JOHNSON CONTROLS
133
SECTION 8 – UNIT OPERATION
FORM 150.73-NM1
ISSUE DATE: 09/04/2020
8
SUCTION PRESSURE CONTROL
The setpoint in Suction Pressure Control is the suction
pressures each individual system on the condensing
unit will control to within +/- the cooling range. The
Setpoint High Limit is the Setpoint plus the Cooling
Range. The Setpoint Low Limit is the Setpoint minus
the Cooling Range. Each system is controlled indepen-
dently of each other according to its setpoint and cool-
ing range.
Each system must have its own zone thermostat. If the
respective zone thermostat is closed, the lead compres-
sor on that system will be energized. In addition the
liquid line solenoid to that system will be energized.
Upon energizing any compressor the 60 second Anti-
Coincidence timer will be initiated.
If after 150 seconds of run-time the suction pressure is
still above the Setpoint High Limit, the next compres-
sor in sequence will be energized. Additional loading
stages are energized at a rate of once every 150 seconds
if the suction pressure remains above the Setpoint High
Limit. Each system will have its own load timer of 150
seconds.
If the suction pressure falls below the Setpoint High
Limit and greater than the Setpoint Low Limit, load-
ing and unloading do not occur. This area of control is
called the control range.
If the suction pressure falls below the Setpoint Low
Limit, unloading (compressors cycling off) occurs at
a rate of 30 seconds per system. The zone thermostat
must be satisfied before the last compressor in the sys-
tem cycles off, even if the suction pressure is below the
Setpoint Low Limit.
Hot Gas Bypass solenoid will be energized as the last
stage of unloading. If the zone thermostat calls for
cooling when the suction pressure is below the Set-
point Low Limit, the first stage compressor will be en-
ergized with the hot gas solenoid.
It should be noted that the zone thermostat has ultimate
control. As long as the zone thermostat is calling for
cooling, at least compressor will be running, regardless
of whether the suction pressure is below Setpoint Low
Limit.
The figure below illustrates loading and unloading in
Suction Pressure Control Mode. See section on Set-
points Keys for programmable values.
FIGURE 24 - SUCTION PRESSURE CONTROL
30 second control range 150 second
unloading (no compressor staging) loading
117 psig (8.0 bar) 126 psig (8.7 bar)
Setpoint
121 psig (8.3 bar)
Setpoint = 121 psig (8.3 bar)
Control Range = 5 psig ( 0.35 bar)
Setpoint
Low Limit
Setpoint
High Limit
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