Famco Danfoss Maneurop MT, Руководство пользователя

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OPERATING ENVELOPES
Zeotropic refrigerant
mixtures
Dew point temperature
and
mean temperature
for R-407C
Phase shift
Temperature glide
Refrigerant mixtures can be either zeo-
tropic or azeotropic.
In a zeotropic mixture (like R-407C)
on the other hand the composition of
vapor and liquid changes during the
phase transition. When the effect of this
phase transition is very small, the mix-
ture is often called a near-azeotropic
mixture. R-404A is such a near-azeotro-
pic mixture.
An azeotropic mixture, on the other
hand, (like R-502 or R-507A) behaves
like a pure refrigerant. During a phase
transition (from vapor to liquid or from
liquid to vapor) the composition of va-
por and liquid stays the same.
The composition change causes phase
shift and temperature glide.
In parts of the system where both vapor
and liquid phase are present (evapora-
tor, condenser, liquid receiver), the pha-
ses do not have the same composition.
In fact both phases form two different
refrigerants. Therefore zeotropic refri-
gerants need some special attention.
Zeotropic refrigerants must always be
charged in liquid phase. Flooded eva-
porators and suction accumulators
should not be applied in systems with
zeotropic refrigerants. This also applies
to near-azeotropic mixtures.
During the evaporating process and the
condensing process at constant pressu-
re, the refrigerant temperature will de-
crease in the condenser and rise in the
evaporator. Therefore when speaking
about evaporating and condensing
temperatures, it is important to indicate
whether the temperature under discus-
sion is a dew point temperature or a
mean 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 on the saturated vapor line.
Points C and D are mean point values.
These are temperatures that corres-
pond more or less with the average
temperature during the evaporating
and condensing process. For the same
R-407C cycle, mean point tempe-
ratures are typically about 3.5°F to
5.5°F lower than dew point tempe-
ratures. In accordance with ASER-
COM recommendations, Danfoss
Commercial Compressors uses dew
point temperatures for selection
tables, application envelopes, etc.
To obtain exact capacity data at mean
point temperatures, the mean point
temperatures must be converted to
dew point temperatures, using refri-
gerant data tables from the refrigerant
manufacturer.