Section 8 - Unit Operation; Capacity Control; Suction Pressure Limit Controls; Discharge Pressure Limit Controls - York YLUA0078 Installation Operation & Maintenance

FORM 150.73-NM1
ISSUE DATE: 09/04/2020

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 XTBC2)
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 ca-
pacity control functions will be initiated.
It should be noted that the evaporator contacts (Ter-
minals 23 and 24 of XTBC2) 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 regu-
late the discharge air temperature or suction pressure
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 cut-
out. 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 min-
ute elapses. This control is only operable if the optional
suction pressure transducers are installed.
JOHNSON CONTROLS
SECTION 8 – UNIT OPERATION

DISCHARGE PRESSURE LIMIT CONTROLS

The discharge pressure limit controls unload a sys-
tem before it reaches a safety limit due to high load
or dirty condenser coils. The micro monitors discharge
pressure and unloads a system, if fully loaded, by one
compressor when discharge pressure exceeds the pro-
grammed 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.

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 23 on page 132.
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 Set-
point 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 23
on page 132 and Table 24 on page 132.
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