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SECTION 8 – UNIT OPERATION
TABLE 15 - RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
*STEP
0
1
2
3
4
5
6
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.
RETURN CHILLED LIQUID SYSTEM LEAD/
LAG AND COMPRESSOR SEQUENCING
A lead/Lag option may be selected to help equalize
average run hours between systems with 2 refriger-
ant systems. This may be programmed under the OP-
TIONS 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 microprocessor starts the systems.
The microprocessor will sequence compressors load
and unload systems according to Table 16 on page
150. The microprocessor will lead/lag compressors
within each circuit to maximize individual compres-
sor run time for the purpose of lubrication. It will also
TABLE 16 - RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
Step
COMP 1
0
OFF
1
ON + HG
2
ON
3
ON
4
ON
5
ON
6
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 less than SP, the Hot Gas
Bypass solenoid is turned OFF when the LWT more than SP plus CR/2.
2. Step 3 is skipped when loading occurs.
3. Step 4 is skipped when unloading occurs.
150
COMPRESSOR
0
1 W/HGB
1 NO HGB
2
2
3
4
LEAD SYSTEM
COMP 2
-
OFF
-
OFF
-
OFF
-
OFF
-
ON
-
ON
-
ON
-
COMPRESSOR ON POINT
SETPOINT
SP + CR/8 (NOTE 1)
SP + CR/4
SP + 2*CR/4 (NOTE 2)
SP + 2*CR/4
SP + 3*CR/4
SP + CR
prevent the same compressor from starting 2 times in
a row. The microprocessor 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
always be the next compressor to start. The running
compressor 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 com-
pressor does not repeatedly cycle ON and OFF.
COMP 1
OFF
See NOTE 1
OFF
OFF
See NOTE 2
ON
See NOTE 3
OFF
ON
ON
FORM 201.26-NM1
ISSUE DATE: 1/19/2018
COMPRESSOR OFF POINT
SETPOINT
SETPOINT
SP +CR/8
SP + CR/4
SP + CR/4 (NOTE 3)
SP + 2*CR/4
SP + 3*CR/4
LAG SYSTEM
COMP 2
-
OFF
-
OFF
-
OFF
-
OFF
-
OFF
-
OFF
-
ON
-
JOHNSON CONTROLS
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