ASHRAE Journal - April 2013 - 67

refrigeration applications

Sit Back, Enjoy the Glide
By Andy Pearson, Ph.D., C.Eng., Member ASHRAE
This article is the fourteenth in a series exploring refrigeration and heat pump
concepts without using jargon

L

ast month, we thought a bit about the effect water has on ammonia
when they are mixed, and it was noted that ammonia refrigeration

systems have been known to carry on working even when as much as
25% of the liquid in the system is water. The effect of the water is to
move the boiling point of the ammonia, so that in an evaporator the
temperature shifts as the liquid turns to gas.

This effect is known as “glide.” It is get a large enough temperature difference
tolerable in an evaporator where the outlet to do the superheating is by lowering the
is intended to be a mixture of liquid and evaporating temperature, but that’s another
gas, for example, in a pumped circulation story (see “The Temperature Lift” in March
system. But, it can be disastrous if the 2012). There are two drawbacks to this apflow control is dependent on superheat proach. First, it assumes that the pressure is
produced at the end of the evaporator.
the same all the way through the evaporator.
The thermostatic expansion valve— Second, it assumes that the temperature is
from the Greek words “thermos,” mean- the same all the way through the evaporator.
ing “hot” (if you don’t
In fact, there needs to be a
drop it*) and “statos,”
little bit of pressure drop to
meaning “standing,” so
make the refrigerant move
“thermostatic” = “keeps
in the right direction, but
it hot”—or its modern
this is not usually enough to
electronic equivalent
cause a problem. The boiling
measures the inlet prestemperature of the refrigerant
sure and the outlet temusually follows the pressure
perature of the evaporator
(see “Temperatures, Pressures,
and adjusts the refriger- *This joke is courtesy of Flan- and Refrigerants” in April
ant flow rate to make sure ders and Swann, English mu- 2012), and so it drops a little
there is only gas in the sic hall comedians of the bit as the pressure reduces. A
evaporator outlet. This is 1950s, from whom I learned traditional mechanically opergood news for the com- all the important bits of ated thermostatic valve can
pressor, which doesn’t thermodynamics.
usually control steadily if there
suffer from unsquashable liquid in the is at least 10 degrees F (5.5 K) superheat. A
place where gas squashing takes place, good electronic valve can manage to keep
and also good news for the evaporator steady at about half this level.
manufacturer who gets to sell a larger
If the refrigerant is actually a cocktail
evaporator to provide the necessary heat of two or more chemicals then sometimes
exchange surface to do this superheating. the boiling temperature does not follow
It is not such good news for system the pressure, but slides from one condiefficiency, particularly if the only way to tion to another as the fluid boils. This is
April 2013

called “glide” and can make your head
hurt. Glide happens because the components of the mixture evaporate at different
rates, so the proportions of the cocktail
change. The pressure drops slightly on
the way through the evaporator but the
boiling temperature rises a lot. If a standard mechanical valve is set to maintain
10 degrees F (5.5 K) difference between
the boiling point at the coil inlet and the
actual temperature at the coil outlet, this
might not be enough to allow the valve
to do its stable control thing. The result
is unsquashable liquid in the compressor.
For example, a coil with liquid entering
at –15°F (–26°C) that has a pressure drop
equivalent to 1 degrees F (0.5 K) and is
set to control the superheat to 10 degrees
F (5.5 K) will have an outlet temperature
of –5°F (–21°C) and the superheat will
actually be 11 degrees F (6 K) on normal
refrigerant. If there is also an 8 degree F
(4.4 K) glide in the refrigerant because
it is a cocktail, then the actual superheat
at the coil outlet will only be 3 degrees F
(1.6 K) and a mechanical expansion valve
will not be able to control accurately. If
the pressure reading is taken at the coil
outlet and a “true” superheat value is
being measured, as is the case with some
electronic valves, then it will work OK,
but with mechanical valves this is not
possible due to the way the valve cleverly
balances internal pressures to vary the
amount of opening.
In principle it is possible to use “glide” to
your advantage, provided the heat exchanger can be designed to give maximum benefit
from the inlet temperature to the stuff being
cooled. This would allow the compressor to
operate at a higher inlet pressure, but this is
tricky in practice, so it’s not normally done,
even with “wide glide” refrigerants.

