F1
KM
HP
LPS
DGT
M
MPM
S
SSH
CB
Fuses
Compressor contactor
High pressure safety switch
Safety pressure switch
Discharge gas thermistor (embedded in compressor)
Compressor motor
Motor Protection Module
Thermistor chain (motor and discharge temperature)
Surface sump heater
Thermal magnetic motor circuit breaker
Specific application recommendations
Low ambient application
Low ambient start-up
Under cold ambient conditions (<0°C), upon start-up the pressure in the condenser may be so low that a sufficient
pressure differential across the expansion device cannot be developed to properly feed the evaporator.
As a result, the compressor may go into a deep vacuum, which can lead to compressor failure due to internal arcing
and instability in the scroll wraps. Under no circumstances should the compressor be allowed to operate under
vacuum. The low-pressure control must be set in accordance with the table section “Low pressure” in order to
prevent this from happening. Early feeding of the evaporator and management of the discharge pressure could
help to attenuate these effects.
Low pressure differentials can also cause the expansion device to «hunt» erratically, which might cause surging
conditions within the evaporator, with liquid spillover into the compressor. This effect is most pronounced during
low load conditions, which frequently occur during low ambient conditions.
Low ambient operations
The Danfoss CH scroll compressor requires a minimum pressure differential of 6 to 7 bar between the suction and
discharge pressures to force the orbiting scroll down against the oil film on the thrust bearing. Anything less than
this differential and the orbiting scroll can lift up, causing a metal-to-metal contact. It is therefore necessary to
maintain sufficient discharge pressure in order to ensure this pressure differential. Care should be taken during low
ambient operations when heat removal from air-cooled condensers is greatest and head pressure control may be
required for low ambient temperature applications. Operation under low pressure differential may be observed by a
significant increase in the sound power level generated by the compressor.
It is recommended that the unit be tested and monitored at minimum load and low ambient conditions as well. The
following considerations should be taken into account to ensure proper system operating characteristics.
Expansion device
The expansion device should be sized to ensure proper control of the refrigerant flow into the evaporator. An
oversized valve may result in erratic control. This can lead to liquid refrigerant entering the compressor if the
expansion valve does not provide stable refrigerant super-heat control under varying loads. The superheat setting
of the expansion device should be sufficient to ensure proper superheat levels during low loading periods. A
minimum of 5 K stable superheat is required.
Head pressure control under low ambient conditions
Several possible solutions are available to prevent the risk of compressor to vacuum and low pressure differential
between the suction and discharge pressures.
Scroll compressors, CH290 and CH485 | Application
© Danfoss | Climate Solutions | 2022.09
AB236986442743en-000501 | 36
