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OMM 1008-1
Explanation of Tower Control Settings
The MicroTech II controller 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 or Stage
Setpoint is selected.
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, which is not recommended. 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 pressure). 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.
1. Tower Fan Staging Only (NONE); This is not a recommended control strategy.
The following settings are used for the Tower Fan Staging Only mode, (SP= setpoint)
a)
TOWER SETPOINT Screen
i)
SP1. Select TEMP if control is based on condenser EWT or LIFT if based on compressor lift
expressed in degrees.
ii)
SP2. Select NONE for no bypass valve or fan VFD control.
iii)
SP3. 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)
SP4. 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)
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.
vi)
If TEMP is selected in SP1, use
(1)
SP6. Select STAGE DIFFERENTIAL in degrees F, start with default of 3 degrees F.
(2)
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.
b)
If LIFT is selected in SP1, use
i)
SP7. Select STAGE DIFFERENTIAL in PSI. Start with default of 6 PSI.
ii)
SP12-15. Start with default setpoints. The number of STAGE ON setpoints used must be the same
as SP3.
See
Figure 8
on page 8 for fan staging field wiring connection points.