Andy Pearson, Ph.D., C.Eng., is group
engineering director at Star Refrigeration in Glasgow, U.K.
ASHRAE Journal

67



ASHRAE Journal - April 2013

Table of Contents for the Digital Edition of ASHRAE Journal - April 2013

ASHRAE Journal - April 2013
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Ground-Coupled Heat Pump and Energy Storage
Fan Efficiency Requirements for Standard 90.1-2013
Technology Award Case Studies:
Geothermal for 5 Ecosystems
Holistic HVAC Design
Standing Columns
Engineer’s Notebook
Data Centers
Emerging Technologies
IAQ Applications
Refrigeration Applications
Special Products
People
International Column
Energy Modeling
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - April 2013 - ASHRAE Journal - April 2013
ASHRAE Journal - April 2013 - Cover2
ASHRAE Journal - April 2013 - 1
ASHRAE Journal - April 2013 - 2
ASHRAE Journal - April 2013 - Contents
ASHRAE Journal - April 2013 - Commentary
ASHRAE Journal - April 2013 - 5
ASHRAE Journal - April 2013 - Industry News
ASHRAE Journal - April 2013 - 7
ASHRAE Journal - April 2013 - 8
ASHRAE Journal - April 2013 - Letters
ASHRAE Journal - April 2013 - Meetings and Shows
ASHRAE Journal - April 2013 - 11
ASHRAE Journal - April 2013 - 12
ASHRAE Journal - April 2013 - 13
ASHRAE Journal - April 2013 - Ground-Coupled Heat Pump and Energy Storage
ASHRAE Journal - April 2013 - 15
ASHRAE Journal - April 2013 - 16
ASHRAE Journal - April 2013 - I1
ASHRAE Journal - April 2013 - I2
ASHRAE Journal - April 2013 - I3
ASHRAE Journal - April 2013 - I4
ASHRAE Journal - April 2013 - 17
ASHRAE Journal - April 2013 - 18
ASHRAE Journal - April 2013 - 19
ASHRAE Journal - April 2013 - 20
ASHRAE Journal - April 2013 - 21
ASHRAE Journal - April 2013 - 22
ASHRAE Journal - April 2013 - 23
ASHRAE Journal - April 2013 - Fan Efficiency Requirements for Standard 90.1-2013
ASHRAE Journal - April 2013 - 25
ASHRAE Journal - April 2013 - 26
ASHRAE Journal - April 2013 - 27
ASHRAE Journal - April 2013 - 28
ASHRAE Journal - April 2013 - 29
ASHRAE Journal - April 2013 - 30
ASHRAE Journal - April 2013 - 31
ASHRAE Journal - April 2013 - Geothermal for 5 Ecosystems
ASHRAE Journal - April 2013 - 33
ASHRAE Journal - April 2013 - 34
ASHRAE Journal - April 2013 - 35
ASHRAE Journal - April 2013 - 36
ASHRAE Journal - April 2013 - 37
ASHRAE Journal - April 2013 - 38
ASHRAE Journal - April 2013 - 39
ASHRAE Journal - April 2013 - Holistic HVAC Design
ASHRAE Journal - April 2013 - 41
ASHRAE Journal - April 2013 - 42
ASHRAE Journal - April 2013 - 43
ASHRAE Journal - April 2013 - 44
ASHRAE Journal - April 2013 - 45
ASHRAE Journal - April 2013 - 46
ASHRAE Journal - April 2013 - 47
ASHRAE Journal - April 2013 - Engineer’s Notebook
ASHRAE Journal - April 2013 - 49
ASHRAE Journal - April 2013 - 50
ASHRAE Journal - April 2013 - 51
ASHRAE Journal - April 2013 - 52
ASHRAE Journal - April 2013 - 53
ASHRAE Journal - April 2013 - Data Centers
ASHRAE Journal - April 2013 - 55
ASHRAE Journal - April 2013 - 56
ASHRAE Journal - April 2013 - 57
ASHRAE Journal - April 2013 - 58
ASHRAE Journal - April 2013 - 59
ASHRAE Journal - April 2013 - Emerging Technologies
ASHRAE Journal - April 2013 - 61
ASHRAE Journal - April 2013 - 62
ASHRAE Journal - April 2013 - 63
ASHRAE Journal - April 2013 - IAQ Applications
ASHRAE Journal - April 2013 - 65
ASHRAE Journal - April 2013 - 66
ASHRAE Journal - April 2013 - Refrigeration Applications
ASHRAE Journal - April 2013 - Special Products
ASHRAE Journal - April 2013 - People
ASHRAE Journal - April 2013 - International Column
ASHRAE Journal - April 2013 - 71
ASHRAE Journal - April 2013 - Energy Modeling
ASHRAE Journal - April 2013 - 73
ASHRAE Journal - April 2013 - Products
ASHRAE Journal - April 2013 - 75
ASHRAE Journal - April 2013 - 76
ASHRAE Journal - April 2013 - 77
ASHRAE Journal - April 2013 - 78
ASHRAE Journal - April 2013 - Classified Advertising
ASHRAE Journal - April 2013 - Advertisers Index
ASHRAE Journal - April 2013 - Cover3
ASHRAE Journal - April 2013 - Cover4
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