Leaving Chilled Liquid System Lead/Lag And Compressor Sequencing; Leaving Chilled Liquid Controloverride To Reduce Cycling; Fig. 58 - Leaving Water Temperature Control Example; Fig. 59 - Setpoint Adjust - York YLAA0285SE Manual

SECTION 8 – UNIT OPERATION
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 (4.4 ºC). 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 Leaving Water Temperature is less than
setpoint. Hot gas is turned off as temperature rises
when Leaving Water Temperature is more than setpoint,
plus the control range divided by two. If temperature
remains below the setpoint low limit on the lowest step
of capacity, the microprocessor board 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 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 microprocessor board
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,
with a maximum value allowed of 50 °F (10 °C) (See
Fig. 59).
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.
30 sec. Control Range
unloading (no compressor staging)
44.0 °F
LWT
(6.7 °C)
Low Limit
Leaving Water Temp. Control – Compressor Staging
Setpoint = 46.0 °F (7.8 °C) Range = +/- 2 °F(1.1 °C)
FIG. 58 – LEAVING WATER TEMPERATURE CONTROL EXAMPLE
168
60 sec.
loading
46.0 °F 48.0°
(7.8 °C) (8.9 °C)
Setpoint High limit
6
5
4
3
2
1
0
0 1 2
LAST RUN TIME OF LEAD SYSTEM (MINUTES)
FIG. 59 – SETPOINT ADJUST
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 refrigerant
systems. This may be programmed under the OPTIONS
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 selects
specifically the sequence which the microprocessor
board starts systems.
On a hot water start, once a system starts, it will turn on all
compressors before the next system starts a compressor.
The microprocessor will sequence compressors within
each circuit to maximize individual compressor run time
on individual compressors within a system to prevent
short cycling.
Each compressor in a system will be assigned an
arbitrary 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 compressor 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 repeatedly cycle
on and off.
Once the second system starts a compressor on a 2
system chiller, the microprocessor board will attempt
to equally load each system as long as the system is not
limiting or pumping down. Once this occurs, loading and
unloading will alternate between systems, loading the
lead system first or unloading the lag system first.
FORM 150.72-NM3 (1020)
ISSUE DATE 10/05/2020
3 4 5 6
LD11415
JOHNSON CONTROLS