Application guidelines
54
FRCC.PC.023.A7.22
Single compressors
Specific application recommendations
Discharge line and reversing
valve
Defrost and reverse cycle
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 3.28 ft 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 during the defrost period.
When the compressor is started again, after
defrost, it will run at 25/30 rps for a 10 seconds
period. After this period it is recommended to
maintain the speed at 50 rps for 10 to 15 seconds.
Thus to avoid excessive liquid refrigerant to come
back to the compressor sump.
Defrost cycle logic
Start of defrost
sequence
End of defrost
sequence
Compressor on
Compressor off
4 way valve position 1
4 way valve position 2
10" 2"
10" 2"
In order to limit liquid amount handled by
the compressor when beginning & ending
defrost,below defrost cycle logic is suggested:
- stop the compressor before moving the 4 way
valve:
step 1: stop the compressor
step 2: wait for 10 seconds
step 3: move the 4 way valve
step 4: wait for 2 seconds
step 5: restart the compressor
Defrost cycle logic must respect all system
components recommendations, in particular 4
way valve Max. Operating Pressure Differential.
EXV can also be opened when the compressor is
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 for 4 way valve moving.
Danfoss recommend above defrost cycle logic,
but the control logic is also system specified.
