Ref-L01
Refrigerants
©Wanchai 30Oct2009
Introduction1
Introduction Ref-L01
The thermodynamic efficiency of a
refrigeration system depends mainly on its
operating
ti temperatures.
t t However,
H
important practical issues such as the
system design, size, initial and operating
costs, safety, reliability, and serviceability
etc. depend very much on the type of
refrigerant selected for a given
application.
application
©Wanchai 30Oct2009
Introduction2
Refrigerant selection criteria: Ref-L01
Selection of refrigerant for a particular
application
pp is based on the followingg
requirements:
i. Thermodynamic and thermo-physical
properties
ii. Environmental and safety properties, and
iii. Economics
©Wanchai 30Oct2009
Introduction3
Thermodynamic and thermo-physical properties:Ref-L01
a)) Suction ppressure: At a ggiven evaporator
p
temperature, the saturation pressure should
be above atmospheric for prevention of air
or moisture ingress into the system and ease
of leak detection. Higher suction pressure is
better as it leads to smaller compressor
displacement
©Wanchai 30Oct2009
Introduction4
Thermodynamic and thermo-physical properties:Ref-L01
b)) Discharge
g pressure:
p At a ggiven condenser
temperature, the discharge pressure should
be as small as possible to allow light-
weight construction of compressor,
condenser etc.
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Introduction5
Thermodynamic and thermo-physical properties:Ref-L01
c)) Pressure ratio:
ratio Should be as small as
possible for high volumetric efficiency and
low power consumption.
d) Latent heat of vaporization: Should be as
large as possible so that the required mass
fl rate
flow t per unitit cooling
li capacity
it will
ill be
b
small
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Introduction6
Thermodynamic and thermo-physical properties:Ref-L01
e) Isentropic index of compression: Should
be as small as p
possible so that the
temperature rise during compression will be
small
f) Liquid specific heat: Should be small so
that degree of subcooling will be large
leading to smaller amount
amo nt of flash gas at
evaporator inlet
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Introduction7
Thermodynamic and thermo-physical properties:Ref-L01
g) Vapour specific heat:
heat Should be large so
that the degree of superheating will be small
h) Thermal conductivity:
conductivity Thermal
conductivity in both liquid as well as vapour
phase should be high for higher heat
transfer coefficients
i) Viscosity:
Viscosity Viscosity should be small in both
liquid and vapour phases for smaller
frictional pressure drops
©Wanchai 30Oct2009
Introduction8
Thermodynamic and thermo-physical properties:Ref-L01
The thermodynamic properties are interrelated
and mainly depend on normal boiling
point,
i critical
i i l temperature, molecular
l l
weight and structure.
©Wanchai 30Oct2009
Introduction9
Thermodynamic and thermo-physical properties:Ref-L01
The normal boiling point indicates the useful
temperature levels as it is directly related to
the
h operating
i pressures.
A high critical temperature yields higher
COP due to smaller compressor superheat
and smaller flash gas losses. On the other
hand since the vapour
p ppressure will be low
when critical temperature is high, the
volumetric capacity will be lower for
refrigerants with high critical temperatures.
©Wanchai 30Oct2009
Introduction10
Thermodynamic and thermo-physical properties:Ref-L01
This once again shows a need for trade-
trade-off
between high COP and high volumetric
capacity.
capacityi It is
i observed
b d that
h for
f most off the
h
refrigerants the ratio of normal boiling
point to critical temperature is in the range
of 0.6 to 0.7. Thus the normal boiling point
is a ggood indicator of the critical
temperature of the refrigerant.
©Wanchai 30Oct2009
Introduction11
Thermodynamic and thermo-physical properties:Ref-L01
The important properties such as latent heat of
vaporization and specific heat depend on the
molecular
l l weighti h andd structure off the
h molecule.
l l
A small value of vapour specific heat indicates
higher degree of superheat. A large value of
vapour specific heat results in a higher value of
liquid
q specific
p heat,, leadingg to higher
g flash gas
g
losses. Studies show that in general the optimum
value of molar vapour specific heat lies in the
range of 40 to 100 kJ/kmol.K.
