York YCUL0045E, Installation Operation & Maintenance

Page: 85 / 144

JOHNSON CONTROLS
85
FORM 150.63-NM8 (1020)
2
UNIT OPERATION
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 evapo-
rator blower contacts (Terminals 23 and 24 of CTB2)
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 contacts (Termi-
nals 23 and 24 of CTB2) are not required to be used to
cycle the evaporator 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
suction pressure(s) to the desired setpoint, and regulate
the discharge air temperature or suction pressure to meet
that desired setpoint.
DISCHARGE AIR TEMPERATURE CONTROL
The setpoint in Discharge Air Temperature Control is the
temperature the condensing unit will control to /-
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 11.
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-
Coincidence 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 Setpoint
Low Limit, unloading occurs at a rate of 60 seconds.
The sequences of Capacity Control (compressor stag-
ing) for loading and unloading are shown in Tables 19
& 20.
FIG. 11 – DISCHARGE AIR TEMPERATURE CONTROL
20 sec. 30 sec. 60 sec. control range 60 sec.
unloading unloading unloading (no compressor staging) loading
51.5°F 52.5°F 53.0°F 55.0°F 57.0°
(10.8°C) (11.4°C) (11.7°C) (12.8°C) (13.9°C)
Low Limit Setpoint High limit
Discharge Air Temperature Control – Compressor Staging
Setpoint = 55.0°F (12.8°C) Range = +/- 5°F (-12.2°C)