Single compressors
Specific application recommendations
Low ambient start-up
Head pressure control under
low ambient conditions
Crankcase heaters
Under cold ambient conditions, 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 abnormal low suction
pressure, which can lead to compressor failure.
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 “Pressure settings” in order to
prevent this from happening.
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.
Several possible solutions are available to prevent
the compressor from drawing down to a vacuum
upon start-up under low ambient conditions.
In air-cooled machines, cycling the fans with
a head pressure controller will ensure that the
fans remain off until the condensing pressure
has reached a satisfactory level. In water-cooled
units, the same can be performed using a water
regulator valve that is also operated by head
pressure, thereby ensuring that the water valve
does not open until the condensing pressure
reaches a satisfactory level.
Note: The minimum condensing pressure must
be set at the minimum saturated condensing
temperature shown in the application envelopes.
Under very low ambient conditions, in which
testing has revealed that the above procedures
might not ensure satisfactory condensing and
suction pressures, the use of a liquid receiver
with condenser and receiver pressure regulators
would be possible.
Condensing pressure control is also strongly
recommended to improve any system efficiency.
The most accurate value is to control the
condensing temperature at 12 K above the
ambient temperature for air cooled condensers.
For further information, please contact Danfoss
Technical support.
A crankcase heating will minimize refrigerant
migration caused by the large temperature
gradient between the compressor and the
remainder of the system.
Belt type crankcase heaters can be used, see
section "Accessory". They can be connected to
CDS803 relay 1 or 2.
Low ambient compressor
operations
Low load operations
It is recommended that the unit is tested and
monitored at minimum load and, if possible,
during low ambient conditions as well. During
conditions of low load on the system, the
following considerations should be taken into
account to ensure proper system operating
characteristics.
• The superheat setting of the expansion
device should be sufficient to ensure proper
superheat levels during low loading periods. 5
to 6 K stable superheat is required. In addition,
the refrigerant charge should be sufficient
to ensure proper sub-cooling within the
condenser so as to avoid the risk of flashing in
the liquid line before the expansion device. The
expansion device should be sized to ensure
proper control of the refrigerant flow into the
evaporator.
32
FRCC.PC.034.A2.02
Application Guidelines
