Danfoss VZH088-G single User Manual

Specic 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 sucient pressure dierential 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 dierentials can also cause the
expansion device to “hunt” erratically, which
might cause surging conditions within the
evaporator, with liquid spillover into the
compressor. This eect 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 o 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 eciency.
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.
Surface sump or belt type crankcase heaters can
be used, see section "Accessory". They must be
connected to the CDS303 relay 2.
Low ambient compressor
operations
Low ambient operations and
minimum pressure
dierential at steady
running conditions
The VZH compressor requires a minimum
pressure dierential between the suction and
discharge pressures (please refer to operation
envelop) to force the orbiting scroll-down against
the oil lm on the thrust bearing. Anything less
than this dierential and the orbiting scroll
can lift up, causing a metal-to-metal contact.
It is therefore necessary to maintain sucient
discharge pressure in order to ensure this
pressure dierential. 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 dierential may be observed
by a signicant 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 sucient to ensure proper superheat
levels during low loading periods. 5 to 6 K stable
superheat is required. In addition, the refrigerant
charge should be sucient to ensure proper
sub-cooling within the condenser so as to avoid
the risk of ashing in the liquid line before
the expansion device. The expansion device
should be sized to ensure proper control of the
refrigerant ow into the evaporator.
50 FRCC.PC.023.A8.02
Application Guidelines

Danfoss VZH088-G single Specifications

General

Brand	Danfoss
Model	VZH088-G single
Category	Air Compressor
Language	English