Specific application recommendations
Discharge temperature
thermostat
Heat pumps frequently utilize high condensing
temperatures in order to achieve a sufficient
temperature rise in the medium being heated. At
the same time, they often require low evaporator
pressures to obtain sufficient temperature
differentials between the evaporator and the
outside temperature. This situation may result
in high discharge temperature; as such, 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 too high discharge
temperatures can result in mechanical damage
to the compressor as well as thermal degradation
of the compressor lubricating oil and a lack of
sufficient lubrication.
The discharge gas thermostat should be set
to shut down the compressor in the event
discharge gas rises above 275°F.
Discharge line
and reversing valve, solenoid
valves
Defrost and reverse cycle
Sump heaters
Sump heaters are mandatory on reversible
cycle applications given the high probability of
liquid migration back to the compressor sump
during off-cycles due to the outdoor location of
most units and operations during low ambient
conditions.
The Danfoss SM/SY/SZ 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 period of time which situation may occur
with slow-acting reversing valves in heat pumps.
Discharge pressures exceeding the operating
envelope may result in nuisance high-pressure
switch cutouts and place excess strain on both
the bearings and motor.
To prevent such occurrences, it is important that
a 1-meter minimum discharge line length be
allowed between the compressor discharge port
and the reversing valve or any other restriction.
This gives sufficient free volume for the discharge
gas to collect and to reduce the pressure peak
during the time it takes for the valve to change
position. At the same time, it is important that
the selection and sizing of the reversing or 4-way
valve ensure that the valve switches quickly
enough to prevent against too high discharge
pressure and nuisance high-pressure cutouts.
Check with the valve manufacturer for optimal
sizing and recommended mounting positions.
In applications with heat recovery or condenser
partialisation, servo piloted solenoid valve has
to be properly sized or associated with a second
small valve in parallel, in order to avoid quick
discharge pressure drops when opening. This
phenomenon could lead to hammering effects
and create constraints on the non return valve
integrated in discharge fitting (SM/SY/SZ180 to
380).
The Danfoss SM/SY/SZ scroll compressor has the
ability to withstand a certain amount of liquid
refrigerant dynamic slug.
When compressors are installed in parallel,
in order to limit liquid amount handled per
compressor when beginning and ending defrost,
it is recommended to avoid running part load
(keep all compressors running or keep them
stopped when moving 4-way valves).
For further details, please refer to Parallel
application guidelines FRCC.PC.005.
EXV can also be opened when compressors are
stopped and before 4 way valve is moving in
order to decrease pressure difference. Opening
degree and time have to be set in order to keep
a minimum pressure difference for 4 way valve
moving.
Application guidelines
38
FRCC.PC.003.A5.22
