Section 8 - Unit Operation; Capacity Control; Suction Pressure Limit Controls; Discharge Pressure Limit Controls - York YLAA0285SE Manual

FORM 150.72-NM3 (1020)
ISSUE DATE 10/05/2020

CAPACITY CONTROL

To initiate the start sequence of the chiller, all run
permissive inputs must be satisfied (flow/remote start/
stop switch), and no chiller or system faults should
exist.
The first phase of the start sequence is initiated by the
Daily Schedule Start or any Remote Cycling Device. If
the unit is shut down on the daily schedule, the chilled
water pump contacts (Terminals 23 and 24 of XTBC2)
will close to start the pump when the daily schedule start
time has been reached. Once flow has been established
and the flow switch closes, capacity control functions are
initiated, if the remote cycling contacts wired in series
with the flow switch are closed.
It should be noted that the chilled water pump contacts
(Terminals 23 and 24 of XTBC2) are not required to
be used to cycle the chilled water pump. However, in
all cases the flow switch must be closed to allow unit
operation.
The control system will evaluate the need for cooling
by comparing the actual leaving or return chilled liquid
temperature to the desired setpoint, and regulate the
leaving or return chilled liquid temperature to meet that
desired setpoint.

SUCTION PRESSURE LIMIT CONTROLS

The anticipatory controls are intended to prevent the
unit from ever actually reaching a low-pressure cutout.
Loading is prevented if the suction pressure drops below
1.15 x suction pressure cutout (15% below the cutout).
Loading may reoccur after suction pressure rises above
the unload point and a period of one minute elapses. This
control is only operable if the optional suction pressure
transducers are installed.

DISCHARGE PRESSURE LIMIT CONTROLS

The discharge pressure limit controls unload a system
before it reaches a safety limit due to high load or dirty
condenser coils. The microprocessor board monitors
discharge pressure and unloads a system, if fully loaded,
by one compressor when discharge pressure exceeds
the programmed cutout minus 10 PSIG (0.69 barg).
Reloading will occur when the discharge pressure on
the affected system drops to 85% of the unload pressure
and 10 minutes have elapsed.
This control is only applicable if optional discharge
pressure transducers are installed.
JOHNSON CONTROLS
SECTION 8 – UNIT OPERATION

LEAVING CHILLED LIQUID CONTROL

The setpoint, when programmed for Leaving Chilled
Liquid Control, is the temperature the unit will control
to within plus or minus the (control) cooling range. The
Setpoint High Limit is the setpoint plus the cooling
range. The Setpoint Low Limit is the setpoint minus
the cooling range. Fig. 58 should be utilized to aid in
understanding the following description of Leaving
Chilled Liquid Control.
If the leaving chilled liquid temperature is above the
Setpoint High Limit, the lead compressor on the lead
system will be energized along with the liquid line
solenoid. Upon energizing any compressor, the 60
second Anti-Coincidence timer will be initiated to
prevent multiple compressors from turning on.
If after 60 seconds of run-time the leaving chilled liquid
temperature is still above the Setpoint High Limit,
the next compressor in sequence will be energized.
Additional compressors will be energized at a rate of
one every 60 seconds, if the chilled liquid temperature
remains above the Setpoint High Limit and the chilled
liquid temperature is dropping less than 3 °F (-16.1°C/
per minute. The lag system will not be allowed to
start a compressor until the lead system has run for 5
minutes.
If the chilled liquid temperature falls below the Setpoint
High Limit, but is greater than the Setpoint Low Limit,
loading and unloading will not occur. This area of control
is called the control range.
If the chilled liquid temperature drops to between the
Setpoint Low Limit and 0.5 °F (.28 °C) below the
Setpoint Low Limit, unloading (a compressor turns off)
occurs at a rate of one every 30 seconds. If the chilled
liquid temperature falls to a value greater than 0.5 °F
(.28 °C) below the Setpoint Low Limit but not greater
than 1.5 °F (-16.9 °C) below the Setpoint Low Limit,
unloading occurs at a rate of 20 seconds. If the chilled
liquid temperature falls to a value greater than 1.5 °F
(-16.9 °C) below the Setpoint Low Limit, unloading
occurs at a rate of 10 seconds. If the chilled liquid
temperature falls below 1 °F above the low chilled
liquid temperature cutout, unloading occurs at a rate of
10 seconds if it is greater than 10 seconds.
In water cooling mode on R-410A chillers, the minimum
low limit of the control range will be 40.0 ºF (4.4 ºC). For
leaving chilled liquid temperature setpoint and control
range combinations that result in the low limit of the
control range being below 40.0ºF (4.4 ºC), the low limit
will be reset to 40.0ºF (4.4 ºC) and the difference will
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