Single compressors
Specific application recommendations
Discharge line and reversing
valve
Defrost and reverse cycle
Suction line accumulator
Water utilizing systems
The VZH 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 can generate excessive load on 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.
It is strongly recommended to reduce the
compressor speed to 25/30 rps before the 4-way
valve is moved from a position to another.
Refer also to high and low pressure protection.
After the 4-way valve is moved to defrost
position, and in order to shorten the defrost
period, the compressor speed can be maintained
at 70 rps or higher speed during the defrost
period.
When the compressor is started again each time,
compressor will run at 50 rps for at least 1 minute.
Thus to avoid excessive liquid refrigerant to come
back to the compressor sump.
The use of a suction line accumulator is strongly
recommended in reversible-cycle applications.
This because 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.
Apart from residual moisture in the system
after commissioning, water could also enter the
refrigeration circuit during operation. Water in
the system should always be avoided. Not only
because it can quickly 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 safe to
use 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.
34
FRCC.PC.034.A2.02
Application Guidelines
