Application guidelines
39
FRCC.PC.014.A4.22
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 ex-
pansion 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 ex-
pansion 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 condi-
tions, 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 regula-
tor 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 rec-
ommended to improve any system efficiency. The
most accurate value is to control the condensing
temperature at 22°F above the ambient tempera-
ture for air cooled condensers.
For further information, please contact Danfoss
Technical support.
A crankcase heating will minimize refrigerant mi-
gration 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 VSH compressor requires a minimum
pressure differential of 87 to 102 psig 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 met-
al-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
