JOHNSON CONTROLS
132
FORM 150.66-NM3
ISSUE DATE: 09/25/2020
SECTION 8 - UNIT OPERATION
high limit. This will result in a control range the same
size as programmed but not allow the unit to run below
40.0ºF. This control will not aect glycol chillers.
Hot gas, if present, will be the nal step of capacity.
Hot gas is energized when only a single compressor
is running and LWT less than SP. Hot gas is turned o
as temperature rises when LWT is more than SP plus
CR/2. If temperature remains below the setpoint low
limit on the lowest step of capacity, the microprocessor
will close the liquid line solenoid or EEV, after turning
o hot gas, and pump the system down before turning
o 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 plus or minus 1.5°F to plus or minus 2.5°F
(plus or minus .83°C to 1.39°C).leaving chilled liquid
control
LEAVING CHILLED LIQUID CONTROL
OVERRIDE TO REDUCE CYCLING
To avoid compressor cycling the microprocessor 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 microprocessor will increase the setpoint
high limit according to the chart at right, with a maxi-
mum value allowed of 50°F (See Fig. 10).
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
dierence will be added to the setpoint low limit.
Once a system runs for greater than 5 minutes, the set-
point adjust will be set back to 0. This will occur while
the system is still running.
FIG. 10 – LEAVING WATER TEMPERATURE
CONTROL EXAMPLE
Leaving Water Temp. Control – Compressor Staging
Setpoint = 46.0°F (7.8°C) Range = +/- 2°F(1.1°C)
30 sec. Control Range 60 sec.
unloading (no compressor staging) loading
44.0°F 46.0°F 48.0°
(6.7°C) (7.8°C) (8.9°C)
Low Limit Setpoint High limit
LWT
LEAVING CHILLED LIQUID SYSTEM
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 allows automatic Lead/
Lag of the two systems based on average run hours of
the compressors in each system. Manual Lead/Lag se-
lects specically the sequence which the microproces-
sor starts systems.
On a hot water start, once a system starts, it will turn
on all compressors before the next system starts a com-
pressor. The microprocessor will Lead/Lag compres-
sors within each circuit to maximize individual com-
pressor run time on individual compressors within a
system.
Each compressor in a system will be assigned an arbi-
trary priority number 1, 2, or 1, 2, 3. 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 o. Whenever a compres-
sor is shut o, the priority numbers of all compressors
will be decreased by 1 with wrap-around. This control
scheme ensures the same compressor does not repeat-
edly cycle on and o.
Once the second system starts a compressor on a 2 sys-
tem chillers, the microprocessor will attempt to equally
load each system as long as the system is not limit-
ing or pumping down. Once this occurs, loading and
unloading will alternate between systems, loading the
lead system rst or unloading the lag system rst.
0 1 2 3 4 5
LAST RUN TIME OF LEAD SYSTEM (MINUTES)
ADJUST (DEG. F)
6
5
4
3
2
1
0
FIG. 11 – SETPOINT ADJUST
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