Danfoss VZH088-G single User Manual

System design recommendations
VZH 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 below 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 eciency of the complete
refrigerant system, optimized R410A heat
exchangers must be used. R410A refrigerant has
good heat transfer properties: it is worthwhile
designing specic heat exchangers to gain in size
and eciency.
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 eciency at
high condensing pressure. In R410A systems
the positive eect of sub-cooling on system
eciency will be signicantly larger than in R22/
R407C systems.
Furthermore, for good operation of the
expansion device and to maintain good
eciency in the evaporator it is important to
have an adequate liquid sub-cooling. Without
adequate sub-cooling, ash gas will be formed at
the expansion device resulting in a high degree
of vapour at the evaporator inlet leading to low
eciency.
Heat exchangers
Model Refrigerant charge limit (kg)
VZH088 5.9
VZH117 7.9
VZH170 13.5
O-cycle migration Liquid refrigerant can nd its way into the
compressor by means of o-cycle migration or
liquid oodback during operation.
O-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 saturate the oil and lead to a ooded 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, and can also take much oil out
of compressor, then result in oil loss in sump.
This will be risky of bearing seizing especially
for system with large refrigerant and without
oil separator. 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 ooded start.
VZH scroll compressors can tolerate occasional
ooded starts as long as the total system charge
does not exceed the maximum compressor
refrigerant charge limit.
O-cycle migration can be prevented by
implementing a crankcase heating or adding a
pump-down cycle to the operation cycle and a
liquid line solenoid valve.
48 FRCC.PC.023.A8.02
Application Guidelines

Danfoss VZH088-G single Specifications

General

Brand	Danfoss
Model	VZH088-G single
Category	Air Compressor
Language	English