°C)
Constant Approach
Reset – Minimum
Setpoint
This input sets the minimum value for the cooling towers Stage 1 Setpoint when using Constant Approach Reset. Resetting the
condenser entering water too low during cold weather could cause chiller operating problems. Range = 40 – 99.5°F (4.4 –
37.5°C). Default = 74°F (23.3°C)
Cooling Tower Bypass Valve
Name Description
Tower Valve
Control Option
This input tells the CSM if there is a cooling tower bypass valve. If there is a valve, you further define what setpoint the valve will
use. Range = None, Valve Setpoint, Stage Setpoint. Default = None
Tower Valve
Setpoint
This input is the user defined cooling tower valve setpoint. If the Tower Valve Control Option = Valve Setpoint, the valve will be
modulated to maintain the Control Temperature at this setpoint. Range = 40 – 99.5°F (4.5 – 37.5°C). Default = 70°F (21.1°C)
Tower Valve
Control Range
Min
This input defines the lowest valve position the CSM will send to the tower bypass valve. Range = 0 – 100%. Default = 0%
Tower Valve
Control Range
Max
This input defines the highest valve position the CSM will send to the tower bypass valve. Range = 0 – 100%. Default = 100%
Min Tower Valve
Position Setpoint
This value is used as a stage-down condition for interstage control when the Tower Valve Control Option = Stage Setpoint. Range
= Tower Valve Control Range Min – 100%. Default = 20%
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