A
PPLICATION
GUIDELINES
S
PECIFIC
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 pro-
vide 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
10°F stable superheat is required.
Head pressure control under low ambient con-
ditions:
Several possible solutions are available to
prevent the risk of compressor to vacuum and low
pressure differential between the suction and dis-
charge 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 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 temperature
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.
Low ambient start-up
Under cold ambient conditions (<30°F), 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 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.
Early feeding of the evaporator and management
of the discharge pressure could help to attenuate
these effects.
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 ef-
fect is most pronounced during low load condi-
tions, 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 appli-
cation rating point. Never make a selection based
on equivalent displacement.
Scroll and reciprocating
29
FRCC.PC.016.A1.22
