ASHRAE Journal - July 2021 - 55

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Slow, Slow,
Quick-Quick, Slow
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
I've been thinking a lot about different forms of compression and the ways they could
be used to advantage in refrigeration systems. I don't recommend you follow me down
this rabbit-hole; it is confusing and misleading. You may be wondering what the
rhythms of the fox-trot have to do with compressors. There are four thermodynamic
processes for compressing a gas; three are slow and one is quick-quick.
The quick process is the one that we all use every day
and take for granted. Known as " isentropic, " or sometimes
" adiabatic, " compression, it is characterized by
a lack of heat transfer during compression. Adiabatic
means no transfer of heat and isentropic means no
change of entropy, so all isentropic processes are adiabatic
but not all adiabatic processes are isentropic.
Almost all the compressors used in vapor compression
refrigeration systems, from single piston
machines, through scrolls and screws
to huge centrifugal machines are of this
type. However, the high speed introduces
frictional losses and other inefficiencies so
there has been a resurgence of interest in
the other processes in recent years.
Fred was impressed by Ginger's
grasp of thermodynamic processes.
This usually means using isothermal compression,
where the pressure increases but the temperature
doesn't change. This is achieved by allowing heat to flow
out of the compression chamber (which takes time)
while the volume is being reduced and the pressure
raised. An oil-injected screw compressor looks like it
could be isothermal since the heat of compression is
removed through the oil cooling circuit. However, the
additional power required to compress the injected
oil (or the injected liquid if liquid-injected oil cooling
is used) offsets the advantage of operating at a lower
discharge temperature and the power input required
is best calculated by ignoring the oil cooling and then
adjusting the discharge temperature after the power has
been calculated. True isothermal compression would be
achieved by compressing the gas inside a special heat
exchanger and allowing time for the heat to transfer.
This is likely to be bulky, expensive and slow.
Other slow processes are even odder. Isochoric compression
occurs when the pressure rises, achieved by
heat input, but the volume doesn't change, so no work
is done on the gas. This sounds far-fetched, but it is
very common in static situations like when a closed
cylinder bursts after being heated under extreme high
pressure. It is more difficult to visualize
what an isochoric flow process would look
like, but that is one of the four steps in a
Stirling engine. Gas is passed through a heat
exchanger without the volume changing. As
heat is added the pressure goes up and the
volume remains the same.
Isochoric compression seems normal in comparison
to the fourth of the quartet of processes; isobaric
compression. This means compression without a
change of pressure. It seems, at first sight, to be crazy
talk to speak of compression at constant pressure,
but again there are numerous examples of static processes
using this process. This is what makes hot air
balloons float; the pressure inside and outside the
canopy is the same but the density inside is less (in
other words the specific volume is greater than the
cold air that it displaces) and so the balloon is buoyant.
An isobaric flow process is even harder to envisage,
but could lead to a step change in efficiency if it
can be achieved economically.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
J U LY 2021 ashrae.org ASHRAE JOURNAL
55

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