JOHNSON CONTROLS
160
FORM 150.67-NM2
ISSUE DATE: 01/31/2019
SECTION 8 – UNIT OPERATION
RETURN CHILLED LIQUID SYSTEM LEAD/
LAG AND COMPRESSOR SEQUENCING
A lead/Lag option may be selected to help equalize av-
erage run hours between systems with 2 refigerant sys-
tems. This may be programmed under the OPTIONS
key. Auto Lead/Lag of the 2 systems based on average
run hours of the compressors in each system. Manual
Lead/Lag selects specifically the sequence which the
micro starts the systems.
The micro will sequence compressors load and unload
systems according to Table 21 on page 160. The mi-
croprocessor will lead/lag compressors within each cir-
cuit to maximize individual compressor run time for
the purpose of lubrication. It will also prevent the same
compressor from starting 2 times in a row. The micro
will not attempt to equalize run time on individual
compressors within a system.
Each compressor in a system will be assigned an ar-
bitrary number 1, or 2. The non-running compressor
within a system with the lowest priority number will al-
ways be the next compressor to start. The running com-
pressor with priority number 1 will always be the next
compressor to shut off. Whenever a compressor is shut
off, 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 off.
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.
TABLE 21 - RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
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 off 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.
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