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A pump-down cycle to control refrigerant migration may have to be used for several reasons, for
example when the compressor is located outdoors without any housing so that cold air blowing over
the compressor makes the crankcase heater ineffective.
If a pump-down cycle is used, a separate external check valve must be added. The scroll
discharge check valve is designed to stop extended reverse rotation and prevent high-pressure gas
from leaking rapidly into the low side after shut-off. The check valve will in some cases leak causing
the scroll compressor to recycle more frequently. Repeated short-cycling of this nature can result in
a low oil situation and consequent damage to the compressor. The low-pressure control differential
has to be reviewed since a relatively large volume of gas will re-expand from the high side of the
compressor into the low side after shutdown.
For pressure control setting, never set the low-pressure control to shut off outside of the
operating envelope. To prevent the compressor from running into problems during such
faults as loss of charge or partial blockage, the control should not be set lower than the
minimum suction pressure allowed by the operating envelope.
5.12 Refrigerant floodback and oil dilution
One of the major causes of compressor failure is damage caused by liquid refrigerant entering the
compressor in excessive quantities. Improper control of liquid refrigerant in a system can cause a
loss of lubrication and compressor failure.
Within the general refrigeration system qualification, Emerson recommends a refrigerant floodback
and oil dilution test. During the system testing and under all possible operation conditions, the
evaporating temperature and the bottom shell temperature shall be recorded.
The bottom shell temperature together with the evaporating temperature gives an indication whether
liquid refrigerant is returning or diluted in the compressor oil sump. The compressor sump
temperature must remain in the (green) safe area, as shown in the oil dilution chart, see Figure 23
below. In case of operation in the (red) unsafe area, adjustments are required in order to modify the
system design, refrigerant charge or superheat setting of the expansion device(s). The bottom shell
temperature should be measured accurately. The thermo-probe must be positioned on the opposite
side of the sight glass or at an angle of 90° clockwise from the suction inlet with view on the top.
Figure 23: Preliminary oil dilution chart for transient operation with R452B and R454B
5.13 Minimum run time
Emerson recommends a maximum of 10 starts per hour. There is no minimum off time because
scroll compressors start unloaded, even if the system has unbalanced pressures. The most critical
consideration is the minimum run time required to return oil to the compressor after start-up. To
establish the minimum run time, obtain a sample compressor equipped with a sight tube (available
from Emerson) and install it in a system with the longest connecting lines that are approved for the
system. The minimum on time becomes the time required for oil lost during compressor start-up to
return to the compressor sump and restore a minimal oil level that will ensure oil pick-up through the
crankshaft. Cycling the compressor for a shorter period than this, for instance to maintain very tight
temperature control, will result in progressive loss of oil and damage to the compressor.
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