APPLICATION GUIDELINES
34
FRCC.PC.007.B5.02
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
off ers very little internal volume for the compres-
sor to draw vapour from on the suction side. The
compressor can then quickly enter into a vacuum
condition. It is therefore important that the ex-
pansion device be sized correctly and that a suf-
fi cient pressure diff erential across the expansion
device be available to ensure adequate refriger-
ant feed into the evaporator. This aspect is of
special concern when operating the unit under
low ambient and load conditions. For further in-
formation 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 re-
quired. 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 ex-
changer, a suffi cient free volume for the discharge
gas to accumulate is required in order to avoid
excess pressure build-up. At least 1 meter of dis-
charge 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 cool-
ing to heating, defrost or low-load short cycles.
These transient modes of operation may lead to
liquid refrigerant carry-over (or fl ood back) or
excessively wet refrigerant return conditions. As
such, reversible cycle applications require spe-
cifi c precautions for ensuring a long compressor
life and satisfactory operating characteristics.
Regardless of the refrigerant charge in the sys-
tem, specifi c tests for repetitive fl ood back are
required to confi rm whether or not a suction ac-
cumulator needs to be installed.
The following considerations cover the most im-
portant issues when dealing with common appli-
cations. Each application design however should
be thoroughly tested to ensure acceptable oper-
ating 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 re-
mote condenser installations. The sump heater
will minimize refrigerant migration caused by the
large temperature gradient between the com-
pressor 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 suffi cient
time to properly return to the compressor sumps
and that the motor has suffi cient time to cool un-
der conditions of lowest refrigerant mass fl ows.
The use of an electronic expansion valve requires
a specifi 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 ac-
cording 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 off ) by the use of a battery back up.
EXV Opened
Closed
Compressor On
Off
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