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 5°C .
Raise the system pressure
equivalent to 20°C. When saturated
condensing temperature reaches
20°C then start the unit.
When all compressors are idle:
• Check in liquid line sight glass that
there is no liquid refrigerant transfer
• Oil superheat must be >10K during
off-cycle
After compressors has started:
• Oil superheat must remain >10K
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 -5°C 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 12 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
34
FRCC.PC.012.A7.02
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