ASHRAE Journal - April 2020 - 90

REFRIGERATION APPLICATIONS
COLUMN

Andy Pearson

Mixed Messages on
Mixed Refrigerants
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE

he idea of mixing two ﬂuorocarbons to provide a
the composition percentages were not quoted and no
refrigerant that behaved like a single ﬂuid but with information was published about the dew and bubble
beneﬁcial properties was commercialized in the
point temperatures. By 2007 it had been removed from
1950s with the introduction of R-500, R-501 and R-502.
Standard 34 and ISO 817 followed suit in 2014.
ASHRAE Standard 34, which was originally published in
In the late 1980s the idea that refrigerant mixtures
1958, classiﬁed these three blends as "azeotropes" and
were a way of combining favorable properties to offset
considered them to be lower toxicity and non-ﬂammable. the basic disadvantage of non-constant boiling really
The idea of putting a mixture that did not form an
took hold. The term "blend" was adopted as preferred
azeotrope into a vapor compression system was slower
terminology for all 400 and 500 refrigerants at this time
to develop, although very early experiments in the ninebecause, it was said, this was more suggestive of an intenteenth century had used hydrocarbon mixtures like this
tion to achieve a speciﬁc desirable effect rather than the
to good effect. When chemists at the Kinetic Chemical
random happenstance implied by "mixture." Several
Company were looking for more applications for the
researchers looked at less common components to deliver
newly-developed CFCs in the 1930s they also tried nonspeciﬁc beneﬁts, for example small amounts of hydroazeotropic mixtures and even assigned them code numcarbon to improve oil miscibility or traces of lower presbers in the 400 range to differentiate
sure components to lower the overall
Double,
double
toil
and
trouble.
from other ﬂuids. However, when the
saturation pressure of the blend. This
main single component refrigerants for
resulted in intense scrutiny of the scivarious applications (R-11, R-12, R-13 and
ence of fractionation and a tidal wave of
R-22) had been identiﬁed and were being
new applications. In general, when they
commercialized, these other experimenwere used in direct expansion systems
tal oddities with their strangely ambivathese blends performed reasonably
lent boiling points were quietly forgotten.
consistently, provided the technician setBy 1978 ASHRAE Standard 34 had been
ting up the expansion valve superheat
around for twenty years and the committee, recognizing
knew to "aim off" to account for the effect of temperature
the theoretical concept of a non-constant boiling temglide. They were less successful in ﬂooded evaporators
perature mixture, added the 400 series to the standard
so have not been used much in industrial refrigeration
to denote mixtures, although no ﬂuids were listed in
or large chillers. By January 2019 one hundred and ﬁve
that category. Ten years later, when they met again, there
different blends were listed in the 400 series in Standard
were still no 400 series refrigerants in widespread com34, and more keep appearing. Paradoxically, this can be
mercial use. The committee felt uncomfortable in having
seen as a great success for this odd idea that was so slow
an empty class so they racked their brains for an example
to catch on, but at the same time a great
that could be used to ﬁll the gap. One of the commitfailure because the strenuous efforts of
tee mentioned that a mixture of CFC-12 and CFC-114
more than thirty years of global developwas used for air conditioning of operator's cabins in the
ment have not yet delivered a universally https://bit.ly/2T2iToK
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overhead cranes used in steel mills-a very arduous, high
acceptable replacement for CFCs.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
ambient application. This became R-400, but in general

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ASHRAE Journal - April 2020

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