Application guidelines
37
FRCC.PC.014.A4.22
System design recommendations
VSH 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.
If the refrigerant charge exceeds the values in be-
low table, a suction line accumulator is strongly
recommended.
More detailed information can be found in the
paragraphs hereafter. Please contact Danfoss
technical support for any deviation from these
guidelines.
Refrigerant charge limits
To obtain optimum efficiency of the complete
refrigerant system, optimized R410A heat
exchangers must be used. R410A refrigerant has
good heat transfer properties: it is worthwhile
designing specific heat exchangers to gain in size
and efficiency.
An evaporator with optimized R410A distributor
and circuit will give correct superheat at outlet
and optimal use of the exchange surface. This is
critical for plate evaporators that have generally
a shorter circuit and a lower volume than shell &
tubes and air cooled coils.
For all evaporator types a special care is required
for superheat control leaving the evaporator and
oil return.
A sub-cooler circuit in the condenser that creates
high sub-cooling will increase efficiency at high
condensing pressure. In R410A systems the posi-
tive effect of sub-cooling on system efficiency will
be significantly larger than in R22/R407C systems.
Furthermore, for good operation of the expan-
sion device and to maintain good efficiency in
the evaporator it is important to have a high
degree of liquid sub-cooling. Without adequate
sub-cooling, flash gas will be formed at the
expansion device resulting in a high degree of
vapour at the evaporator inlet leading to low
efficiency.
Heat exchangers
Model
Refrigerant charge limit (lb)
VSH088
13
VSH117
17
VSH170
30
Liquid refrigerant can find its way into the compressor by means of off-cycle migration or liquid flood-
back during operation.
Off-cycle migration
Off-cycle refrigerant migration is likely to occur
when the compressor is located at the coldest
part of the installation, when the system uses
a bleed-type expansion device, or if liquid is
allowed to migrate from the evaporator into the
compressor sump by gravity. If too much liquid
refrigerant accumulates in the sump it will satu-
rate the oil and lead to a flooded start: when the
compressor starts running again, the refrigerant
evaporates abruptly under the sudden decrease
of the bottom shell pressure, causing the oil to
foam. In extreme situations, this might result
in liquid slugging (liquid entering the scroll
elements), which must be avoided as it causes
irreversible damage to the compressor.
The presence of liquid in the crankcase can be
easily detected by checking the sump level
through the oil sight glass. Foam in the oil sump
indicates a flooded start.
VSH scroll compressors can tolerate occasional
flooded starts as long as the total system charge
does not exceed the maximum compressor
refrigerant charge.
Off-cycle migration can be prevented by imple-
menting a crankcase heating or adding a pump-
down cycle to the operation cycle and a liquid
line solenoid valve.
