A
PPLICATION
GUIDELINES
FRCC.PC.007.B5.02
33
S
PECIFIC
APPLICATION
RECOMMENDATIONS
The Performer® SH scroll compressor requires a
minimum pressure diff erential of 6 to 7 bar be-
tween the suction and discharge pressures to
force the orbiting scroll down against the oil fi lm
on the thrust bearing. Anything less than this dif-
ferential and the orbiting scroll can lift up, causing
a metal-to-metal contact. It is therefore necessary
to maintain suffi
cient discharge pressure in order
to ensure this pressure diff erential. Care should
be taken during low ambient operations when
heat removal from air-cooled condensers is great-
est and head pressure control may be required for
low ambient temperature applications. Operation
under low pressure diff erential may be observed
by a signifi cant increase in the sound power level
generated by the compressor.
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.
Expansion device
: The expansion device should
be sized to ensure proper control of the refriger-
ant fl ow 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 expan-
sion 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.
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.
Low ambient start-up
Low ambient application
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 section "Low pressure" in order to
prevent this from happening.
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.
Low ambient operations
