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CAPACITY CONTROL
To initiate the start sequence of the unit, all run permis-
sive inputs must be satisfied (air proving/remote start/
stop switch), and no unit or system faults exist.
The first phase of the start sequence is initiated by the
Daily Schedule Start or a Remote Cycling Device. If the
unit is shut down on the daily schedule, the evaporator
blower microboard contacts (TB5 3-4) will close when
the daily schedule start time has been reached. Once
the air proving switch closes, capacity control functions
are initiated.
If unit cycling is accomplished with a remote cycling
device wired in series with the air proving switch, the
evaporator contacts will always be energized as long as
the unit switch is turned ON. When the air proving switch
and remote cycling contacts are closed, the capacity
control functions will be initiated.
It should be noted that the evaporator blower contacts
(TB5 3-4) are not required to be used to cycle the evapo-
rator blower. However, in all cases, the air proving switch
must be closed to allow unit operation.
The control system will evaluate the need for cooling by
comparing the actual discharge air temperature or suc-
tion pressure(s) to the desired setpoint, and regulate the
discharge air temperature or suction pressure to meet
that desired setpoint.
20 sec.
unloading
51.5°F
DAT
(10.8°C)
FIG. 10 – DISCHARGE AIR TEMPERATURE CONTROL
YORK INTERNATIONAL
UNIT OPERATION
30 sec.
60 sec.
unloading
unloading
52.5°F
53.0°F
(11.4°C)
(11.7°C)
Low Limit
Discharge Air Temperature Control – Compressor Staging
Setpoint = 55.0°F (7.8°C) Range = +10°F (5.5°C)
DISCHARGE AIR TEMPERATURE CONTROL
The setpoint in Discharge Air Temperature Control is the
temperature the condensing unit will control to within
+/- the control range. The Setpoint High Limit is the
Setpoint plus the Cooling Range. The Setpoint Low Limit
is the Setpoint minus the Cooling Range. See Figure 5.
If the Discharge Air 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-Coinci-
dence timer will be initiated.
If after 180 seconds of run time the discharge air tem-
perature is still above the Setpoint High Limit, the next
compressor in sequence will be energized. Additional
loading stages are energized at a rate of once every 180
seconds if the discharge air temperature remains above
the Setpoint High Limit.
If the discharge air 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 discharge air temperature drops below the Set-
point Low Limit, unloading occurs at a rate of 60 sec-
onds.
The sequences of Capacity Control (compressor stag-
ing) for loading and unloading are shown in Table 17
through Table 20.
control range
(no compressor staging)
55.0°F
(12.8°C)
Setpoint
FORM 150.63-NM2
60 sec.
loading
57.0°
(13.9°C)
High limit
121
3
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