JOHNSON CONTROLS
151
SECTION 8 – UNIT OPERATION
FORM 150.72-ICOM6
ISSUE DATE: 12/29/2017
8
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
0 1 2 3 4 5
LAST RUN TIME OF LEAD SYSTEM (MINUTES)
ADJUST (DEG. F)
6
5
4
3
2
1
0
FIGURE 39 - SETPOINT ADJUST
LD11415
LEAVING CHILLED LIQUID CONTROL
OVERRIDE TO REDUCE CYCLING
To avoid compressor cycling the microprocessor will
adjust the setpoint upward temporarily. The last run
time of the system will be saved. If the last run time
was greater than 5 minutes, no action is to be taken.
If the last run time for the lead system was less than 5
minutes, the microprocessor will increase the setpoint
high limit according to the chart at right, with a maxi-
mum value allowed of 50 °F (see Figure 29 on page
152).
If adding the setpoint adjust value to the setpoint high
limit causes the setpoint high limit to be greater than 50
°F, the setpoint high limit will be set to 50 °F, and the
difference will be added to the setpoint low limit.
Once a system runs for greater than 5 minutes, the set-
point adjust will be set back to 0. This will occur while
the system is still running.
TABLE 20 - RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
*STEP COMPRESSOR COMPRESSOR ON POINT COMPRESSOR OFF POINT
0 0 SETPOINT SETPOINT
1 1 W/HGB SP + CR/8 (Note 1) SETPOINT
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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