McQuay WSC Operating Manual page 31

Explaination of Tower Control Settings
MicroTech II 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)
ii)
iii) SP3. Select one to four fan outputs depending on the number of fan stages to be used.
iv) SP4. Select STAGE UP TIME from 1 to 60 minutes. The default value of 2 minutes is
v)
2) If TEMP is selected in SP1, use
i)
ii)
3) If LIFT is selected in SP1, use
i)
ii)
See Figure 2, Field Wiring Diagram on page 11 for fan staging field wiring connection points.
OM Centrif Micro ΙΙ-5
SP1. Select TEMP if control is based on condenser EWT or LIFT if based on
compressor lift expressed in psi.
SP2. Select NONE for no bypass valve or fan VFD control.
More than one fan can be used per stage through the use of relays.
probably a good starting point. The value may need to be adjusted later depending on
actual system operation.
SP5. 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.
SP6. Select STAGE DIFFERENTIAL in degrees F, start with default of 3 degrees F.
SP8-11. 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 SP3.
SP7. Select STAGE DIFFERENTIAL in PSI. Start with default of 6 PSI.
SP12-15. Start with default setpoints. The number of STAGE ON setpoints used must be
the same as SP3.
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