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35AB218186428184en-US1501
GENERAL INFORMATIONPRODUCT INFORMATIONINTEGRATION INTO SYSTEMORDERING INFORMATION SYSTEM DESIGN
Test, criteria and
solutions
Test N° Purpose Test condition Pass criteria Solutions
Migration test
Check that
there is no
migration of
refrigerant into
the compressor
(either liquid
or vapour
condensating)
Energize CCH*.
Stabilize the non-running system at
a pressure equivalent to 41°F .
Raise the system pressure
equivalent to 68°F. When saturated
condensing temperature reaches
68°F then start the unit.
When all compressors are idle:
• Check in liquid line sight glass that
there is no liquid refrigerant transfer
• Refer to flood back chart criteria
After compressors has started:
• Oil superheat must remain the
criteria refer to flood back chart.
1. Check bulb position, tightness of
expansion device,
2. add LLSV**
3. add pump down cycle***
4. Check Crank Case Heater efficiency
* Crank case heater (CCH)
The Crank case heaters are designed to protect
the compressor against off-cycle migration of
refrigerant.
Additional heater power or thermal insulation
might be needed in case of ambient temperature
below 23°F and a wind speed above 5m/second.
The heater must be energized whenever all the
compressors are off.
Crank case heater accessories are available from
Danfoss (see section “Accessories”).
Provide separate electrical supply for the
heaters so that they remain energized even
when the machine is out of service (e.g. seasonal
shutdown).
It’s recommended that the heater be turned on
for a minimum of 8 hours prior to starting the
compressor.
**Liquid line solenoid valve (LLSV)
A LLSV is used to isolate the liquid charge
on the condenser side, thereby preventing
against charge transfer to the compressor
during off -cycles. The quantity of refrigerant
on the low-pressure side of the system can be
further reduced by using a pump-down cycle in
association with the LLSV.
***Pump-down cycle
By decreasing pressure in the sump, pump down:
• evacuates refrigerant from oil
• set the sump saturating pressure much
lower than ambiance temperature and due
to that, avoid refrigerant condensation in the
compressor.
For more details on pump-down cycle see
section “Control Logic”.
Oil temperature sensor must be placed between
oil sight glass and compressor baseplate and be
insulated.
Optimum
location area
Manage off cycle migration
On/off cycling (cycle
rate limit)
Danfoss recommends a restart delay timer to
limit compressor cycling. The timer prevents
reverse compressor rotation, which may occur
during brief power interruptions.
The system must be designed in a way that
guarantees a minimum compressor running
time of 2 minutes so as to provide for sufficient
motor cooling after start-up along with proper
oil return. Note that the oil return may vary since
it depends upon system design.
There must be no more than 12 starts per hour,
a number higher than 12 reduces the service life
of the motor-compressor unit. A three-minute
(180-sec) time out is recommended.