26
C6.2.38/0719/E
CAUTION
Vacuum operation! Compressor damage!
Compressor operation outside
the operating envelope is not allowed.
A pump-down cycle to control refrigerant migration may have to be used in conjunction with the
crankcase heating function when the compressor is located outside without any housing so that cold
air blowing over the compressor makes the crankcase heating function 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.
5.14 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, a sample compressor equipped with a sight tube can be ordered
from Emerson for oil return qualification and minimum oil level
– see
Chapter 2.6.3 "Oil filling 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.
5.15 Shut-off sound
Scroll compressors incorporate a device that minimizes reverse rotation. The residual momentary
reversal of the scrolls at shut off will cause a clicking sound, but it is entirely normal and has no effect
on compressor durability.
5.16 Oil level
YBVH* compressors are not equipped with any oil sight glass to ensure maximum hermeticity.
During the system development phase, adequate oil return in any operation should be checked
whatever the compressor model. For this purpose, a sample compressor equipped with a sight tube
can be ordered from Emerson for lab testing. Oil return check test recommendations are also
available on request from the Application Engineering department at Emerson.