ASHRAE Journal - May 2020 - 75

REFRIGERATION APPLICATIONS
COLUMN

Andy Pearson

Getting the Best From
Zeotropes
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE

The two most recent columns in this series may have seemed unnecessarily gloomy
about zeotropic refrigerant blends. However, like most things in life that have unusual
features, there are ways in which their oddities can be used to advantage.
A zeotrope is a blend of refrigerant ﬂuids with differto separate alcohol (more volatile) from water (less volaent boiling points which behaves normally. In abnormal tile) in order to strengthen fermented liquor. When used
blends, also known as azeotropes, the components of
in refrigeration systems it's troublesome because the
the blend interfere with each other and the mixture
composition of the gas ﬂowing through the compressor is
behaves as if it were a single ﬂuid with the various comnow different from the composition in the evaporator.
ponents boiling in proportion to the original composiThe change in boiling point is known as "temperature
tion. The ingredients of zeotropes (the
glide" and the change in proportion of the
Bob loved his temperature glide.
normal ones) don't interfere with each
components is called "composition shift."
other. This results in one of two things,
These are the oddities that can be turned
depending on the circumstances.
to advantages when a zeotrope is used. If a
If the refrigerant is evaporated inside a
ﬂuid is being cooled through a large temtube during a ﬂow process then the presperature range, for example chilled water
sure will be relatively constant, subject
for a production process, then the temonly to the usual pressure drop required
perature glide can enable the compressor
to create the ﬂow. The boiling temperato run with a higher inlet pressure than
ture will drift as the more volatile comwould otherwise be possible. This is only
ponent of the blend boils more enthusiastically than its
possible if the ﬂuid ﬂow directions on either side of the
less vigorous companion, changing the composition of
evaporator can be arranged suitably. Unfortunately this
the liquid ﬂow. This distillation is not a problem for the
can sometimes make the evaporator more expensive, so
compressor provided the liquid is completely evapothere is a capital cost versus operating cost trade-off to
rated before it leaves the tube since the composition of
be considered.
the dry gas will be the same as the liquid inlet. Trouble
In more extreme cases a single circuit system with a far
comes if the liquid is not completely evaporated, which
higher range of boiling points could be used instead of
can happen if the control system regulating the ﬂow
a cascade plant to reach very low temperatures. This is
through the expansion valve doesn't take account of the
known as an "auto-cascade." Where there is a wide range
shift in boiling temperature of the liquid.
in air temperature, for example along the product path
When the liquid is in a pool and is boiling (imaginain an in-line tunnel freezer, a similar technique could be
tively called "pool boiling") the situation is different. The
used. As far as I am aware, this approach
more volatile component (the one which boils more eashas not been commercialized, perhaps
ily-recognized as the one with the lower boiling tempera- because the glide in commercially availture for a given pressure) will evaporate ﬁrst, leaving the
able zeotropes is not large enough to be https://bit.ly/2V8P31I
Rate this Column
less volatile component behind. This is the well-underworth it.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
stood, time-honored process used in whisky distillation
75

ASHRAE JOURNAL

ashrae.org

M AY 2020

https://www.bit.ly/2V8P31I https://www.ashrae.org/

ASHRAE Journal - May 2020

Table of Contents for the Digital Edition of ASHRAE Journal - May 2020