OMM WCT
Centrifugal Chillers
D-EOMWC01302-16EN - 36/84
Explanation of Tower Control Settings
MicroTech III control can control cooling tower fan stages, a tower bypass valve, and/or a tower fan VFD
if the chiller has a dedicated cooling tower.
The Tower Bypass Valve position will always control the Tower Fan Staging if Valve Setpoint, Stage
Setpoint is selected. Fan staging is determined by Min & Max Tower Valve Position.
There are five possible tower control strategies as noted below and explained in detail later in this section.
They are selected from SETPOINT TOWER SP2.
1.
NONE, Tower fan staging only. In this mode the tower fan staging (up to 4 stages) is controlled by either
the condenser Entering Water Temperature (EWT) or LIFT pressure (difference between the condenser
and evaporator pressures). Tower bypass or fan speed are not controlled.
2.
VALVE SP, Tower staging with low-limit controlled bypass valve. In this mode the tower fans are
controlled as in #1 plus a tower bypass valve is controlled to provide a minimum condenser EWT. There
is no interconnection between the fan control and the valve control.
3.
VALVE STAGE, Tower staging with stage controlled bypass valve. In this mode the bypass valve controls
between fan stages to smooth the control and reduce fan cycling
4.
VFD STAGE. In this mode a VFD controls the first fan. Up to 3 more fans are staged on and off and there
is no bypass valve.
5.
VALVE/VFD, Tower fan control with VFD plus bypass valve control.
Tower Fan Staging Only (NONE)
The following settings are used for the Tower Fan Staging Only mode, (SP= setpoint)
1)
TOWER SETPOINT Screen
i)
Tower Control. Select TEMP if control is based on condenser EWT or LIFT if based on
compressor lift expressed in psi.
ii)
Valve/VFD Control. Select NONE for no bypass valve or fan VFD control.
iii)
Tower Stages. Select one to four fan outputs depending on the number of fan stages to be used.
More than one fan can be used per stage through the use of relays.
iv)
Stage up Time. Select STAGE UP TIME from 1 to 60 minutes. The default value of 2 minutes is
probably a good starting point. The value may need to be adjusted later depending on actual
system operation.
v)
Stage down Time. Select STAGE DOWN TIME from 1 to 60 minutes. The default value of 5
minutes is probably a good starting point. The value may need to be adjusted later depending on
actual system operation.
2)
If TEMP is selected in Tower Control, use
i)
Stage Diff. Select STAGE DIFFERENTIAL in °F, start with default of 3 °F.
ii)
Stage #1-4 On (Temp). Set the STAGE ON temperatures consistent with the temperature range
over which the condenser EWT is desired to operate. The default values of 70
F, 75
F, 80
F and
85
F are a good place to start in climates with moderate wet bulb temperatures. The number of
STAGE ON setpoints used must be the same as Tower Stages.
3)
If LIFT is selected in Tower Control, use
i)
Stage Diff. Select STAGE DIFFERENTIAL in PSI. Start with default of 6 PSI.
ii)
Stage #1-4 On (Lift). Start with default setpoints. The number of STAGE ON setpoints used must
be the same as Tower Stages.