JOHNSON CONTROLS
134
FORM 150.66-NM3
ISSUE DATE: 09/25/2020
SECTION 8 - UNIT OPERATION
RETURN CHILLLED 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 refrigerant 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 specically the sequence which the
microprocessor starts the systems.
The microprocessor will sequence compressors load
and unload systems according to Table 21. The micro-
processor 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 2 times in a row. The micro-
processor will not attempt to equalize run time on indi-
vidual 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 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 ensures the same compressor does
not repeatedly cycle on and o.
ANTI-RECYCLE TIMER
The programmable anti-recycle timer ensures 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 rst compressor in a system starts.
The timer begins to count down. If all of the compres-
sors in a circuit cycle o, 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.
ANTI-COINCIDENCE TIMER
Two timing controls are present in software to ensure
compressors within a circuit or between systems do not
start simultaneously. The anti-coincidence timer en-
sures there is at least a one minute delay between sys-
tem starts on 2-circuit systems. This timer is NOT pro-
grammable. The load timers further ensure that there
is a minimum time between compressor starts within
a system.
EVAPORATOR PUMP CONTROL
The evaporator pump dry contacts (CTB2 – terminals
23 and 24) are energized when any of the following
conditions are true:
1. Low Leaving Chilled Liquid Fault
2. Any compressor is running
3. Daily Schedule is not programmed OFF and Unit
Switch is ON
The pump will not run if the micro panel has been pow-
ered up for less than 30 seconds or if the pump has run
in the last 30 seconds to prevent pump motor overheat-
ing.
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 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/ SP + 2*CR/4
6 4 SP + CR SP + 3*CR/4
* STEP can be viewed using the OPER DATA key and scrolling to COOLING DEMAND.
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