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Application Guidelines
16
AB141686422149en-US0401
System design recommendations
Reversible heat pump
systems
Minimum sump superheat
Preventing liquid flood back
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. Specific tests for repetitive flood
back are required to confirm whether or not
special system design considerations should be
considered.
The minimum sump temperature is in the range
from 10°F to 30°F above saturated suction
temperature. Refer to the flood back test criteria.
Danfoss recommends the use of an electronic
expansion valve for all air conditioning and heat
pump applications with VRJ compressors. An
EXV 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.
Exceptions to the use of EXV’s with Danfoss
VRJ must be approved by Danfoss application
engineering.
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.
Refrigerant charge limit
0
5
10
15
20
25
30
35
40
-10
0
10
20
30
40
50
55
Acceptable
Unacceptable
Floodback requirement
Sump superheat, °F
Saturated suction temp. °F
Содержание VRJ
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