© Danfoss | Climate Solutions | 2022.03
Sump heater
Surface Sump heater (SSH)
System design recommendations
Liquid line solenoid valve
(LLSV): This feature is
very convenient and can be used on all types of
applications.
An LLSV is used to isolate the liquid charge in the
high pressure side, thereby preventing against
charge transfer or excessive migration to the
compressor during off-cycles. The quantity of
refrigerant remaining in the low-pressure side
of the system can be further reduced by using a
pump-down cycle in association with the LLSV.
The surface sump heaters are designed to protect
the compressor against off-cycle migration of
refrigerant.
When the compressor is idle, the oil temperature
in the sump of the compressor must be
maintained at no lower than 10 K above the
saturation temperature of the refrigerant on the
low-pressure side. This requirement ensures that
the liquid refrigerant is not accumulating in the
sump. A sump heater is only effective if capable
of sustaining this level of temperature difference.
Tests must be conducted to ensure that the
appropriate oil temperature is maintained under
all ambient conditions (temperature and wind).
Note that below -5°C ambient temperature
and a wind speed of above 5 m/second, we
recommend that the heaters be thermally
insulated in order to limit the surrounding energy
losses. Since the total system charge may be
undefined, a sump heater is recommended on
all stand-alone compressors and split systems.
In addition, any system containing a refrigerant
charge in excess of the maximum recommended
system charge for compressors requires a sump
heater.
At initial start-up or after power shortage, it is
recommended to energize surface sump heater
to remove refrigerant 6 hours in advance. A
quicker start-up is possible by "jogging" the
compressor to evacuate refrigerant in the
compressor. Start the compressor for 1 second,
then wait for 1 to 2 minutes. This operation must
be repeated for each compressor individually:
on
off
on
off
on
off
Compressor 1
Compressor 2
Compressor 3
After 3 or 4 jogs the compressor can be started.
The heater must be energized whenever the
compressor is off to avoid liquid refrigerant
entering the compressor.
Provide separate electrical supply for the heaters
so that they remain energized even when
the machine is out of service (e.g. seasonal
shutdown).
Surface sump heater accessories are available
from Danfoss (see section "Accessories").
The surface sump heaters are designed to
protect the compressor against off-cycle
migration of refrigerant.
Additional heater power or thermal insulation
around compressor might be needed in case
of ambient temperature below -5°C and a wind
speed above 5m/second.
R
MLM/MLZ090-240 model Extra SSH is
necessary when ambiance extra SSH is necessary
when ambiance is below -13°C and it should be
located on the middle shell next to oil sump.
The heater must be turned on whenever all the
compressors are off.
Surface sump heater accessories are available
from Danfoss.
1
2
Model
T ambiance SSH Location
MLM/MLZ
090-130-160-200-240
-13~-5
80W
1
MLM/MLZ
090-130-160-200-240
-13~-23
80W
1+2
29
AB243586442698en-000601
Application Guidelines
Single compressors
