FORM 150.72-NM1 (1020)
155
JOHNSON CONTROLS
8
CAPACITY CONTROL
To initiate the start sequence of the chiller, all run
permissive inputs must be satised (ow/remote start/
stop switch), and no chiller or system faults exist.
The rst 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 ow has been established
and the ow switch closes, capacity control functions are
initiated, if the remote cycling contacts wired in series
with the ow 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 ow 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 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 aected 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.
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. 26 should be utilized to aid
in understanding the following description of Leaving
Chilled Liquid Control.
SECTION 8 – UNIT OPERATION
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
once 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/min. 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 do not occur. This area of control
is called the control range.
If the chilled liquid temperature drops to between
Setpoint Low Limit and 0.5 °F (.28 °C) below the
Setpoint Low Limit, unloading (a compressor turns o)
occurs at a rate of 1 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
(.83 °C) below the Setpoint Low Limit, unloading occurs
FIG. 26 – LEAVING WATER TEMPERATURE
CONTROL EXAMPLE
44.0 ºF
(6.7 ºC)
Low Limit
46.0 ºF
(7.8 ºC)
Setpoint
48.0 ºF
(8.9 ºC)
High Limit
Leaving Water Temp. Control - Compressor Staging
Setpoint = 46.0 ºF (7.8 ºC) Range = +/-2 ºF (1.1 ºC)
30 sec.
unloading
60 sec.
loading
Contol Range
(no compressor staging)
LWT
LD14404
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