chapter 3 refrigerants and mixture...
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CHAPTER 3
REFRIGERANTS AND MIXTURE PREPARATION
Refrigeration is the process of removing heat from the space to be
cooled and transferring it to a place where it is unobjectionable. The
primary purpose of refrigeration is producing and maintaining the
temperature which is lower than that of the surroundings.
In prehistoric times, man found that his game would last during
times when food was not available unless it is stored in snow or in the
coolness of a cave for use during the seasons of unavailability. In
China, before the first millennium, ice was harvested and stored in
insulated houses. Greeks, Hebrews, and Romans placed large amounts
of snow into storage pits dug into the ground and insulated with straw
and wood. In India, evaporative cooling was employed in early days.
For the vaporization of the liquid latent heat is exhausted the
surroundings so that surroundings get cooled.
The intermediate stage in the history of cooling foods was to add
chemicals like potassium nitrate or sodium nitrate to water causing the
temperature to fall. Cooling wine by this method was recorded in 1550,
as were the words "to refrigerate‖.
Commercial refrigeration is believed to have been initiated by
an American business person, Alexander C. Twinning, in 1856. Shortly
afterwards, an Australian, James Harrison, examined the refrigerators
used by Twinning and Gorrie and introduced vapour-compression
refrigeration to the meatpacking and brewing industries.
Ferdinand Carre of France developed a somewhat more complex
system in 1859. Unlike earlier compression-compression machines,
which use air as a coolant agent, Carre‘s equipment contains rapidly
expanding ammonia because boiling point of ammonia is at much lower
temperature than water and is thus can absorb more heat. Carre's
refrigerators have been widely used, and vapour compression
refrigeration has become, the most widely used method of cooling.
Methods of refrigeration can be classified as non-cyclic, cyclic
and non-conventional methods.
3.1 NON-CYCLIC REFRIGERATION
In these methods, refrigeration can be accomplished by melting
ice or by subliming dry ice. These methods are used for small-scale
refrigeration such as in laboratories and portable coolers, or in
workshops. Foodstuffs preserved at this temperature or slightly above
have an increased storage life. Solid carbon dioxide, known as dry ice,
can also used as a refrigerant.
3.2 CYCLIC REFRIGERATION
This consists of a refrigeration cycle, where heat is removed from
a low-temperature space or source and rejected to a high-temperature
sink with the help of external work.
The most common types of refrigeration systems use the vapour-
compression refrigeration cycle. Absorption heat pumps are used in a
minority of applications.
Cyclic refrigeration can be classified as:
Vapour Cycle
Gas Cycle
3.2.1Vapour Cycle Refrigeration
Vapour cycle refrigeration can further be classified as:
1. Vapour Compression Refrigeration
2. Vapour Absorption Refrigeration
1. Vapour Compression Refrigeration
Figure 3.1 Single stage vapour compression refrigeration system A) Schematic diagram B) p-h diagram
1 – Outlet of Evaporator/ Inlet to the Compressor
2 – Outlet of Compressor/ Inlet to Condenser
3 – Point in the Condenser where phase change starts
4 – Outlet of Condenser/ Inlet to Throttling Device
5 – Outlet of Throttling Device/ Inlet to Evaporator
W
Capillary Tube
Condenser Evaporator
Compressor
12
45
QEva
QCon
1
2
34
5
W
P
h
PCon
EvaP
h h1
2QCon
Q Eva
=h5 h4
1 – 2 : Isentropic Compression
2 – 4 : Isobaric Heat Rejection
4 – 5 : Isenthalpic Expansion
5 – 1 : Isobaric Heat Absorption
Figure 3.1(a) shows a schematic diagram of a vapour
compression refrigerator, which consists essentially of a hermetic
reciprocating compressor, an evaporator, an air cooled condenser and a
capillary tube [66]. These components are connected by pipelines in
which a refrigerant with suitable thermodynamic properties circulates.
