McQuay WGS130AA: WGS140AW, Operating Manual

Page: 21 / 98

OM WGS-5 WGS 130A to 190A 21
Circuit Controller Sequence of Operation
Next on?
Start Compressor
Yes
Unit power up
Circuit is in Off state
No
Stage up now?
Yes
No
When the compressor starts, the EXV will open (if not already open) and hold at
3000 steps for 15 seconds.
If the unit is aircooled, 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 after the compressor starts.
If this compressor has the least starts, the other compressor is disabled for some
reason, or the other compressor is already running, then this compressor will be
designated as the next one on.
If there is a need for more cooling capacity, the compressor designated as next on
can start.
When the circuit is in the Off state the EXV is closed, compressor is off, and all fans
are off. The oil heater will be on at this time as long as oil is detected in the
separator.
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 .
Preopen EXV
Is unit watercooled?
Yes
No
If the unit is configured as watercooled, then the EXV preopen process must be
performed. Otherwise, this is skipped and the compressor can be started.
At this point, the EXV will perform a preopen. While the compressor is off, the EXV
is opened for a period of time to prime the evaporator and avoid low pressure at
startup. The circuit status will display
EXV Preopen .