Page 76 OM 780-2
The current pump stage can be viewed on the Flow screen.
Interstage Timers
The CSM uses a stage-up timer and a stage-down timer to coordinate staging. After any pump stage change or chiller stage
change, both timers reset and start counting down. The stage-up timer is set equal to the Pump Stage-Up Delay Time
variable (default is 2 minutes), and the stage-down timer is set equal to the Pump Stage-Down Delay Time variable (default
is 5 minutes). A pump stage change (up or down) cannot occur while the applicable timer is counting down. The timers are
reset after a chiller is enabled or disabled so that the system has a chance to stabilize after a primary pump starts or stops.
Sequencing Logic, Without Loop Bypass Valve
Pump stage 1 is turned on when the chilled water system is turned on, and it is turned off when the system is turned off. If
the Chilled Water Loop Modulation Control Option variable is “None” (no loop bypass valve), the other stages are
controlled as described below.
Stage-Up Control: The CSM stages up when the differential pressure across the cooling loads is too low. This occurs
whenever the following two conditions are satisfied:
1. The stage-up timer has expired. (See “Interstage Timers” above.)
2. The differential pressure is less than the Loop Differential Pressure Setpoint.
Stage-Down Control: The CSM stages down when the differential pressure across the cooling loads is too high. This
occurs whenever the following two conditions are satisfied:
1. The stage-down timer has expired. (See “Interstage Timers” above.)
2. The differential pressure is greater than or equal to the sum of the Loop Differential Pressure Setpoint and the Pump
Stage Differential.
Sequencing Logic, With Loop Bypass Valve
Pump stage 1 is turned on when the CSM has been set to On (Occupied) or the Recirculate Mode has been turned on, and it
is turned off when the CSM is set to Off (Unoccupied) or the Recirculate Mode has been turned off. If the Chilled Water
Loop Modulation Control Option variable is “Chilled Water Bypass Valve,” the other stages are controlled as described
below.
Stage-Up Control: The CSM stages up when the differential pressure across the cooling loads is too low. This occurs
whenever the following four conditions are satisfied:
1. The stage-up timer has expired. (See “Interstage Timers” above.)
2. The differential pressure is less than the Loop Differential Pressure Setpoint.
3. The bypass valve position is less than the Minimum Loop Bypass Valve Position setting.
4. Condition 3 above has been true longer than the Pump Stage-Up Delay Time setting. (The stage-up timer continuously
resets whenever condition 3 is not true.)
Stage-Down Control: The CSM stages down when the valve is bypassing more water than the stage to be turned off is
supplying. This occurs whenever the following three conditions are satisfied:
1. The stage-down timer has expired. (See “Interstage Timers” above.)
2. The valve position is greater than the Maximum Loop Bypass Valve Position setting. (This setting must be determined
by trial and error.)
3. Condition 2 above has been true longer than the Pump Stage-Down Delay Time setting. (The stage-down timer
continuously resets whenever condition 2 is not true.)
Pump Failure
After the CSM enables any pump, it continually checks the pump’s status to verify that it is operating. If the CSM finds that
a pump is not operating (switch open), it immediately starts a timer, which is set equal to the Pump Status Check Delay
Time variable (default is 30 seconds). If the status returns before the timer expires, the timer resets and stops, and the
system continues to operate normally. If the timer expires before the status returns:
• A forced stage-up occurs.
• The Cooling Load Pump #X Fail alarms occurs.
To Next Page
Loading...
Need help?
General
