Trane SCXG-SVX01B-EN Compressors, sequence of, Operation operation, Compressor Lead/Lag Operation

sequence of

Operation operation

Compressors

Units use two sizes of hermetic scroll compressors, 10 and 15 hp, and can use from two to six compressors. When viewing the front of the unit, compressors are identified A through B from left to right. The second compressor from the left, or B compressor, is always the first to come on, unless locked out for a malfunction or shut off on frost protection. Refer toTable O-SO-1 for compressor cycling stages andTable O-SO-3 on page 78 for percent cooling capacity by stage.

The control system logic permits com- pressor operation only after the supply fan is on. If the supply fan shuts down, compressors will not operate. Units without head pressure control (units with intermediate piping packages) will lock out mechanical cooling when the entering condenser water temperature falls below 54°F. Mechanical cooling will resume when the entering condenser water temperature exceeds 58°F.

When there are more than two compres- sors in an air cooled unit, the first two

compressors are manifolded together. If there are four compressors, the second two are manifolded.

Compressor Cycling Compressors cycle to maintain the operating state required by the temperature controls. In the event of a compressor failure, the next available compressor turns on. Refer toTable O- SO-1 for compressor cycling by unit model and tons.

During normal conditions, compressors will not shut off until they have been on for at least three minutes and will not turn on until they have been off for at least three minutes. Normal operating condi- tions are established on an individual compressor basis. When a compressor starts, its timer also starts. The compres- sor evaporator circuit frost protection can override the “minimum” timer and reduce the five minute minimum re- quired time period.

When the unit is powered up, or manually reset there will be a three to eight minute delay before the first compressor may be

turned on as requested by the unit temperature control algorithm.

Compressor Lead/Lag Operation

Compressor lead/lag is a user-selectable feature at the HI panel and is available on all units. After each request for compressor operation, the lead refrigeration circuit or compressor switches, thereby causing a more equitable or balanced run time among compressors.

When lead/lag is enabled, each time the system cycles, it will alternate between the standard compressor staging and the lead/lag staging. UsingTable O-SO-1, a SXWG 30-ton unit will first stage com- pressor B then A, then AB for first cycle and A, then AB for the second cycle. Appropriate condenser valves (water- cooled and condenser fans (air-cooled) will stage with appropriate compressors to maintain saturated condensing temperature. Enabling lead/lag may drop a cooling stage when compared to standard staging. SeeTable O-SO-1 for compressor staging.

Table O-SO-1. Compressor Stages.

Unit	Refrigerant	Compressor HP		Standard	Lead/Lag	SCM
Size	Circuit Type	by Stage		Compressor	Compressor	or
	Model # Digit 5	A	B	Staging	Staging	MCM
SXWG 20, 25	Independent	10	10	B/AB	A/AB	MCM
SXRG 20
SXWG 30	Independent	15	10	B/A/AB	A/AB	MCM
SXRG 25
SXWG 35	Independent	15	15	B/AB	A/AB	MCM
SXRG 32

88 SCXG-SVX01B-EN