Specific application recommendations
Low ambient application
Low ambient operations
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.
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 consideration is especially
important in manifolded units where low load
conditions may require the frequent cycling of
compressors. 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.
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. Variable speed
fans can also be used to control the condensing
pressure. 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.
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 head pressure
control valve is recommended. Note: This
solution requires extra refrigerant charge, which
can introduce other problems. A non-return
valve in the discharge line is recommended and
special care should be taken when designing the
discharge line.
For further information, please contact Danfoss.
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 “High and low
pressure protection” 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.
Unlike the reciprocating compressor, a scroll
doesn’t have dead volume. Neither does it have a
suction valve causing pressure drop. As a result a
scroll compressor has a high volumetric efficiency
even at low suction pressure. In systems such
as ice makers and milk cooling tanks this high
capacity at low temperature shortens the cooling
time.
When moving from a reciprocating compressor
to a scroll compressor, the selection shall always
be made based on cooling capacity at the
application rating point. Never make a selection
based on equivalent displacement.
Scroll and reciprocating
38
FRCC.PC.015.A8.02
Application Guidelines
