169
JOHNSON CONTROLS
FORM 150.72-NM3 (1020)
ISSUE DATE 10/05/2020
SECTION 8 – UNIT OPERATION
8
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 o) when the
return water temperature is equal to the Cooling Setpoint
(See sample in Table 14). At return water temperatures
between the Cooling Setpoint and Cooling Setpoint
plus Range, compressor loading and unloading will be
determined by the formulas in Table 15.
Return Chilled Liquid Control MUST
only be used when constant chilled
liquid ow 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 ow 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 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
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).
As an example of compressor staging (refer to Table 14),
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 15, 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 o, and Cooling Setpoint plus Range is the point
all compressors are on. Specically, 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 evaporator.
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 continue
to cycle o 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
o, 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 not only provides 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 the increment in the sequence of
the capacity control scheme that can be viewed under
the OPER DATA key. Refer to the section in this IOM
on DISPLAY/PRINT keys for specic information on the
OPER DATA key.
TABLE 14 – SAMPLE COMPRESSOR STAGING FOR RETURN WATER CONTROL
*Unloading only
Compressor Staging for Return Water Control
4 Compressor
Cooling Setpoint = 45 °F (7.2 °C) Range = 10 °F(5.6 °C)
# of Comp ON 0 * 1+HG 1 2 3 4
RWT
45 °F
(7.2 °C)
46.25 °F
(7.9 °C)
47.5 °F
(8.6 °C)
50.0 °F
(10.0 °C)
52.5 °F
(11.4 °C)
55.0 °F
(12.8 °C)
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