ASHRAE Journal - March 2023 - 64

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Economize!
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
The original " free lunch " was served in western saloons in the nineteenth century to
anyone who had bought at least one drink. The food was plentiful and without charge,
but tended to be salt beef, salted ham, salty peanuts and so on. Hence the phrase,
popularized by Milton Friedman in the 1970s but at least 50 years older than that,
" there's no such thing as a free lunch. "
Economizers fitted to refrigeration plants seem like a
free lunch. More cooling capacity from the same size of
compressor; what's not to like? I am a fan of the concept,
but without a clear understanding of what's going on
it is easy to make expensive mistakes. Some of the high
pressure liquid coming out of the condenser is separated
from the main flow and expanded to an intermediate
pressure to be used to refrigerate the remaining
liquid. This achieves a greater cooling effect by the main
flow when it is expanded and evaporated and increases
the capacity of the compressor (although
it handles the same suction volume
with or without an economizer). This
technique is regularly used with screw
compressors, which can be equipped
with a side port to enable the intermediate
pressure gas coming from the economizer
to be added to the compressor
Lunch was thirsty work.
flow along the compression path. This means that less
work is required to compress that portion of the total
compressor flow which increases efficiency.
The economizer can be " open flash, " meaning that
all of the condenser outlet is expanded to intermediate
pressure in a receiver and the resulting flash gas is
diverted to the economizer port, or " closed flash, " meaning
that only the economizer port flow is expanded
initially and is used in a heat exchanger to refrigerate
the high-pressure liquid stream. There are merits and
disadvantages with each system, but the concept is often
thought to be only applicable to screw compressors.
However, the same principle applied to reciprocating
compressors has been patented several times over
the last 120 years and delivers the same capacity and
efficiency benefits. It's often called a Voorhees system,
after G.T. Voorhees, who was granted a patent on a
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ASHRAE JOURNAL ashrae.o rg M A R CH 2023
ported reciprocating compressor in 1905. An equivalent
arrangement was patented in the early 1920s by the
Haslam Company in England for carbon dioxide compressors
discharging at or above the critical pressure.
My father was granted a patent (now expired) on a
similar concept where the inlet of one of the cylinders
of a multicylinder reciprocating compressor was connected
to the intermediate pressure source and the rest
were connected to the suction chamber. This allowed
the economizer gas to be compressed in parallel with
the main gas flow, with the two gas streams
only meeting in the discharge gallery of the
compressor. The concept, named " parallel
compression, " was intended for use in
single compressor transcritical CO2 systems
such as heat pump water heaters. It didn't
take off in the single compressor variant
but the name stuck and the technique is
now frequently applied to multiple compressor systems
with one or more machines on " economizer duty " and
the rest on the main stream. A quirk of this arrangement
is that the cooling effect can be increased by up
to 20% when the suction mass flow is reduced by 25%
because the extra subcooling of the transcritical gas
cooler outlet is so useful.
However, this is not really a free lunch. The capacity
is increased but so is the system complexity, with the
requirement for additional heat exchangers, additional
control valves and additional pipework. Rising energy
costs might make it easier to justify that capital expenditure,
but the complexity also makes it harder to monitor
performance and ensure that it is paying back fast
enough.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
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ASHRAE Journal - March 2023

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