OMM 1008-1
69
The stage up timer starts when the condenser pump starts. The first stage turns ON when the following
conditions are met:
•
The stage up timer completes
•
The ECWT is > Stage #1 ON (Temp) setpoint (only if the Tower Control setpoint = Temperature)
•
The Lift pressure is > Stage #1 ON (Lift) setpoint (only if the Tower Control setpoint = Lift)
•
Bypass valve position is > the Stage Up @ setpoint (only if Valve/VFD Control setpoint = Valve Stage)
Additional stages can turn on (up to the number specified by the Tower Stages setpoint) when above
conditions are met for the next stage plus the following condition:
•
VFD Speed is > the Stage Up @ setpoint (only if Valve/VFD Control setpoint = VFD Stage OR Valve
SP/VFD Stage)
Down staging occus when the the following conditions are met:
•
The stage down timer completes
•
The ECWT is < Stage #X ON (Temp) setpoint – Stage Differential (Temp) setpoint point
(only if the Tower Control setpoint = Temperature).
•
The Lift pressure is < Stage #X ON (Lift) setpoint – Stage Differential (Lift) setpoint point
(only if the Tower Control setpoint = Lift)
•
Bypass valve position is < the Stage Down @ setpoint (only if Valve/VFD Control setpoint = Valve
Stage)
•
VFD Speed is < the Stage Down @ setpoint (only if Valve/VFD Control setpoint = VFD Stage OR Valve
SP/VFD Stage)
Each stage up or stage down event restarts both the stage up and stage down timers. Only one fan output is
switched at a time (except that all outputs switch OFF when the condenser pump state equals OFF).
Analog Output Control
Each analog output is controlled according to the following rules/algorithms and in accordance with whether
the Compressor Mode setpoint is set to AUTO or MANUAL (normal operation) or TEST (test mode). All
outputs are initialized to 0 at power on.
Cooling Tower Bypass Valve
When the Valve/VFD Control setpoint is set to None OR VFD Stage, this output is set to 0. Otherwise, it is
controlled as described below.
Initial Valve Position
When the condenser pump is not in the RUN state, the valve output is set as a function of entering condenser
water temperature (ECWT) per the following graph.
Figure 32, Bypass Valve Position
Max Position @
Set Point
(90°F)
Min Position @
Set Point
(60°F)
Min Start Position
Set Point (10%)
Initial Valve Position
(values are examples only)
Max Start Position
Set Point (90%)
