Instruction manual

94 McQuay OM 920
Operator’s Guide
Four Equal Sized Compressors, Four Circuits, No WRV
Only one sequence is provided so the user makes no selections
• Total cooling stage = 4.
• Maximum cooling stages is always set to 4.
• The Staging sequence is the Standard sequence that contains the enabled Stage 1
Compressor with the fewest run hours. Compressors in disabled circuits are ignored when
only enabled compressor run hours are compared. Maximum cooling stages is set to 4.
• The Compressor Sequence changes only when the compressor stage equals Maximum
cooling stages or zero. This means that disabling and re-enabling the compressors or turning
the unit off and on may be necessary to cause sequence changes.
• The same staging sequence is used whether a circuit is disabled or not. This means that units
will remain at a stage for an extra stage time when staging up if a disabled circuit is added at
that stage. It also means that units will remain at a stage for an extra stage times when
staging down if a disabled circuit is turned off at that stage.
• Any compressor on the disabled circuit stays off if it is already off and is turned off if it is
on.
Four Equal Sized Compressors, Four Circuits, Water Regulating Valve
Only one sequence is provided so the user makes no selections
• Total cooling stages = 4.
• Maximum cooling stages is always set to 4.
• If neither circuit # 1 nor circuit # 2 is disabled, the staging sequence is Std-1 if compressor #
1 has fewer hours than compressor # 2, and the staging sequence is Std-2 if compressor # 1
does not have fewer hours than compressor # 2.
• If Circuit # 1 is disabled, the staging sequence is Std-2.
• If Circuit # 2 is disabled, the staging sequence is Std-1.
• If both circuit # 1 and circuit # 2 are disabled, the staging sequence is Std-1. Only
compressors 3 and 4 are used.
• The Compressor Sequence changes based on Run Time only when the compressor stage
equals Maximum cooling stages or zero. This means that disabling and re-enabling the
compressors or turning the unit off and on may be necessary to cause sequence changes.
• The compressor sequence changes immediately if a circuit is disabled to keep either
compressor # 1 or # 2 on. This will result in compressors being turned off and others being
turned on in this emergency condition.
• The same staging sequence is used whether a circuit is disabled or not. This means that units
will remain at a stage for an extra stage time when staging up if a disabled circuit is added at
that stage. It also means that units will remain at a stage for an extra stage time when staging
down if a disabled circuit is turned off at that stage.
Table 61: Standard Staging
Standard Staging
Staging Sequence Stage 1
Compressor
Stage 2
Compressors
Stage 3
Compressors
Stage 4
Compressors
Std-1 1 1,2 1,2,3 1,2,3,4
Std-2 2 2,3 2,3,4 1,2,3,4
Std-3 3 3,4 1,3,4 1,2,3,4
Std-4 4 1,4 1,2,4 1,2,3,4
StgdClgCap 25% 50% 75% 100%