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8.8 ACCUMULATORS
Due to the inherent ability of scroll to handle liquid refrigerant in flooded start and defrost cycle operation
conditions, accumulators may not be required. An accumulator is suggested on single compressor 2-6
hp systems when the charge limitations exceed 4.5kg, and on the single compressor 7-12 hp systems
when charge limitations exceed 8kg. On systems with defrost schemes or transient operations that allow
prolonged uncontrolled liquid return to the compressor, an accumulator is required unless a suction header
of sufficient volume to prevent liquid migration to the compressor is used.
Excessive liquid flood back or repeated flooded starts will dilute the oil in the compressor causing inadequate
lubrication and bearing wear. Proper system design will minimize liquid flood back, thereby ensuring
maximum compressor life.
In order to assure that liquid refrigerant does not return to the compressor during the running cycle,
attention must be given to maintaining proper superheat at the compressor suction inlet. Recommended
is 5-8K superheat, measured on the suction line 150mm from the suction port, to prevent liquid refrigerant
flood back.
Another method to determine if liquid refrigerant is returning to the compressor is to accurately measure
the temperature difference between the compressor bottom shell and the suction line. During continuous
operation we recommend that this difference be a minimum of 25°C. To measure oil temperature through
the compressor shell, place a thermocouple on the bottom center (not the side of the suction) of the
compressor shell and insulate from the ambient.
During rapid system changes, such as defrost or ice harvest cycles, this temperature difference may drop
rapidly for a short period of time. When the crankcase temperature difference falls below the recommended
25°C; duration should not exceed a maximum (continuous) time period of two minutes and should not go
lower than a 12°C difference.
8.9 PUMP DOWN RECOMMENDATIONS
Refrigeration scroll compressors use a low-leak discharge valve to prevent high pressure backflow into
the low side. Typically, this check valve prevents system pressures from equalizing and pump down can
be achieved. If short cycling cannot be avoided, using a 3-minute time delay will limit the cycling of the
compressor to an acceptable level.
For the single phase MBP compressors, there is a small balance port on the discharge check valve to
balance the high and low side inside of the compressor when the compressor is stoped, if pump down is
applied, an external check valve on the discharge line is suggested.
