ASHRAE Journal - August 2013 - 99

Column RefRigeRation applications

What’s Up With CO2?
By andy peaRson, ph.d., c.eng., MeMBeR ashRae

but they were relatively expensive compared to lower
pressure refrigerants, so they fell out of favor.
CO2 systems cooled by the surrounding air operate on
their high pressure side above the maximum pressure at
which the gas can be liquefied by cooling. This is a confusing thing and has led to a lot of latter-day mythology about
these so-called “transcritical” systems. In reality, they are
not so different from what we are used to; their major challenge is that the system capacity is greatly reduced on a hot
day, which is not just a bad thing, but bad timing, too.
However, this high pressure makes CO2 gas very dense,
which delivers the opportunity to do amazing things with
system design. Pushing the discharge pressure above the
condensing zone results in a temperature drop as the gas
cools, but on the low pressure side of the system the temCO2 contributes more than any other gas to global warm- perature remains constant as the liquid evaporates. This is
ing—this is clearly a bad thing. However, without global
a “best of both worlds” deal: glide on the heating side and
warming the earth would be uninhabitable, with an averno glide on the chiller side—allowing CO2 to be used in high
temperature heat pumps very effectively.
age surface temperature of –2°F (19°C).
King Harold couldn’t see why there was
High operating pressure delivers the
This is a very bad thing, so global warming
such a fuss about CO2.
good things about CO2—the small comis a good thing provided, as Ralph Waldo
pressor size, the low pressure loss in
Emerson said, we have “moderation in all
suction pipes and the good heat transthings, especially moderation.” So CO2 as a
substitute for high-GWP HFC refrigerant can
fer in heat exchangers, so it ought to
help reduce the additional global warming
be welcomed. The high density allows
that pushes the thermometer beyond the
crazy things to be done in the evaporator
tolerable band to which we have been accusdesign department, even for very low
tomed for at least the last thousand years.
temperature systems such as cooling to
CO2 systems have to run at very high pressure, typi–50°F (–45°C) without suffering large pressure drop.
cally eight to 10 times higher than an average ammonia
I have never yet been able to design a CO2 evaporator
that suffered from the circuits being too long, despite
system, for them to be cooled by the surrounding air.
trying several times. The circuits are almost impossible
Equipment needs to be designed to contain this presto overload, especially in chill applications. As we push
sure safely; this is clearly a good but expensive thing.
Many people seem to be put off exploring the possibility toward making system efficiency better, this radical feaof using CO2 for this reason, so it has a bad consequence. ture of CO2 will be crucial to the success of new system
However, those who persevere and spend the little extra designs. Unfortunately, system designers and particuit takes to accommodate higher operating pressures
larly evaporator designers are following old rules for old
have learned that there are lots of reasons to like CO2—
refrigerants and are not capitalizing on this advantage.
more of this good thing later. One hundred years ago
This is a bad thing, but it is not irredeemable.
CO2 systems were commonplace, often capable of operAndy Pearson, Ph.D., C.Eng., is group engineering director at Star Refrigeration in
Glasgow, U.K.
ating at pressures up to 1,500 psig (over 100 bar gauge),

There has probably been more written
about possible applications for CO2 as a
refrigerant in the last 20 years than about
any other refrigerant at any time in the
history of mechanical refrigeration.
The classic text “1066 And All That,” a
spoof history of England, characterizes all
history as either “a good thing” or “a bad
thing,” or occasionally “a very good thing”
or “a very bad” thing. Here is the same
technique used to analyze the prospects
for CO2 refrigeration.

Au g ust 2013

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