25
speciFic application RecoMMendations
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
low load conditions, implement the
following to ensure proper system
operating characteristics.
•
The superheat setting on the
expansion valve must be set to ensure
proper superheat operation during
low load periods. A minimum of
9
o
F stable superheat is required.
In addition, the refrigerant charge
should be sufficient to ensure proper
sub-cooling in the condenser to avoid
the risk of flashing in the liquid line
ahead of the expansion device. The
expansion device should be sized to
ensure proper control of 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 frequent
cycling of compressors. This can lead
to liquid refrigerant entering the
compressor if the expansion valve
does not provide stable refrigerant
superheat control under varying
loads.
•
Condenser fans cycling should ensure
that minimum pressure differential is
maintained between the suction and
discharge pressures. Variable-speed
fans can also be used to control the
amount of heat removed from the
condenser.
•
Compressors should be run for a
minimum period to ensure proper oil
return and also to allow sufficient time
to cool the motor under conditions of
lowest refrigerant mass flow.
Brazed plate
heat exchangers
Brazed plate heat exchange needs very
little internal volume to satisfy heat
transfer requirements. Consequently,
the heat exchanger offers very little
internal volume for the compressor to
draw vapor from on the suction side.
The compressor can then quickly enter
into a vacuum condition; it is therefore
important
that
the
expansion
device be sized correctly and that a
sufficient pressure differential across
the expansion device be available
to ensure adequate refrigerant feed
into the evaporator. This aspect is
of special concern when operating
the unit under low ambient and load
conditions. For further information on
these conditions, please refer to the
previous sections.
Due to the small volume of a brazed
plate heat exchanger, no pump-down
cycle is normally required. The suction
line running from the heat exchanger
to the compressor must be trapped
to avoid refrigerant migration to the
compressor.
When the condenser is a brazed
plate heat exchanger, a sufficient
free volume is required for the
accumulation of discharge gas in order
to avoid excess pressure buildup. At
least 3 ft of discharge line is necessary
to generate the free volume. To help
reduce the gas volume even further
immediately after start-up, the supply
of cooling water to the heat exchanger
may be opened before the compressor
starts up. This will remove
superheat,
and the incoming discharge gas will
condense more quickly.
Reversible heat pump
systems
Anytime during change over from
cooling to heating, or during
defrost or during low load short
cycling, there is a possibility of liquid
refrigerant
floodback.
Reversible
cycle applications therefore require
specific precautions for ensuring
long compressor life and satisfactory
operating characteristics. Regardless
of the system refrigerant charge,
specific tests for repetitive floodback
are required in order to determine
whether or not a suction accumulator
needs to be installed. A crankcase
heater and discharge gas thermostat
are required for reversible heat pump
applications.
The following considerations cover the
Summary of Contents for Performer SM Series
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