JOHNSON CONTROLS
132
FORM 150.27-NM1
ISSUE DATE: 09/30/2015
SECTION 8 – UNIT OPERATION
If the return water temperature drops to 53.4 °F (11.9
°C), one compressor would cycle off leaving five com-
pressors running. The compressors would continue to
cycle off approximately every 1.7 °F (.94 °C), with the
exception of Hot Gas Bypass. Notice that the Hot Gas
Bypass would cycle on when the return water tempera-
ture dropped to 46.25 °F (7.9 °C). At this point one
compressor would be running with hot gas.
Should the return water temperature rise from this
point to 46.7 °F (8.2 °C), the Hot Gas Bypass would
shut off, still leaving one compressor running. As the
load increased, the compressors would stage on every
1.7 °F (.94 °C).
Also note that
not only pro-
vides the formulas for the loading (On Point) and un-
loading (Off Point) of the system, the “STEP” is also
shown in the tables. The “STEP” is the increment in
the sequence of the capacity control scheme that can be
viewed under the OPER DATA key.
Refer to Display/
Print Keys on Page 107 for specific information on
the OPER DATA key.
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
. The microprocessor will lead/lag compressors
within each circuit to maximize individual compressor
run time for the purpose of lubrication. It will also pre-
vent the same compressor from starting two 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 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.
ANTI-RECYCLE TIMER
The programmable anti-recycle timer assures that sys-
tems do not cycle. This timer is programmable under
the PROGRAM key between 300 and 600 seconds.
Whenever possible, to reduce cycling and motor heat-
ing, the anti-recycle timer should be adjusted to 600
seconds. The programmable anti-recycle timer starts
the timer when the first compressor in a system starts.
The timer begins to count down. If all of the compres-
sors in a circuit cycle off, a compressor within the cir-
cuit will not be permitted to start until the anti-recycle
timer has timed out. If the lead system has run for less
than 5 minutes, 3 times in a row, the anti-recycle timer
will be extended to 10 minutes.
TABLE 18 -
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.
2. Step 3 is skipped when loading occurs.
3. Step 4 is skipped when unloading occurs.
