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Unit Controls
RETURN CHILLED LIQUID CONTROL
(DUAL SYSTEM 4 AND 6 COMP UNITS ONLY)
Return chilled liquid control is based on staging the com-
pressors to match the cooling load. The chiller will be
fully loaded when the return water temperature is equal
to the Cooling Setpoint plus the Range Setpoint. The
chiller will be totally unloaded (all compressors off) when
the return water temperature is equal to the Cooling
Setpoint. At return water temperatures between the
Cooling Setpoint, and Cooling Setpoint plus Range
Setpoint, compressor loading and unloading will be de-
termined by the formulas in Table 34 or Table 35.
Normal loading will occur at intervals of 60 seconds
according to the temperatures determined by the for-
mulas. Unloading will occur at a rate of 30 seconds
according the temperatures determined in the formu-
las.
The return chilled liquid setpoint is programmable from
40°F to 70°F (4.4°C to 21.1°C) in water chilling mode
and from 10°F to 70°F (-12.2°C to 21.1°C) in glycol chill-
ing mode. In both modes, the cooling range can be from
4°F to 20°F (2.2° to 11.1°C).
As an example of compressor staging (refer to Table 33
and Table 34), a chiller with six compressors using a
Cooling Setpoint programmed for 45°F (7.20°C) and a
Range Setpoint of 10°F (5.56°C). Using the formulas in
Table 34, the control range will be split up into six (seven
including hot gas) segments, with the Control Range de-
termining the separation between segments. Note also
that the Cooling Setpoint is the point at which all com-
pressors are off, and Cooling Setpoint plus Range
82
Setpoint is the point all compressors are on. Specifi-
cally, if the return water temperature is 55°F (12.8°C),
then all compressors will be on, providing full capacity.
At nominal gpm, this would provide approximately 45°F
(7.2°C) leaving water temperature out of the evapora-
tor.
If the return water temperature drops to 53.3°F (11.8°C),
one compressor would cycle off leaving five compres-
sors running. The compressors would continue to cycle
off approximately every 1.7°F (.94°C), with the excep-
tion of hot gas bypass. Notice that the hot gas bypass
would be available when the return water temperature
dropped to 46.25°F (7.9°C). At this point one compres-
sor would be running.
Should the return water temperature rise from this point
to 46.7°F (8.2°C), the hot gas bypass would shut off, still
leaving one compressor running. As the load increased,
the compressors would stage on every 1.7°F (.94°C).
Also notice that Tables 34 and 35 not only provide the
formulas for the loading (ON POINT) and unloading (OFF
POINT) of the system, the "STEP" is also shown in the
tables. The "STEP" is that sequence in the capacity con-
trol scheme that can be viewed under the OPER DATA
key. Please refer to the section on the DISPLAY/PRINT
keys for specific information on the OPER DATA key.
YORK INTERNATIONAL
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