System design recommendations
Cycle rate limit
R
No more than 12 starts per hour (6 when a
soft start accessory is used) are allowed. A higher
number would reduce the service life of the
motor-compressor unit. If necessary, use an anti-
short-cycle timer within the control circuit.
The system must be designed in a way that
guarantees minimum compressor running time
so as to provide sufficient motor cooling after
start-up as well as proper oil return from the
system to the compressor.
A 5-minute delay between two successive
compressor starts is being proposed herein,
with a 2-minute runtime after each start and a
3-minute idle time between each stop and start.
Only during the pump-down cycle may the
compressor run for much shorter intervals.
Liquid floodback during
operation
Off-cycle migration
Liquid refrigerant control
and charge limits
During normal and stable system operation,
refrigerant will leave the evaporator in a
superheated condition and enter the compressor
as a superheated vapour. Normal superheat
values at compressor suction are 5 to 30 K.
However the refrigerant leaving the evaporator
can contain an amount of liquid refrigerant due
to different reasons:
• wrong dimensioning, wrong setting or
malfunction of expansion device
• evaporator fan failure or frosted-up evaporator
coils.
In these situations, liquid refrigerant will
continuously enter the compressor. The negative
effects from continuous liquid floodback are:
• permanent oil dilution
• in extreme situations with high system
refrigerant charge and large amounts of
floodback, liquid slugging could occur.
During system standstill and after pressure
equalisation, refrigerant will condensate in
the coldest part of the system which may be
the compressor when it is placed in a cold
environment. Ultimately, the full system
refrigerant charge can condensate in the
compressor crankcase. A large amount will
dissolve in the compressor oil until the oil is
completely saturated with refrigerant. When
the compressor is started, the pressure in the
crankcase decreases rapidly and refrigerant
will violently evaporate, causing the oil to foam
(boiling). Both dilution and foaming reduce
the lubrication properties of the oil. In extreme
situations liquid could enter the compressor
cylinders with immediate compressor break-
down as a result.
Refrigeration compressors are basically
designed as gas compressors. Depending on the
compressor design and operating conditions,
most compressors can also handle a limited
amount of liquid refrigerant. Maneurop® NTZ
compressors have a large internal volume and
can therefore handle relatively large amounts of
liquid refrigerant without major problems.
However even when a compressor can handle
liquid refrigerant, this will not be favourable to
its service life.
Liquid refrigerant will dilute the oil, wash out the
bearings causing wear and eventually seizure.
Furthermore, high oil carry over will cause lack of
oil in the sump.
Good system design can limit the amount of
liquid refrigerant in the compressor, which will
have a positive effect on the compressor service
life.
R
Liquid refrigerant can enter a compressor
in various ways, with different effects on the
compressor as described in the following points.
With R454C/R455A, liquid migration to the
compressor must be avoided by maintaining
adequate superheat setting of min. 8-10K.
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AB245486497018en-000601
Application Guidelines
