McQuay AGS 225D, Operating Manual

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48 OM AGSD-1
error. At any time, the adjusted target pressure cannot go below the low pressure inhibit
setpoint or above 52 psi.
When the EXV transitions to the pressure control state, the target will start at the current
evaporator pressure value. The pressure target will then decrement 0.2 psi every second
until reaching the normal calculated target. If the pressure at transition is less than the
calculated target, then pressure control will start immediately with the calculated target.
Superheat Control
In superheat control, suction superheat is controlled directly by the EXV. The superheat
target varies linearly from 6 to 10 degrees F as discharge superheat changes from 30 to 22
degrees F. A PID logic will be used to control the superheat to the target value.
When the EXV transitions to the superheat control state, the target will start at the current
suction superheat value. This target will then decrement 0.1 degree F every second until
reaching the normal target.
Closed
Any time the EXV is not in pressure control or superheat control, it will be in a closed state.
At this time, the EXV position is 0 steps and the EXV close signal (digital output) is active.
EXV State Transitions
Power ON
Closed
Superheat
Pressure
EXV States
T1 T4 T5
T2
T3
Transitions:
T1 – Transition from Closed to Pressure Control requires all of the following
•
Unit State = Auto
•
Evap State = Run
•
Compressor is available
•
Compressor is next on
•
Stage up now flag is set
•
Compressor State = Offr