JOHNSON CONTROLS
159
SECTION 8 – UNIT OPERATION
FORM 150.67-NM2
ISSUE DATE: 01/31/2019
8
FIGURE 42 - LEAVING WATER TEMPERATURE CONTROL EXAMPLE
Leaving Water Temp. Control – Compressor Staging
Setpoint = 46.0 °F (7.8 °C) Range = +/- 2 °F(1.1 °C)
30 sec. Control Range 60 sec.
unloading (no compressor staging) loading
44.0 °F 46.0 °F 48.0°
(6.7 °C) (7.8 °C) (8.9 °C)
Low Limit Setpoint High limit
LWT
Return Chilled Liquid Control MUST
only be used when constant chilled liquid
ow is ensured.
The range MUST always be programmed
to equal the temperature drop across
the evaporator when the chiller is “fully
loaded”. Otherwise, chilled liquid tem-
perature 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 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 capacity.
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 (refer to Table
22 on page 163), 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, provid-
ing full capacity. At nominal gpm, this would provide
approximately 45 °F (7.2 °C) leaving water tempera-
ture out of the evaporator.
If the return water temperature drops to 53.4 °F (11.9
°C), one compressor would cycle off leaving five com-
pressors running. The compressors would continue 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 tempera-
ture 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 20 on page 160 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 incriment in
the sequence of the capacity control 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.
TABLE 19 - SAMPLE COMPRESSOR STAGING FOR RETURN WATER CONTROL
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)
*Unloading only
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