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SECTION 8 - UNIT OPERATION
Hot gas, if present, will be the final step of capacity. Hot
gas is energized when only a single compressor is running
and LWT<SP. Hot gas is turned off as temperature rises
when LWT>SP + CR/2. If temperature remains below
the setpoint low limit on the lowest step of capacity, the
micro will close the liquid line solenoid or EEV, after
turning off hot gas, and pump the system down before
turning off the last compressor in a system.
The leaving chilled liquid setpoint is programmable from
40°F to 70°F (4.4°C to 21.1°C) in water chilling mode
and from 10°F to 70°F (-12.2°C to 21.1°C) in glycol
chilling mode. In both modes, the cooling range can be
from +/-1.5°F to +/-2.5°F (+/-.83°C to 1.39°C).leaving
chilled liquid control
LEAVING CHILLED LIQUID CONTROL
OVERRIDE TO REDUCE CYCLING
To avoid compressor cycling the micro will adjust the
setpoint upward temporarily. The last run time of the
system will be saved. If the last run time was greater
than 5 minutes, no action is to be taken. If the last run
time for the lead system was less than 5 minutes, the
lWT
FIG. 16A – LEAVING WATER TEMPERATURE CONTROL EXAMPLE
LEAVING CHILLED LIQUID SYSTEM
COMPRESSOR SEQUENCING
Each compressor in a system will be assigned an arbitrary
priority number 1, 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 to shut off. Whenever a compressor is shut off, the
priority numbers of all compressors will be decreased
by 1 with wrap-around. This control scheme ensures the
same compressor does not repeatedly cycle on and off.
RETURN CHILLED LIQUID CONTROL
(Should not be used on Single System Chill-
ers)
130
30 sec.
Control Range
unloading
(no compressor staging)
44.0°F
(6.7°C)
low limit
leaving Water Temp. Control – Compressor Staging
Setpoint = 46.0°F (7.8°C) Range = +/- 2°F(1.1°C)
microprocessor will increase the setpoint high limit
according to the chart below; with a maximum value
allowed of 50°F (see FIG. 16).
If adding the setpoint adjust value to the setpoint high
limit causes the setpoint high limit to be greater than
50°F, the setpoint high limit will be set to 50°F, and the
difference will be added to the setpoint low limit.
Once a system runs for greater than 5 minutes, the
setpoint adjust will be set back to 0. This will occur
while the system is still running.
6
5
4
3
2
1
0
0
1
2
LAST RUN TIME OF LEAD SYSTEM (MINUTES)
FIG. 16 – SETPOINT ADJUST
60 sec.
loading
46.0°F
48.0°
(7.8°C)
(8.9°C)
Setpoint
high limit
ANTI-RECYCLE TIMER
The programmable anti-recycle timer ensures that
systems do not cycle. This timer is programmable
under the PROGRAM key between 300 - 600 seconds.
Whenever possible, to reduce cycling and motor heating,
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 compressors in a
circuit cycle off, a compressor within the circuit 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.
FORM 150.66-NM2
ISSUE DATE: 09/25/2020
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JOHNSON CONTROLS
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