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Application guidelines
40
FRCC.PC.014.A4.22
Specific application recommendations
Heat pumps frequently utilize high condensing
temperatures in order to achieve a sufficient tem-
perature rise in the medium being heated. At the
same time, they often require low evaporating
pressures to obtain sufficient temperature differ-
entials between the evaporator and the outside
temperature. This situation may result in high
discharge temperature; as such, it is mandatory
that a discharge gas safety control is carried to
protect the compressor from exces
Discharge temperature
monitoring
A brazed plate heat exchanger needs very little
internal volume to satisfy the set of heat transfer
requirements. Consequently, the heat exchanger
offers very little internal volume for the com-
pressor to draw vapour 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 ex-
pansion 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 the 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 using a brazed plate condenser heat
exchanger, a sufficient free volume for the dis-
charge gas to accumulate is required in order to
avoid excess pressure build-up. At least 1 meter
of discharge line is necessary to generate this vol-
ume. To help reduce the gas volume immediately
after start-up even further, the supply of cooling
water to the heat exchanger may be opened
before the compressor starts up so as to remove
superheat and condense the incoming discharge
gas more quickly.
Because of the large compressor capacity varia-
tion and VSH capability to run at low condensing
temperature an EXV (electronic expansion valve)
is mandatory.
Transients are likely to occur in reversible heat
pump systems, i.e. a changeover cycle from cool-
ing to heating, defrost or low-load short cycles.
These transient modes of operation may lead
to liquid refrigerant carry-over (or flood-back)
or excessively wet refrigerant return conditions.
As such, reversible cycle applications require
specific precautions for ensuring a long com-
pressor life and satisfactory operating character-
istics. Regardless of the refrigerant charge in the
system, specific tests for repetitive flood-back
are required to confirm whether or not a suction
accumulator needs to be installed. The following
considerations cover the most important issues
when dealing with common applications. Each
application design however should be thor-
oughly tested to ensure acceptable operating
characteristics.
Brazed plate heat
exchangers
Reversible heat pump
systems
levels during low loading periods. 9 to 10.8°F
stable superheat is required. In addition, the
refrigerant charge should be sufficient to ensure
proper sub-cooling within the condenser so as to
avoid the risk of flashing in the liquid line before
the expansion device. 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 can lead to liquid refrigerant entering the
compressor if the expansion valve does not pro-
vide stable refrigerant super-heat control under
varying loads.
• Condenser fans should be cycled in such a
way that the minimum pressure differential is
maintained between the suction and discharge
pressures. Variable speed fans can also be used
to control the amount of heat to be removed
from the condenser.
• The compressors should be run for a minimum
period in order to ensure that the oil has suffi-
cient time to properly return to the compressor
sump and that the motor has sufficient time
to cool under conditions of lowest refrigerant
mass flows.
Refer to section “Oil return management
function”.