User manual

Operation
24
Figure 20 Second cooling source and two-step compressorized cooling
Minimum Chilled Water Temperature—This feature permits the user to select the minimum
chilled water temperature that allows simultaneous operation of the second cooling source
(chilled-water valve control) and compressor control. This feature is enabled in the Service menu
under Setpoints, parameter Minimum CW Temp.
If the water temperature is below this minimum chilled water setpoint, parameter Minimum CW
Temp Value, (Service Menu, Setpoints), the control will operate ONLY the second cooling source
control, i.e., the compressor is locked out. Above the minimum chilled water setpoint, assuming the
fluid temperature is below the return room air temperature (delta T between room and glycol = true),
the control will operate the second cooling source control and compressor control simultaneously if
needed.
If the Minimum CW Temp is disabled, the second cooling source temperature is ignored, the control
will always operate the second cooling source and compressors simultaneously when the load requires
it.
GLYCOOL
™
Cooling—Free-Cooling
When GLYCOOL cooling is available, the temperature control will calculate a total cooling
requirement of 200% rather than 100%. Assuming that full GLYCOOL capacity is available, the
GLYCOOL valve opens proportionally as the requirement for cooling rises from 0 to 100%. If more
than 100% cooling is required, then the compressors are activated their normal activation settings
within the 100-200% proportional band, 150% and 200%, respectively, on two-step systems (133%,
163%, 180% and 200% for a four-step system). If full GLYCOOL capacity is not available, then the
GLYCOOL valve will be opened proportionally over a cooling requirement band equal to the available
GLYCOOL capacity. The compressors would be activated when the GLYCOOL capacity is exceeded.
For example, if the GLYCOOL capacity is 60%, then the GLYCOOL valve would be fully open at 60%
cooling requirement. The compressors would continue to activate or deactivate based on shifting the
activation points from 0-100% to 100-200% in relation to the cooling output band within the 100-200%
proportional band. To reduce compressor cycling and prevent hunting, GLYCOOL capacity first
becomes available when the entering glycol temperature is at least 8°F (4.4°C) (22% capacity) below
the return air temperature, or 3°F (1.7°C) below the return air temperature for two hours. GLYCOOL
capacity is 100% when the glycol temperature is 25°F (13.9°C) below the return air temperature. The
system will continue to operate in Econ-O-Cool mode as necessary as long as the entering glycol
temperature remains at least 3°F (1.7°C) (0% capacity) below the return air temperature. If
GLYCOOL is not available, the compressors’ activation and deactivation points are not shifted as
explained above.
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Increasing Temperature
Cool 2
On
Cool 1
On
+ 100%
Cooling
Valve 100%
Open
Valve
Closed
½ Dead-
band
76 77 78 79
+ 200%
Cooling
0%
Cooling
2nd
Source
Comp
½ Proportional Band ½ Proportional Band
Band1: 2
nd
Source Band 2: Compressors
Temp Setpoint: 70°F
Proportional Band: 8°F
Deadband: 2°F