Danfoss MLM Series Application Manuallines page 27

Application Guidelines
Zeotropic refrigerant
mixtures
Phase shift
Temperature glide
PVE
Alkylbenzene oil
Operating conditions
R407A / R407F are zeotropic mixture (like R407C).
The composition of vapour and liquid changes
during the phase transition. The composition
In system components where both vapour and
liquid phases are present (evaporator, condenser,
liquid receiver), the liquid phase and vapour
phase do not have the same composition. In
fact both phases form two different refrigerants.
During the evaporating and the condensing
process at constant pressure, the refrigerant
temperature will decrease in the condenser
and rise in the evaporator. Therefore when
speaking about evaporating and condensing
temperatures, it is important to indicate whether
this is a dew point temperature or a mid point
value. In the figure below, the dotted lines are
lines of constant temperature.
They do not correspond to the lines of constant
pressure.
Points A and B are dew point values. These are
temperatures on the saturated vapour line.
Points C and D are mid point values. These
are temperatures which correspond more or
less with the average temperature during the
evaporating and condensing process.
Polyvinyl ether (PVE) is an innovative refrigeration
lubricant for HFC refrigerant systems. PVE is as
hygroscopic as existing polyolester lubricants
(POE), but PVE doesn't chemically react with
water; no acids are formed and compressor
evacuation is easier.
As PVE can be mixed with POE, oil top up can be
done with up to 25% POE.
Alkylbenzene oil can be applied in systems
using HCFC refrigerants (R22). Compared to
a mineral oil it provides distinct advantages:
excellent miscibility, excellent thermal stability,
compatibility with mineral oils and constant
quality.
change causes phase shift and temperature
glide.
Therefore zeotropic refrigerants need some
special attention. Zeotropic refrigerants must
always be charged in liquid phase. Flooded
evaporators should not be applied in systems
with zeotropic refrigerants.
For the same R407A/F cycle, mid point
temperatures are typically about 2°C lower
than dew point temperatures. According to
EN12900 recommendations, Danfoss Commercial
Compressors use dew point temperatures for
selection tables and application envelopes etc.
The compressor technology applied in MLZ
compressors in combination with PVE lubricant
provides the best possible result in terms of
reliability and compressor lifetime.
The PVE lubricant is compatible with R22 which
makes the MLZ compressors a very versatile
multi- refrigerant solution. Very high care has
to be taken for vacuum as PVE is much more
hygroscopic than alkylbenzene or mineral oil.
MLM series compressors are charged with
Alkylbenzene oil and herewith offer an
economically interesting alternative to the
MLZ series in regions where R22 is still the
predominant refrigerant. Note however that
MLM compressors can not be used with HFC
refrigerants.
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FRCC.PC.015.A8.02