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If a pumpdown 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
f rom leaking rapidly into the low side after shut-off. The check valve will in some cases leak causing
the scroll compressor to recycle more f requently. Repeated short cycling of this nature can result in
a low oil situation and consequent damage to the compressor.
The hysteresis of the
low-pressure
control differential has to be reviewed since a relatively large volume of gas will re-expand f rom 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.
5.14 Minimum run time
Emerson recommends a maximum of 10 starts per hour. The most critical consideration is the
minimum run time required to return oil to the compressor af ter start -up. To establish the minimum
run time, a sample compressor equipped with
external oil sight glass
can be ordered from Emerson
f or lab testing. 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, f or instance to maintain very
tight temperature control, will result in progressive loss of oil and damage to the compressor.
5.15 Oil level
There is no oil sight glass on YHV*RG/RT compressors to ensure maximum hermeticity.
The oil return depends on the system design. During the system development phase, adequate oil
return in any operation should be checked whatever the compressor model. Oil return in continuous
operation and in on/off cycling has to be verified see also section 5.14 "Minimum run time". For
this purpose, a sample compressor for lab testing, equipped with an external oil sight tube, can be
ordered from Emerson.
If an oil separator is used the returning oil should be injected into the suction line 5 to 10 cm from the
compressor. Check with supplier whether the oil separator requires additional charge of oil. If it is
needed, pre-charge additional oil to the system accordingly.
Oil return check test recommendations are also available on request f rom the Application
Engineering department at Emerson.
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AGL_HP_VS_YHV_RG_RT_EN_Rev00
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