Specific application recommendations
The use of a suction line accumulator is strongly
recommended in reversible cycle applications as
a result of 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 slugging 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
Apart from residual moisture in the system
after commissioning, water could also enter the
refrigeration circuit during operation. Water in
the system shall always be avoided. Not only
because it can shortly lead to electrical failure,
sludge in sump and corrosion but in particular
because it can cause serious safety risks.
Common causes for water leaks are corrosion and
freezing.
Corrosion
: Materials in the system shall be
compliant with water and protected against
corrosion.
Freezing
: When water freezes into ice its volume
expands which can damage heat exchanger
walls and cause leaks. During off periods water
inside heat exchangers could start freezing when
ambient temperature is lower than 0°C. During
on periods ice banking could occur when the
circuit is running continuously at too low load.
Both situations should be avoided by connecting
a pressure and thermostat switch in the safety
line.
Water utilizing systems
Discharge line, reversing
valve, solenoid valves
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).
Defrost and reverse cycle
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.
Each application design however should
be thoroughly tested to ensure acceptable
operating characteristics.
39
AB237986441643en-010801
Application Guidelines
Summary of Contents for R134a
Page 2: ......
