Low Load Operations; Brazed Plate Heat Exchangers; Water Utilising Systems - Danfoss MLM Series Application Manuallines

A G
PPLICATION UIDELINES

Low load operations

Brazed plate
heat exchangers

Water utilising systems

32
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PECIFIC APPLICATION RECOMMENDATIONS
The compressor should be run for a minimum pe-
riod to ensure that the oil has suffi cient time to
properly return to the compressor sump and that
A brazed plate heat exchanger needs very little in-
ternal volume to satisfy the heat transfer require-
ments. Consequently, the heat exchanger off ers
very little internal volume for the compressor to
draw vapour from the suction side. The compres-
sor can then quickly enter into a vacuum condi-
tion. It is therefore important that the expansion
device be sized correctly and that a suffi cient
pressure diff erential across the expansion device
be available to ensure adequate refrigerant feed
into the evaporator. This aspect is of special con-
cern when operating the unit under low ambi-
ent and load conditions. For further information
on these conditions, please refer to the previous
sections.
Apart from residual moisture in the system after
commissioning, water could also enter the re-
frigeration circuit during operation. Water in the
system shall always be avoided. Not only because
it can shortly lead to electrical failure, sludge in
sump and corrosion but in particular because it
can cause serious safety risks.
Common causes for water leaks are corrosion and
freezing.
FRCC.PC.015.A4.02
the motor receives enough cooling under condi-
tions of lowest refrigerant mass fl ow.
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 discharge gas volume imme-
diately after start-up, the supply of cooling water
to the heat exchanger may be opened before the
compressor starts, to remove superheat and con-
dense the incoming discharge gas more quickly.
Corrosion: Materials in the system shall be compli-
ant with water and protected against corrosion.
Freezing: When water freezes into ice its volume
expands which can damage heat exchanger walls
and cause leaks. During off periods water inside
heat exchangers could start freezing when ambi-
ent temperature is lower than 0°C. During on pe-
riods ice banking could occur when the circuit is
running continuously at too low load. Both situa-
tions should be avoided by connecting a pressure
and thermostat switch in the safety line.