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FORM QWC3-NM1
ISSUE DATE:11/20/2020
The running compressor with priority number 1 will al-
ways be the next to shut OFF. Whenever a compressor
is shut OFF, the priority numbers of all compressors
will be decreased by 1 with wrap-around. This control
scheme assures the same compressor does not repeat-
edly cycle ON and OFF.
Once the second system starts a compressor on a 2 sys-
tem chillers, the microprocessor will attempt to equally
load each system as long as the system is not limit-
ing or pumping down. Once this occurs, loading and
unloading will alternate between systems, loading the
lead system first or unloading the lag system first.
RETURN CHILLED LIQUID CONTROL
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 14 on page 147).
At return water temperatures between the Cooling
Setpoint and Cooling Setpoint plus Range, compres-
sor loading and unloading will be determined by the
formulas in Table 15 on page 148.
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 ac-
cording to the temperatures determined by the formulas.
Unloading will occur at a rate of 30 seconds according
to the temperatures determined in the formulas used to
calculate the ON and OFF points for each step of ca-
pacity.
TABLE 14 - SAMPLE COMPRESSOR STAGING FOR RETURN WATER CONTROL
# OF COMP ON
RWT
45 °F
(7.2 °C)
*Unloading only
QUANTECH
COMPRESSOR STAGING FOR RETURN WATER CONTROL
4 COMPRESSOR
COOLING SETPOINT = 45 °F (7.2 °C) RANGE = 10 °F(5.6 °C)
0
* 1+HG
46.25 °F
(7.9 °C)
The return chilled liquid setpoint is programmable
from 40 °F to 70 °F (4.4 °C to 21.1 °C) in water chill-
ing 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).
As an example of compressor staging (see Table 17
on page 150), 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 including hot gas) segments, with the Control
Range determining the separation between segments.
Note also that the Cooling Setpoint is the point at which
all compressors 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 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 53.4 °F (11.9
°C), one compressor would cycle OFF leaving five
compressors running. The compressors would contin-
ue to cycle OFF approximately every 1.7 °F (.94 °C),
with the exception 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 compressor 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 15 on page 148 not only provides
the formulas for the loading (ON POINT) and unload-
ing (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 under the OPER DATA key. See the section on
the DISPLAY/PRINT keys for specific information on
the OPER DATA key.
1
2
47.5 °F
50.0 °F
(8.6 °C)
(10.0 °C)
SECTION 8 – UNIT OPERATION
3
4
52.5 °F
55.0 °F
(11.4 °C)
(12.8 °C)
147
8
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