Instruction manual

92 McQuay OM 920
Operator’s Guide
Three Equal Sized Compressors, Three Circuits, No WRV
Only one sequence is provided so the user makes no selections
• Total cooling stages = 3.
• Maximum cooling stages is always set to 3.
• 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.
• The Compressor Sequence changes only when the compressor stage is 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 time 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.
Three Equal Sized Compressors, Three Circuits, Water Regulating Valve
Only one sequence is provided so the user makes no selections
• Total cooling stages = 3.
• Maximum cooling stages is always set to 3.
• If both circuit # 1 and circuit # 2 are enabled, 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
compressor # 3 is used.
• The Compressor Sequence changes based on Run Time only when the compressor stage
equals Maximum Cooling stage 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.
• In order to keep either compressor # 1 or # 2 on, the compressor sequence changes
immediately if a circuit is disabled. This will result is 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 times when
staging down if a disabled circuit is turned off at that stage.
• Any compressor on a disabled circuit stays off if it is already off and is turned off if it is on.
Table 57: Standard Staging
Standard Staging
Staging Sequence Stage 1 Compressor Stage 2 Compressors Stage 3 Compressors
Std-1 1 1,2 1,2,3
Std-2 2 2,3 1,2,3
Std-3 3 1,3 1,2,3
StgdClgCap 33% 67% 100%
Table 58: Standard Staging
Standard Staging
Staging Sequence Stage 1 Compressor Stage 2 Compressors Stage 3 Compressors
Std-1 1 1,3 1,2,3
Std-2 2 2,3 1,2,3