McQuay WSC Operating Manual page 95

2. Once the lead compressor has satisfied the Soft Loading requirement, and judged to be at
Full Load, the Lag compressor (on multiple compressor applications) will determine when to
stage up as follows.
3. The Lag compressor will initiate a start sequence when the following events occur: a) Full
Load flag received from lead compressor, b) Evap LWT slope is less than minimum pull
down rate, c) Evap LWT exceeds the Stage-Delta-T setpoint.
4. The initial step in the Lag start sequence is to send a flag to the lead compressor causing it to
continuously unload the vanes for the duration of two Postlube time periods. This reduces
the head pressure that the lag compressor will have to overcome when it starts. Note that
after the two Postlube periods, the lead compressor will load back up regardless of the lag
compressor's run status.
5. The lag compressor will wait for a period of time equal to the Postlube minus the Prelube
time before starting it's oil pump. This coordinates the two compressors so that as the lead
unloads to the vanes closed condition, the lag compressor is completing the Prelube state and
both compressor are released to load up together. One minute after the lag compressor starts,
Amp balancing takes affect to share the load.
Determining Full Load Status
Since the chiller's Full Load status doesn't correspond to %RLA, a direct comparison cannot be
made. That is to say that the chiller can be running flat out (vanes fully open) at 90 %RLA. The
%RLA number is greatly affected by the chiller's operating conditions (i.e. condenser water,
evap Delta-T).
The following describes the six parameters used to set the Full Load indication for a compressor.
1. Vane position – Vane position is not measured directly. On Chillers with a VFD, a switch is
2. VFD Speed - A flag is set if the VFD is equal or above 100% speed.
3. Max_Amps - A flag is set if %RLA is equal to, or above Max_Amps.
4. Demand Limit - A Flag is set if %RLA is equal to, or above a % demand limit (either 4-
5. Max Capacity based on pressure - A Flag is set if Evap Saturated Pressure is equal to, or
6. Soft loading - If the operator has enabled the SoftLoad feature, the first compressor (on the
OM Centrif Micro ΙΙ-5
used to detect "Vanes_Open". Chiller's without a VFD use a "Full Load" timer (Set Comp
SPs (4). This timer sums the time that the load vanes solenoid is pulsed. Any unload pulses
will reset the timer.
Vanes_Open flag is set.
20mA signal or network amp limit).
below the Evap Inhibit loading setpoint "LowEvPrHold" found under SET ALARM LMTs
(1).
A low refrigerant charge can cause this flag to be set at lower than expected capacity, but it
is still an indication that the chiller has reached maximum capacity.
pLAN network) to run will inhibit the following flags while the SET UNIT SPs (6)
"SoftLoadRamp" timer is running; Vanes_Open, Max_Amps, Demand Limit, and Max
Capacity.
When the continuous load pulse time exceeds the setpoint the
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