ASHRAE Journal - February 2020 - 72

COLUMN REFRIGERATION APPLICATIONS
Andy Pearson

The Art of Compromise
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE

When I was a small kid, whenever there was a discussion or disagreement about what
was happening, my mother would say "Well, we will compromise and do it my way." It
was years before I learned that this is not really what "compromise" means. I guess it's
a quote from somewhere but I've been unable to trace it.
is the norm. The change in gas temperature is sufﬁciently
Refrigeration is full of compromises. Even the basic
vapor compression cycle is less than ideal in order to
large for the cross-ﬂow element to be minimized, so again
make it practical. Everyone knows that the best that can
the compromise is accepted unchallenged in most cases.
be hoped for in extracting heat from a substance to cool it
When evaporators are used to provide superheated gas
below ambient temperature is the Carnot efﬁciency; the
to the compressor inlet for added reliability a further
ratio of heat extracted to work done under ideal condicompromise is required. An air-cooler is a pretty poor
tions. A Carnot cycle; however, would require isentropic
way of providing gas heating and often up to one third
compression of a two-phase ﬂuid, which is not really
of the total surface area is required for superheating the
feasible. The Perkins cycle (what we all use, sometimes
suction gas. What's even worse is that the only way to get
calling it the reverse-Rankine cycle) is a
enough temperature difference to provide
Not all compromise is sensible.
practical adaptation of Carnot, with the
the necessary gas heating is to lower the sucliquid phase evaporated to dryness so
tion temperature, making the plant even less
that the compressor is protected from
efﬁcient. If the refrigerant is a non-azeotrope
the effects of ingesting liquid, which is
then temperature glide adds insult to injury;
"incompressible." If we can't have isentropic compresthe suction pressure has to be even lower to get enough
sion of a gas-liquid mixture, then isothermal compressuperheat into the gas.
sion would be nice, enabling more efﬁcient operation.
Even more compromise is evident in the expansion
However, this too is sacriﬁced in order to provide a polyprocess. This was called the "internal hemorrhage" of the
tropic compressor that is both simple and reliable, hoperefrigeration cycle by Professor Gustav Lorentzen because
fully without losing too much efﬁciency.
it bleeds away efﬁciency without even being noticed.
Compromise abounds even before the refrigerant
Ideally, the Perkins cycle should use an expansion engine,
reaches the compressor. If evaporators were truly "councapable of handling two-phase ﬂow and with capacity
tercurrent," or counterﬂow, then better efﬁciency could
modulation to match the compressor in part-load operabe achieved and if non-azeotropic ﬂuids were used then
tion. In practice the expansion device is normally no
further advantage could be gained by taking the beneﬁt
more sophisticated than a hole in a metal disc. Yet again,
of the temperature glide on board. This would be like
efﬁcient operation is sacriﬁced to gain lower capital cost.
starting to climb the stairs from the third or fourth step.
The unusual physical properties of transcritical CO2 are
However, such evaporators, although theoretically posalso drawing attention to more efﬁcient expanders but
sible, would be bulky and expensive so usually a more
cost and complexity are still big challenges.
compact cross-ﬂow design is used. This has become
One of the most fascinating aspects of working in refrigmore important in recent years with the resurgence of
eration is the way in which the balance of factors that
transcritical CO2 systems because the gas-cooler would
drive these compromises is constantly shifting. That's
also beneﬁt from a counterﬂow design. In a few cases a
what keeps things interesting; long may it continue!
tube-in-tube arrangement can be used for water-cooled
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
gas-coolers, but usually for air-cooled designs cross-ﬂow
72

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