Application guidelines
52
FRCC.PC.023.A7.22
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. Refer to crankcase
heating section “Crankcase heating function” for
details and settings.
Low ambient compressor
operations
Low ambient operations and
minimum pressure differential
at steady running conditions
The VZH compressor requires a minimum
pressure differential of 87 to 101 psi 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.
Low load operations
It is recommended that the unit be 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.
