JOHNSON CONTROLS
184
FORM 150.67-NM1
ISSUE DATE: 4/28/2017
SECTION 8 – UNIT OPERATION
control range being below 40.0ºF, the low limit will be
reset to 40.0ºF and the difference will be added to the
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 affect glycol chillers.
Hot gas, if present, will be the final step of capacity.
Hot gas is energized when only a single compressor
is running and LWT is less than SP. Hot gas is turned
OFF 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 micropro-
cessor will close the liquid line solenoid, 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 chill-
ing 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 mi-
nus 2.5 °F (plus or minus 0.83 °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 Figure 42).
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 set-
point adjust will be set back to 0. This will occur while
the system is still running.
LEAVING 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 allows automatic Lead/
Lag of the two systems based on average run hours of
the compressors in each system. Manual Lead/Lag se-
lects specifically 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 sequence compres-
sors within each circuit to maximize individual com-
pressor run time on individual compressors within a
system to prevent short cycling.
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 off. Whenever a compres-
sor is shut off, the priority numbers of all compressors
will be decreased by 1 with wrap-around. This control
scheme assures the same compressor does not repeat-
edly cycle ON and OFF.
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 first or unloading the lag system first.
FIGURE 41 - LEAVING WATER TEMPERATURE CONTROL EXAMPLE
Leaving Water Temperature Control – Compressor Staging
Setpoint = 46.0 °F (7.8 °C) Range = plus or minus 2 °F (1.1 °C)
LWT
60 sec.
loading
30 sec.
unloading
Control Range.
(no compressor staging)
44.0 °F
(6.7 °C)
Low Limit
46.0 °F
(7.8 °C)
Setpoint
48.0 °F
(8.9 °C)
High Limit
0 1 2 3 4 5
LAST RUN TIME OF LEAD SYSTEM (MINUTES)
SETPOINT ADJUST
6
5
4
3
2
1
0
FIGURE 42 - SETPOINT ADJUST
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