Pecific Application Recommendations; Low Ambient Application; Scroll And Reciprocating - Danfoss MLM Series Application Manuallines

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PPLICATION UIDELINES

Low ambient application

Low ambient start-up
Low ambient operations

Scroll and reciprocating

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PECIFIC APPLICATION RECOMMENDATIONS

Under cold ambient conditions (<0°C), upon start-
up the pressure in the condenser may be so low
that a suffi cient pressure diff erential across the
expansion device cannot be developed to prop-
erly 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 com-
pressor be allowed to operate under vacuum. The
low-pressure control must be set in accordance
with the table on page 24 in order to prevent this
from happening.
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 fl ow into the
evaporator. An oversized valve may result in er-
ratic 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 suffi cient 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 avail-
able to prevent the risk of compressor to vacuum
and low pressure diff erential between the suction
and discharge pressures.
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 effi ciency
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.
FRCC.PC.015.A4.02
Early feeding of the evaporator and management
of the discharge pressure could help to attenuate
these eff ects.
Low pressure diff erentials 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 ef-
fect is most pronounced during low load condi-
tions, which frequently occur during 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. Variable speed fans
can also be used to control the condensing pres-
sure. In water-cooled units, the same can be per-
formed using a water regulator valve that is also
operated by head pressure, thereby ensuring that
the water valve does not open until the condens-
ing pressure reaches a satisfactory level.
The minimum condensing pressure must be set
at the minimum saturated condensing tempera-
ture shown in the application envelopes.
Under very low ambient conditions, in which test-
ing 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 ex-
tra 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.
When moving from a reciprocating compressor
to a scroll compressor, the selection shall always
be made based on cooling capacity at the appli-
cation rating point. Never make a selection based
on equivalent displacement.
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