33
FRCC.PC.007.B8.02
Application guidelines
Specific application recommendations
Brazed plate
heat exchangers
Electronic expansion
valve
A brazed plate heat exchanger needs very little
internal volume to satisfy the set of heat transfer
requirements. Consequently, the heat exchanger
o ers very little internal volume for the
compressor 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 is sized correctly and
that a su cient pressure di erential across the
expansion device is 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 su cient free volume for the
discharge 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 volume. 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.
Reversible heat pump
systems
Transients are likely to occur in reversible heat
pump systems, i.e. a changeover cycle from
cooling to heating, defrost or low-load short
cycles. These transient modes of operation
may lead to liquid refrigerant carry-over (or
ood back) or excessively wet refrigerant return
conditions. As such, reversible cycle applications
require speci c precautions for ensuring a long
compressor life and satisfactory operating
characteristics. Regardless of the refrigerant
charge in the system, speci c tests for repetitive
ood back are required to con rm 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 thoroughly tested to ensure
acceptable operating characteristics.
Sump heaters
Low load operation
Sump heaters are strongly recommended on
all systems where the compressor is exposed to
low ambient temperatures, especially split and
remote condenser installations. The sump heater
will minimize refrigerant migration caused by
the large temperature gradient between the
compressor and the remainder of the system,
please refer to section "Accessories".
The compressors should be run for a
minimum period in order to ensure that the
oil has su cient time to properly return to the
compressor sumps and that the motor has
su cient time to cool under conditions of lowest
refrigerant mass ows.
The use of an electronic expansion valve requires
a speci c compressor start / stop control.
A specific compressor start sequence control has
to be set when an electronic expansion valve
(EXV) is used. The sequence must be adjusted
according to the EXV step motor speed to allow
time for the EXV to open before the compressor
starts to avoid running under vacuum conditions.
The EXV should be closed at compressor stop
not to let refrigerant in liquid phase entering the
compressor. Ensure that the EXV closes when the
supply voltage to the controller is interrupted (ie
power cut o ) by the use of a battery back up.
EXV Opened
Closed
Compressor On
O
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