JOHNSON CONTROLS
170
FORM 150.72-NM3 (1020)
ISSUE DATE 10/05/2020
SECTION 8 – UNIT OPERATION
RETURN CHILLLED LIQUID SYSTEM LEAD/
LAG AND COMPRESSOR SEQUENCING
A lead/lag option may be selected to help equalize
average run hours between systems with two refrigerant
systems. This may be programmed under the OPTIONS
key. Auto Lead/Lag of the two systems based on average
run hours of the compressors in each system. Manual
Lead/Lag specically selects the sequence in which the
microprocessor board starts the systems.
The microprocessor board will sequence compressors
load and unload systems according to Table 16. The
microprocessor will lead/lag compressors within each
circuit to maximize individual compressor run time
for the purpose of lubrication. It will also prevent the
same compressor from starting two times in a row. The
microprocessor board will not attempt to equalize run
time on individual compressors within a system.
Each compressor in a system will be assigned an
arbitrary number 1 or 2. The non-running compressor
within a system with the lowest priority number will
always be the next compressor to start. The running
compressor with priority number 1 will always be the
next compressor to shut o. Whenever a compressor is
shut o, the priority numbers of all compressors in each
system will be decreased by 1 with the wrap around.
This control scheme assures the same compressor does
not repeatedly cycle on and o.
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.
TABLE 15 – RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
* STEP can be viewed using the OPER DATA key and scrolling to COOLING DEMAND.
*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
LEAD SYSTEM LAG SYSTEM
Step COMP 1 COMP 2 -
See NOTE 1
See NOTE 2
See NOTE 3
COMP 1 COMP 2 -
0 OFF OFF - OFF OFF -
1 ON + HG OFF - OFF OFF -
2 ON OFF - OFF OFF -
3 ON OFF - ON OFF -
4 ON ON - OFF OFF -
5 ON ON - ON OFF -
6 ON ON - ON ON -
NOTES
1. Step is Hot Gas Bypass and is skipped when loading occurs. Hot Gas Bypass operation is inhibited during pumpdown. For Leaving Chilled
Liquid Control the Hot Gas Bypass solenoid is energized only when the lead compressor is running and the LWT < SP, the Hot Gas Bypass
solenoid is turned o when the LWT > SP + CR/2.
1. Step 1 is not used for loading or unloading.
2. Step 3 is skipped when loading occurs.
3. Step 4 is skipped when unloading occurs.
TABLE 16 – RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
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