Refrigerant Charge Limit; Reversible Heat Pump Systems - Danfoss HHP Series Application Manuallines

Application Guidelines

Refrigerant charge limit

Packaged units
System with remote heat exchanger
Recommended
REC
Note: for special conditions such as low ambient temperature, low load operation or brazed plate heat exchangers please refer to corresponding sections
Reversible heat pump
systems
Split unit heating mode
20
FRCC.PC.017.A2.02
System design recommendations
Danfoss Heat Pump scroll compressors can
tolerate liquid refrigerant up to a certain extend
without major problems. However, excessive
liquid refrigerant in the compressor is always
unfavourable for service life. Besides, the
installation cooling capacity may be reduced
because of the evaporation taking place in the
compressor and/or the suction line instead of the
evaporator. System design must be such that the
amount of liquid refrigerant in the compressor
is limited. In this respect, follow the guidelines
given in the section: "Essential piping design
recommendations" in priority.
Depending on test results, crankcase heaters, Liquid Line Solenoid Valve or suction accumulator must
be applied see below.
BELOW charge limit
 No test or additional safeties required
Off cycle migration test
REC
Required

REQ
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 carryover (or flood back)
or excessively wet refrigerant return conditions. As
such, reversible cycle applications require specific
precautions for ensuring a long compressor
life and satisfactory operating characteristics.
Regardless of the refrigerant charge in the
system, specific tests for repetitive flood back
are required to confirm whether or not a suction
Repeat the test, but with the system in heating
mode and the outdoor temperature at -18°C
dry bulb. If the sump superheat is not in the
"ACCEPTABLE ZONE" shown in the Flood back
Use the tables below to quickly evaluate the
required compressor protection in relation
with the system charge and the application.
More detailed information can be found in the
paragraphs hereafter. Please contact Danfoss for
any deviation from these guidelines.
Notes: for reversible heat pump systems and
other specific applications, please refer to section
"Specific application recommendations".
HHP015-019-021-026
HHP030-038-045
REQ
REQ
REQ
REQ
No test or additional safeties required
accumulator needs to be installed. A crankcase
heater and discharge gas thermostat are required
for reversible heat pump applications.
These considerations cover the most important
issues in the realm of common applications.
Each application design however should be
thoroughly tested to ensure acceptable operating
characteristics.
Requirement graph on the next page, a suction
accumulator is required.
Compressor models Refrigerant charge limit (kg)
ABOVE charge limit
Off cycle migration test
Liquid flood back test
Off cycle migration test
Liquid flood back test
3.6
5.4