JOHNSON CONTROLS
73
SECTION 5 – SEQUENCE OF OPERATION
FORM 100.50-NOM12
ISSUE DATE: 04/02/2019
5
LD28117
FIGURE 33 - VAV — UNOCCUPIED
COMPRESSORS: CONTROL AND OPERATION
Compressor Systems
• The Series 100 units have three completely inde-
pendent compressor systems. Each compressor
system has two compressors piped in a tandem
arrangement.
• Each system has a low pressure cutout (LPCO)
switch, a compressor safety chain, liquid line îš¿l-
ter driers, and two thermostatic expansion valves
(TXVs) (one each for the upper and lower evapo-
rator coils).
• Each compressor in a tandem arrangement can be
staged on independently of the other compressor.
This staging allows better control of the SAT and
keeps undercooling and overcooling to a mini-
mum under normal operating conditions.
Compressor Control
• The IPU board has internal logic and program-
ming to stage compressors ON/OFF to achieve
and maintain the active cooling SAT setpoint.
• Under normal operation, each compressor has a
3.5 minute guaranteed ON/OFF timer.
• Under normal operating conditions, there is a 3.5
minute Interstage Time Delay after any compressor
is started/stopped before starting/stopping another
compressor unless fast compressor start is User En-
abled.
Compressor Staging
• The Series 100 units have three sets of tandem
scroll compressors. This allows the unit to have
six stages of cooling.
• When the unit enters an Active Cooling mode, the
IPU board starts to stage compressors ON.
• The IPU board stages compressors ON/OFF to
achieve and maintain the active SAT setpoint.
• If a compressor system has experienced a Safety
Lockout, that circuit is removed from the compres-
sor staging circuit until the Safety Lockout clears.
• Compressor staging is also affected when only
one or two of the three systems have a low ambi-
ent package.
• See Table 27 on page 74 for compressor stag-
ing.
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