26
crankcase heaters
Most reversible cycle units are
located outdoors and operate
during conditions of low ambient
temperature. In these applications,
crankcase heaters are mandatory
because of the high probability
of liquid migrating back to the
compressor sump during off-cycles.
discharge temperature
thermostat
Heat pumps frequently use high
condensing temperatures in order to
achieve sufficient temperature rise
in heated medium. They also often
require low evaporator pressures in
order to obtain sufficient temperature
differentials between the evaporator
and the outside ambient temperature.
These can result in high discharge
temperature and so it is mandatory
that a discharge gas thermostat
be installed on the discharge line
to protect the compressor from
excessive temperatures. Operating the
compressor at discharge temperatures
that are too high can result in
mechanical compressor damage as
well as thermal degradation of the
lubricating oil and lack of sufficient
lubrication.
The discharge gas thermostat should
be set to shut down the compressor in
the event discharge gas temperature
rises above 275°F.
discharge line
and reversing valve
The Performer® scroll compressor is
a high volumetric machine and, as
such, can rapidly build up pressure
in the discharge line if gas in the line
becomes obstructed even for a very
short time. This situation may occur
with slow-acting reversing valves in
heat pumps. Discharge pressures
exceeding the operating envelope may
result in nuisance high pressure switch
cut-outs and may place excess strain
on both bearings and motor.
To prevent such problems, it is
important to allow a 3 ft minimum
discharge line length between the
compressor discharge port and
the reversing valve or any other
restriction. This gives sufficient free
volume for collection of discharge
gas and reduction of the pressure
peak during the time it takes for the
valve to operate. At the same time,
it is important to select and size a
4-way reversing valve so as to ensure a
quick switch over. This would prevent
from building up to a high discharge
pressure and the resulting nuisance
high pressure cut-outs.
Check with the valve manufacturer
for optimal sizing and recommended
mounting positions.
The use of a suction line accumulator
is strongly recommended in reversible
cycle applications due to the
possibility of a substantial quantity
of liquid refrigerant remaining in the
evaporator, which acts as a condenser
during the heating cycle.
This liquid refrigerant can then return
to the compressor, either flooding the
sump with refrigerant or as a dynamic
liquid slug when the cycle switches
back to a defrost cycle or to normal
cooling operations.
Sustained and repeated liquid slug-
ging and floodback can seriously
impair the oil’s ability to lubricate the
compressor bearings. This situation
can be observed in wet climates where
it is necessary to frequently defrost
the outdoor coil in an air source
heat pump. In such cases a suction
accumulator becomes mandatory.
suction line accumulator
speciFic application RecoMMendations
most important issues for common
applications. Each application design ,
though, should be thoroughly tested
to ensure acceptable operating
characteristics.
Summary of Contents for Performer SM Series
Page 39: ......
