Instruction manual

McQuay OM 920 93
Operator’s Guide
Four Equal Sized Compressors, Two Circuits
The user selects the following:
1. Staging Type = Standard or Alternate Staging
2. Lead Circuit = Auto, # 1 or # 2.
• Total cooling stages = 4.
• If Lead Circuit is set to Auto and neither circuit is disabled, the Staging sequence is the
Standard or Alternate sequence that contains the Stage 1 Compressor with the fewest run
hours. Maximum cooling stages is set to 4.
• If Lead Circuit is set to # 1 and Circuit # 2 is enabled, the Staging sequence is Std-1 or Std-3
that contains the Stage 1 Compressor with fewer run hours.
• If Lead Circuit is set to # 1 and Circuit # 2 is disabled, the Staging sequence is Alt-1 or Alt-
3 that contains the Stage 1 Compressor with fewer run hours.
• If Lead Circuit is set to # 2 and Circuit # 1 is enabled, the Staging sequence is Std-2 or Std-4
that contains the Stage 1 Compressor with fewer run hours.
• If Lead Circuit is set to # 2 and Circuit # 1 is disabled, the Staging sequence is Alt-2 or Alt-
4 that contains the Stage 1 Compressor with fewer run hours.
• 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.
• If a circuit is disabled, Maximum cooling stages is set to two. Any compressor on the
disabled circuit stays off if it is already off and is turned off if it is on.
• If dehumidification is active, Alternate Staging is used regardless of the Staging Type=
setting.
StgdClgCap 33% 67% 100%
Table 59: 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%
Table 60: Alternate Staging
Alternate Staging
Staging Sequence Stage 1
Compressor
Stage 2
Compressors
Stage 3
Compressors
Stage 4
Compressors
Alt-1 1 1,3 1,2,3 1,2,3,4
Alt-2 2 2,4 1,2,4 1,2,3,4
Alt-3 3 1,3 1,3,4 1,2,3,4
Alt-4 4 2,4 2,3,4 1,2,3,4
StgdClgCap 25% 50% 75% 100%
Table 58: Standard Staging
Standard Staging
Staging Sequence Stage 1 Compressor Stage 2 Compressors Stage 3 Compressors