Return Chilled Liquid Control; Table 19 - Compressor Staging For Return Water Control - York YCAL Series Installation Operation & Maintenance

FORM 150.66-NM3
ISSUE DATE: 09/25/2020

RETURN CHILLED LIQUID CONTROL

(Can be used on Dual System 4, 5 & 6 Comp
Units Only)
Return chilled liquid control is based on staging the
compressors 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. The
chiller will be totally unloaded (all compressors off)
when the return water temperature is equal to the Cool-
ing Setpoint (See sample in Table 19). At return water
temperatures between the Cooling Setpoint and Cool-
ing Setpoint plus Range, compressor loading and un-
loading will be determined by the formulas in Table 20.
Return Chilled Liquid Control MUST
only be used when constant chilled
liquid flow is ensured.
The RANGE MUST always be pro-
grammed to equal the temperature
drop across the evaporator when the
chiller is "fully loaded". Otherwise,
chilled liquid temperature will over or
under shoot. Variable flow must never
be used in return chilled liquid mode.
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 ac-
cording to the temperatures determined in the formulas
used to calculate the on and off points for each step of
capacity.
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 chilling mode. In both modes, the cooling range
can be from 4°F to 20°F (2.2° to 11.1°C).
TABLE 19 – COMPRESSOR STAGING FOR RETURN WATER CONTROL
COOLING SETPOINT = 45°F (7.2°C) RANGE = 10°F (5.6°C)
# OF COMP ON
45°F
RWT
(7.2°C)
* Unloading Only
JOHNSON CONTROLS
4 COMPRESSORS
0 *1+HG 1
46.25°F 47.5°F
(7.9°C) (8.6°C)
As an example of compressor staging (refer to Table
19), 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
20, the control range will be split up into six (seven in-
cluding 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 is the
point all compressors are on. Specifically, if the return
water temperature is 55°F (12.8°C), then all compres-
sors will be on, providing full capacity. At nominal
gpm, this would provide approximately 45°F (7.2°C)
leaving water temperature out of the evaporator.
If the return water temperature drops to 53.4°F (11.9°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 cycle on when the return water temperature
dropped to 46.25°F (7.9°C). At this point one compres-
sor would be running with hot gas.
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 note that Table 20 not only provides the formu-
las for the loading (ON POINT) and unloading (OFF
POINT) of the system, the "STEP" is also shown in the
tables. The "STEP" is the increment in the sequence
of the capacity control scheme that can be viewed un-
der the OPER DATA key. Refer to the section on the
DISPLAY/PRINT keys for specific information on the
OPER DATA key.
2 3
50.0°F 52.5°F 55.0°F
(10.0°C) (11.4°C) (12.8°C)
SECTION 8 - UNIT OPERATION
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