McQuay WSC Operating Manual page 57

Staging Parameters
Full Load Determination
Each compressor determines if it is at its maximum capacity (or maximum allowed capacity) and,
if so, set its Full Load flag. The flag shall be set (full load) when one or more of the following
conditions are met.
• The compressor is at its physical limit of capacity which means:
For VFD Set Point = NO: The load output has been pulsed ON for a cumulative time equal to
or greater than the Full Load set point. Any unload pulse shall reset the cumulative time to
zero.
For VFD Set Point = YES: Load pulsing has exceeded the Full Load set point (as described
above) AND the VFD speed = 100%
OR
The Vanes Open digital input is On AND the VFD speed = 100%.
• The %RLA is above or equals the Maximum Amp limit set point.
• The %RLA is above or equals the Demand Limit analog input value
• The %RLA is above or equals the Network Limit value
• The evaporator pressure is below the Low Evap Pressure-Inhibit set point.
When none of the above conditions are met, the Full Load flag shall be cleared.
Absolute Capacity
Each compressor shall estimate its absolute capacity from the present value of %RLA and the
Absolute Capacity set point from the equation:
Absolute Capacity = (%RLA Factor) * (Absolute Capacity set point)
Where the %RLA Factor is interpolated from the following table.
%RLA
%RLA Factor
Multiple Compressor Staging
• This section defines which compressor is the next one to start or stop. The next section defines when the
start, or stop, is to occur.
Functions
• Can start/stop compressors according to an operator defined sequence.
• Can start compressors based on # of starts (run hours if starts are equal) and stop on run hours.
• The above two modes can be combined so that there are two or more groups where all
compressors in the first group are started (based on number of starts/hours) before any in the
second group, etc. Conversely, all compressors in a group are stopped (based on run hours)
before any in the preceding group, etc.
• An "efficiency priority" mode can be selected for two or more chillers where one compressor
is started on each chiller in the group before a second is started on any of them.
• A "pump priority" mode can be selected for one or more chillers where all compressors on a
given chiller are started before going to the next chiller in the group.
• One or more compressors can be defined as "standby" where it is never used unless one of the
normal compressors is unavailable.
OM Centrif Micro ΙΙ-5
0 50 75 100
0 0.35 0.75 1.00
150
1.50
57