©Wanchai 30Oct2009
Introduction12
Thermodynamic and thermo-physical properties:Ref-L01
The freezing point of the refrigerant should be
lower than the lowest operating temperature of
the
h cyclel to prevent blockage
bl k off refrigerant
fi
pipelines.
©Wanchai 30Oct2009
Introduction13
Ref-L01
Environmental and safety properties
a) Ozone Depletion Potential (ODP): the Montreal
protocol => phased-out (e.g. R 11, R 12) in near-
f
future(e.g.
( R22).
22) Si
Since O
ODP ddepends
d mainly
i l on
the presence of chlorine or bromine in the
molecules, refrigerants having either chlorine
(i.e., CFCs and HCFCs) or bromine cannot be
used under the new regulations
g
©Wanchai 30Oct2009
Introduction14
Ref-L01
Environmental and safety properties
b) Global Warming Potential (GWP): Refrigerants
should have as low a GWP value as possible to
minimize
i i i the
h problem
bl off global
l b l warming.
i
Refrigerants with zero ODP but a high value of
GWP (e.g. R134a) are likely to be regulated in
future.
©Wanchai 30Oct2009
Introduction15
Ref-L01
Environmental and safety properties
c) Total Equivalent Warming Index (TEWI): The
factor TEWI considers both direct (due to
release
l into
i atmosphere)
h ) andd indirect
i di (through
(h h
energy consumption) contributions of
refrigerants to global warming. Naturally,
refrigerants with as a low a value of TEWI are
preferable from gglobal warming
p g ppoint of view.
©Wanchai 30Oct2009
Introduction16
Ref-L01
Environmental and safety properties
d) Toxicity: Ideally, refrigerants used in a
refrigeration system should be nontoxic. In
generall the
h degree
d off hazard
h d depends
d d on:
- Amount of refrigerant used vs total space
- Type of occupancy
- Presence of open flames
- Odor of refrigerant, and
- Maintenance condition
©Wanchai 30Oct2009
Introduction17
Ref-L01
Environmental and safety properties
e) Flammability: The refrigerants should
preferably be non-flammable and nonexplosive.
AS A hhas di
ASHRAE divided
id d refrigerants
fi into
i six
i safety
f
groups (A1 to A3 and B1 to B3). Refrigerants
belonging to Group A1 (e.g. R11, R12, R22,
R134a, R744, R718) are least hazardous, while
refrigerants
g belonging
g g to Group p B3 ((e.g.
g R1140))
are most hazardous
©Wanchai 30Oct2009
Introduction18
Ref-L01
Environmental and safety properties
f) Chemical stability: The refrigerants should be
chemically stable as long as they are inside the
refrigeration
fi i system.
g) Compatibility with common materials of
construction (both metals and nonmetals)
h) Miscibility with lubricating oils: Oil separators
have to be used if the refrigerant is not miscible
with lubricating oil (e.g. ammonia). Precautions
should be taken to ensure oil return to the
compressor.
©Wanchai 30Oct2009
Introduction19
Ref-L01
Environmental and safety properties
i) Dilelectric strength: This is an important
property for systems using hermetic compressors.
For these
h systems theh refrigerants
fi should
h ld have
h as
high a dielectric strength as possible
j) Ease of leak detection: In the event of leakage of
refrigerant from the system, it should be easy to
detect the leaks.
©Wanchai 30Oct2009
Introduction20
Ref-L01
Economic properties:
The refrigerant used should preferably be
inexpensive and easily available.
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Introduction21
Ref-L01
Classification of fluids used as refrigerants
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Introduction22
Ref-L01
Designation of refrigerants
Halocarbon compounds The halocarbon group
includes refrigerants which contain one or more
of the three halogens chlorine, fluorine, and
bromine. All the refrigerants are designated by R
followed by a unique number.These refrigerants
are designated by R XYZ, where:
X+1 indicates the number of Carbon (C) atoms
Y 1 indicates number of Hydrogen (H) atoms
Y-1 atoms, and
Z indicates number of Fluorine (F) atoms
The balance indicates the number of Chlorine atoms. Only
2 digits indicates that the value of X is zero.
