McQuay AGS 225D, Operating Manual

Page: 13 / 82

OM AGSD-1 13
Figure 5, Circuit Sequence of Operation
Next on?
Next off?
Start Compressor
Stage down now?
Pumpdown circuit
Yes
Yes
Yes
No
No
Unit power up
Circuit is in Off state
No
When in the Run state, the compressor will load/unload as needed to satisfy the
load. The compressor will also load/unload to load balance with other compressors
that are running and not in a limited condition. However, it cannot load up until the
discharge superheat has been over 22 F for at least 30 seconds. After this, the
circuit status will be
Run:Normal .
The EXV will operate in either Pressure Control or Superheat Control. In Pressure
Control, the evaporator pressure is controlled to a target pressure, which is adjusted
based on LWT and discharge superheat. In Superheat Control, the suction
superheat is controlled to a target that varies with discharge superheat.
Fans will be staged on and off, and if available the VFD speed will be modulated, to
control the condenser pressure. The condenser pressure is controlled to a target
that is based on evaporator pressure, with the target getting higher as the
evaporator pressure gets higher.
Stage up now?
Yes
No
Run Compressor
When the compressor starts, the EXV will open and begin controlling the evaporator
pressure. Depending on the OAT, a number of fans may be started with the
compressor to keep condenser pressure from climbing too fast. The circuit status
will normally be
Run: Disc SH Low immediately after the compressor starts.
This compressor will be next on if it has the least starts of the compressors that are
not running and are available to run.
If there is a need for more cooling capacity, the compressor designated as next on
can start.
If this compressor has the most run hours of the running compressors, then this
compressor will be designated as the next one off.
If less cooling capacity is needed, the compressor designated as next off can shut
down. This condition may arise when either the LWT has dropped far enough below
the active set point or all circuits are running at a low capacity.
When the circuit does a normal shutdown, a pumpdown is performed. The EXV is
closed while the compressor continues to run. As soon as the pumpdown is
initiated, the compressor is unloaded to the minimum. The condenser fans
continue to control normally during this process. The circuit status during this time
is
Run:Pumpdown .
After the evaporator pressure drops below the pumpdown pressure or enough time
has passed, the compressor and fans are shut off to end the pumpdown process.
The circuit status will normally be
Off:Cycle Timers at this time.
When the circuit is in the Off state the EXV is closed, compressor is off, and all fans
are off.
If the compressor is ready to start when needed, the circuit status will be
Off:Ready . When the circuit switch is off, the circuit cannot start and the status will
be Off:Pumpdown Switch .