JOHNSON CONTROLS
157
SECTION 8 – UNIT OPERATION
FORM 150.72-ICOM7
ISSUE DATE: 10/22/2020
8
TABLE 24 - SAMPLE COMPRESSOR STAGING FOR RETURN WATER CONTROL
COMPRESSOR STAGING FOR RETURN WATER CONTROL
4 COMPRESSOR
COOLING SET POINT = 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
If adding the set point adjust value to the set point high
limit causes the set point high limit to be greater than
50°F, the set point high limit will be set to 50°F, and
the difference will be added to the set point low limit.
Once a system runs for greater than 5 min, the set point
adjust will be set back to 0. This will occur while the
system is still running.
0 1 2 3 4 5
LAST RUN TIME OF LEAD SYSTEM (MINUTES)
ADJUST (DEG. F)
6
5
4
3
2
1
0
FIGURE 35 - SET POINT ADJUST
LD11415
The leaving chilled liquid set point 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 plus or minus 1.5°F to plus or minus 2.5°F
(plus or minus 83°C to 1.39°C) leaving chilled liquid
control.
LEAVING CHILLED LIQUID CONTROL
OVERRIDE TO REDUCE CYCLING
To avoid compressor cycling the microprocessor will
adjust the set point upward temporarily. The last run
time of the system will be saved. If the last run time
was greater than 5 min, no action is to be taken. If the
last run time for the lead system was less than 5 min,
the microprocessor will increase the set point high lim-
it according to the chart at right, with a maximum value
allowed of 50°F (see Figure 35 on page 157).
TABLE 25 - RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
*STEP COMPRESSOR COMPRESSOR ON POINT COMPRESSOR OFF POINT
0 0 SET POINT SET POINT
1 1 W/HGB SP + CR/8 (Note 1) SET POINT
2 1 NO HGB SP + CR/4 SP + CR/8
3 2 SP + 2*CR/4 (Note 2) SP + CR/4
4 2 SP + 2*CR/4 SP + CR/4 (Note 3)
5 3 SP + 3*CR/4 SP + 2*CR/4
6 4 SP + CR SP + 3*CR/4
NOTES:
1. Step 1 is Hot Gas Bypass and is skipped when loading occurs. Hot Gas Bypass operation is inhibited during Pumpdown.
2. Step 3 is skipped when loading occurs.
3. Step 4 is skipped when unloading occurs.
* STEP can be viewed using the OPER DATA key and scrolling to COOLING DEMAND.
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