Return Chilled Liquid System Lead/Lag And Compressor Sequencing; Anti-Recycle Timer; Anti-Coincidence Timer; Evaporator Pump Control And York Hydro Kit Pump Control - York YLAA Series Installation Operation & Maintenance

FORM 150.72-ICOM6
ISSUE DATE: 12/29/2017
As an example of compressor staging (See Table 20
on page 151), a chiller with six compressors using a
Cooling Setpoint programmed for 45 °F (7.20 °C) and
a Range Setpoint of 10 °F (5.56 °C). Using the formu-
las in Table 20 on page 151, the control range will be
split up into six (seven including hot gas) segments,
with the Control Range determining the separation be-
tween segments. Note also that the Cooling Setpoint is
the point at which all compressors are OFF, and Cool-
ing Setpoint plus Range is the point all compressors are
ON. Specifically, if the return water temperature is 55
°F (12.8 °C), then all compressors will be ON, provid-
ing full capacity. At nominal gpm, this would provide
approximately 45 °F (7.2 °C) leaving water tempera-
ture out of the evaporator.
If the return water temperature drops to 53.4 °F (11.9
°C), one compressor would cycle OFF leaving five
compressors running. The compressors would contin-
ue to cycle OFF approximately every 1.7 °F (.94 °C),
with the exception of hot gas bypass. Notice that the
hot gas bypass would cycle ON when the return water
temperature dropped to 46.25 °F (7.9 °C). At this point
one compressor would be running with hot gas.
Should the return water temperature rise from this
point to 46.7 °F (8.2 °C), the hot gas bypass would
shut OFF, still leaving one compressor running. As the
load increased, the compressors would stage ON every
1.7 °F (.94 °C).
Also note that Table 20 on page 151 not only pro-
vides the formulas for the loading (ON POINT) and
unloading (OFF POINT) of the system, the "STEP" is
also shown in the table. The "STEP" is the increment
in the sequence of the capacity control scheme that can
be viewed under the OPER DATA key. See Display/
Print Keys on page 115 for specific information on
the OPER DATA key.
RETURN 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 of the 2 systems based on
average run hours of the compressors in each system.
Manual Lead/Lag selects specifically the sequence
which the microprocessor starts the systems.
The microprocessor will sequence compressors load
and unload systems according to Table 21 on page
152. The microprocessor will lead/lag compressors
within each circuit to maximize individual compres-
JOHNSON CONTROLS
sor run time for the purpose of lubrication. It will also
prevent the same compressor from starting 2 times in
a row. The microprocessor will not attempt to equalize
run time on individual 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 OFF. Whenever a compressor
is shut OFF, the priority numbers of all compressors
in each system will be decreased by 1 with the wrap
around. This control scheme assures the same com-
pressor does not repeatedly cycle ON and OFF.

ANTI-RECYCLE TIMER

The programmable anti-recycle timer assures 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 first compressor in a system starts.
The timer begins to count down. If all of the compres-
sors in a circuit cycle OFF, 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

This timer is not present on single-system units. Two
timing controls are present in software to assure com-
pressors within a circuit or between systems, do not
start simultaneously. The anti-coincidence timer as-
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 assure that there
is a minimum time between compressor starts within
a system.
EVAPORATOR PUMP CONTROL AND YORK
HYDRO KIT PUMP CONTROL
The evaporator pump dry contacts (XTBC2 – Termi-
nals 23 and 24) are energized when any of the follow-
ing conditions are true:
1. Low Leaving Chilled Liquid Fault
2. Any compressor is running
3. Daily Schedule is ON, Unit Switch is ON and Re-
mote Stop is closed
SECTION 8 – UNIT OPERATION
153
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