©Wanchai 30Oct2009
Introduction23
Ref-L01
Halocarbon refrigerants
©Wanchai 30Oct2009
Introduction24
Ref-L01
Refrigerants
Inorganic compounds Many of the early refrigerants were
inorganic compounds, and some have maintained their prominence
to this day.
©Wanchai 30Oct2009
Introduction25
Ref-L01
Refrigerants
Hydrocarbons Many hydrocarbons are suitable as
refrigerants, especially for service in the petroleum and
petrochemical
t h i l industry.
i d t
©Wanchai 30Oct2009
Introduction26
Ref-L01
Refrigerants
Azeotropes An azeotropic mixture of two substances
is one which cannot be separated into its components
b di
by distillation.
till ti An A azeotrope
t evaporates
t andd
condenses as a single substance with properties that
are different from those of either constituent. The
most popular azeotrope is refrigerant 502, which is a
mixture of 48.8 percent refrigerant 22 and 51.2
percent refrigerant 115.
©Wanchai 30Oct2009
Introduction27
Ref-L01
Thermodynamic comparison of some common refrigerants
(ammonia)
COP of a Carnot cycle is 5.74
©Wanchai 30Oct2009
Introduction28
Physical and chemical comparison Ref-L01
Two important characteristics of refrigerants from a safety
standpoint are its flammability and toxicity.
Ammonia is listed as flammable with 16 to 25 percent
ammonia by volume in air, while the others are considered
nonflammable.
R-12 is considered nontoxic in concentrations up to 20
percent by volume for an exposure period of less than 2 h,
while ammonia is assigned to a group of refrigerants
considered injurious or lethal in concentrations of 0.5 to 1
percent for exposures of 0.5 h duration. R-11, 22, and 502 are
in a class slightly more toxic than R-12.
©Wanchai 30Oct2009
Introduction29
Ref-L01
Basis of choice of refrigerant
Air. The major use of air as a refrigerant is in aircraft, where the light
weight of an air system compensates for its low COP.
Ammonia. Large industrial low-temperature installations are the
applications where ammonia is most frequently used.
C b di
Carbon dioxide.
id This
Thi refrigerant
fi t is
i sometimes
ti usedd for
f direct-contact
di t t t
freezing of food.
R-11.
11. Along with R-113 this refrigerant is popular for centrifugal
compressor systems.
R-12.
12 This refrigerant is used primarily with reciprocating compressors for
service in domestic refrigeration appliances and in automotive air
conditioners.
R-22.
22 Because a smaller and lower-cost compressor
p can be used with R-
22 than with R-12, this refrigerant has taken over many air-conditioning
applications from R-12.
R-502. This is one of the newer refrigerants, with some of the advantages
of R-22 but with the further advantage of better behavior with oil and
lower compressor discharge temperatures than R-22.
©Wanchai 30Oct2009
See table Introduction30
Secondary refrigerants Ref-L01
Secondary refrigerant does not change phase. Secondary
refrigerant such as brines and antifreezes, are solutions with
freezing temperatures below 0oC.
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Introduction31
Phase diagram of an anti-freeze. Ref-L01
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Introduction32
Phase diagram of an anti-freeze. Ref-L01
The percent of ice in the mixture at C is given by
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Introduction33
Phase diagram of an anti-freeze. Ref-L01
the percent liquid is given
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Introduction34
Properties of ethylene glycol solutions Ref-L01
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Introduction35
Properties of ethylene glycol solutions Ref-L01
©Wanchai 30Oct2009
Introduction36
Properties of ethylene glycol solutions Ref-L01
©Wanchai 30Oct2009
See refrigerant properties
Introduction37
Assignment Ref-L01
Problems in Stoecker & Jones
15-1, 15-2, 15-3, 15-4, 15-5
©Wanchai 30Oct2009
Introduction38
