ASHRAE Journal - July 2019 - 37

COLUMN REFRIGERATION APPLICATIONS
Andy Pearson

Part One

Be More Speciﬁc
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE

For years I have been hearing that coefﬁcient of performance (COP), sometimes
called energy efﬁciency ratio (EER), is the most important parameter in refrigeration
systems if you care about energy performance. These are the ratio of cooling effect (or
heating effect for heat pumps) to the energy input, often expressed as a dimensionless
ratio because both parameters can be measured in kW.
Sometimes it can be given as Btu per kWh or even,
reasons exist why a designer may want to keep the commore confusingly, hp per ton; horsepower being the
pressor discharge pressure relatively high, for example
shaft input to the compressor, so a measure of work
to ensure a good defrost, to make the oil cooling system
input and ton being the "ton of refrigeration," equal
work or to provide steady ﬂow through the expansion
to 12,000 Btu/h or 3.52 kW. Note that COP and EER are
valve. All of these constraints can be eliminated by good
heat divided by work, so a big number is a good thing,
system design, and it should be possible to allow the
whereas hp/ton is work divided by heat, so the inverse,
compressor to run to as low a discharge pressure as it
which confuses me no end.
can handle. Anything higher than that will indicate
An obvious problem with COP as a measure of system wasted energy.
"goodness" is that it doesn't say anything
A further difﬁculty, and one which has been
Be more specific.
about how well the system is achieving its
bothering me more and more in recent years,
mission, only about how effectively it does
is that the investment decision is based on
what it is doing. These might not seem so difstatements given about the operation of the
ferent, but consider that it is easy to improve
system when it is brand-new and squeakythe COP of a cold store system by leaving
clean. No consideration seems to be given to
all the store doors open. This will cause the
the way in which the plant ages or is mainroom temperature to rise and so the comtained. Many systems are installed without
pressors will operate with a higher suction pressure
much thought having been given to this topic, so, for
and will be "more efﬁcient." However, the room temexample, there is no way to clean the ﬁns of the evapoperature will no longer satisfy the design requirement, rator or condenser, no way to stop oil from fouling the
and the plant will probably run all day and all night
inside of the evaporator and no attempt to measure
(at least in summer), so the electricity bill will be very
performance degradation over time. Again, good system
high and the performance will be very unsatisfactory.
design can avoid these pitfalls, but despite the possibilA more subtle difﬁculty is that the COP gives a number ity they are all too often ignored.
for comparison under certain operating conditions. It's
When buying a refrigeration plant, purchasers should
very common when project bids are being compared to
be far more speciﬁc about their expectations if they
see emphasis placed on the performance of the plant
want to avoid long-term disappointment. Next month,
under the maximum expected ambient conditions.
in Part Two of this column, we will look at a better way to
Sometimes, buying decisions are based on the energy
judge cold store efﬁciency and consider the beneﬁts and
performance at that operating point alone. However, if
disadvantages of the "speciﬁc" approach.
the system is designed to run that way year-round, its
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
performance will again be very unsatisfactory. Many
J U LY 2 0 19

ashrae.org

ASHRAE JOURNAL

https://www.ashrae.org/

ASHRAE Journal - July 2019

Table of Contents for the Digital Edition of ASHRAE Journal - July 2019