System design recommendations
Crankcase heater
Crankcase heaters provide extra compressor
protection, and should be considered for all
applications. For initial installation of precharged
systems and for any extended power interruptions,
the crankcase heater should be energized for 24
hours prior to compressor startup.
The minimum sump temperature is in the
range from 6K to 17K above saturated suction
temperature. Refer to the flood back test criteria
section "System design recommendations".
Minimum sump
superheat
Loss of charge protection
Danfoss Heat Pump scroll compressors do not
include a thermal valve protection; therefore,
all applications should consider loss of charge
protection :
• A low pressure switch in the low pressure side of
the system is required.
• A discharge line thermostat set no higher than
140°C is recommended. The thermostat must
be a manual lockout type device (or electrical
lockout circuit) and be located within 150 mm
of the compressor discharge connection. The
discharge line thermostat must be insulated to
insure proper sensing and operation.
In installations with good oil return and line
runs up to 15 m, no additional oil is required. If
installation lines exceed 15 m, additional oil may
be needed. 1 or 2% of the total system refrigerant
charge (in weight) can be used to roughly define
the required oil top-up quantity.
Always use oil from new cans.
Top-up the oil while the compressor is idle. Use
any accessible connector on the compressor
suction line and a suitable pump.
Oil level checking and
top-up
Danfoss Heat Pump scroll compressors are
machines with fixed volume ratio, and operate
more efficiently near the design pressure ratio. In
the extreme, do not exceed a 11:1 pressure ratio
(absolute discharge pressure to absolute suction
pressure) for extended periods. The Danfoss Heat
Pump scroll compressor is equipped with an
internal pressure relief valve for protection against
blocked condenser and fan failure conditions.
High pressure ratio
Danfoss recommends the use of a thermostatic
expansion valve for all air conditioning and heat
pump applications. A TXV has two key benefits:
it provides modulating control of the system
under varying load conditions, and it protects
the compressors from flood back during adverse
running conditions.
Excessive liquid refrigerant flood back during
steady state operation is a major system design
consideration for all types of compressors. Oil
dilution that occurs with excessive flood back
can have a significant adverse effect on bearing
reliability. Suction accumulators may be required
in some applications to prevent flood back.
When the use of fixed orifice devices is specified
in the system design, and when a TXV is applied
at the limit of its control range, the following tests
should be conducted to determine if a suction
accumulator is needed. Refer to the flowcharts
section "System design recommendations" to
determine when to apply the excessive liquid
flood back test.
Preventing liquid flood
back
Testing for excessive
liquid flood back
Water utilising systems
Apart from residual moisture in the system
after commissioning, water could also enter the
refrigeration 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.
Corrosion: Materials in the system shall be
compliant 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
ambient temperature is lower than 0°C. During on
periods ice banking could occur when the circuit
is running continuously at too low load. Both
situations should be avoided by connecting a
pressure and thermostat switch in the safety line.
22
FRCC.PC.017.A2.02
Application Guidelines