The corresponding pressure-enthalpy (p–h) diagram is shown in Figure
3.1(b). In order to simulate the vapour compression refrigerator, a
number of assumptions are made. They are (a) steady state operation
(b) no frictional pressure drop through pipelines, i.e. pressure changes
only through the capillary tube and the compressor (c) heat gains or
heat losses from or to the system are neglected (d) no superheating or
sub cooling takes place and (e) the compressor has isentropic efficiency
of 75% [13], It must be noted that the above diagrams are either for
single component refrigerants or azeotropic mixtures. The T-s diagram
for zeotropic mixtures will be discussed at a later stage in the
calculations part. However the expressions for various parameters
would remain the same.
The pressure ratio (PR) is defined as the ratio of the condensing
pressure (P2) to the evaporating pressure (P1), i.e.
PR = 1
2
P
P 3.1
The condensing and evaporating pressures are determined
corresponding to the condensation and evaporation temperatures. The
condensation temperature is decided by the temperature of the ambient
air, whereas the evaporation temperature is determined by the load
temperature based on the required freezer air temperature.
The performance parameters of the VCR system are Co-efficient
of Performance (COP), is the ratio of refrigeration effect (RE) to the work
done (W)
COP = W
RE 3.2
The energy balance of the evaporator and compressor give
refrigeration effect and work done.
Refrigeration effect = (h1 – h5) kJ/kg 3.3
Work done = (h2 – h1) kJ/kg 3.4
2. Vapour Absorption Refrigeration (VAR)
In the early years of the twentieth century, the vapour absorption
cycle using ammonia-water systems was popular and widely used. After
the development of the vapour compression cycle, the vapour
absorption cycle lost much of its importance because of its low
coefficient of performance (about one fifth of that of the VCR cycle).
Today, the vapour absorption cycle is used mainly where fuel for
heating is available but electricity is not, such as in recreational
vehicles that carry LP gas. It is also used in industrial environments
where plentiful waste heat overcomes its inefficiency.
The absorption cycle is similar to the VCR cycle, except for the
method of raising the pressure of the refrigerant vapour. In the
absorption system, the compressor is replaced by an absorber which
dissolves the refrigerant vapour in a suitable liquid, a liquid pump is
used which raises the pressure and a generator which, on heat
addition, drives off the refrigerant vapour from the high-pressure
solution. Some work is required by the liquid pump but, for a given
quantity of refrigerant, it is much smaller than needed by the
compressor in the vapour compression cycle. In a VAR system, a
suitable combination of refrigerant and absorbent is used. The most
common combinations are NH3-H2O and H2O -lithium bromide.
3.2.2 Gas Cycle
When the working fluid is a gas that is compressed and expanded
but does not change its phase, the refrigeration cycle is called a gas
cycle. Air is most often used as the working fluid in gas cycle
refrigeration.
The gas cycle is less efficient than the vapour compression cycle
because in the gas cycle the heat is carried in the form of sensible heat
only. A gas cycle refrigeration system will require a large mass flow rate
and it would be bulky. Because of their lower efficiency and larger
dimensions, air cycle coolers are not used now-a-days in global cooling
devices. Air cycle refrigeration finds its application in air craft
refrigeration because the compressed air is already available and there
is no need of a separate compressor for refrigeration process which
reduces the weight per ton of refrigeration.
3.3 NON-CONVENTIONAL METHODS
There are some special methods to produce low temperatures which are
thermoelectric refrigeration, vortex tube and steam jet refrigeration.
1. Thermoelectric Refrigeration
Thermoelectric cooling uses the Peltier effect to create a heat flux
between the junctions of two different types of materials. This effect is
commonly used in camping and portable coolers and for cooling small
instruments and electronic components.
2. Vortex Tube
The vortex tube used for spot cooling, when compressed air is
available and thermo-acoustic refrigeration using sound waves in a
pressurized gas to drive heat transfer and heat exchange.
3. Steam Jet Refrigeration
It is quite similar to more conventional refrigeration cycles, with
an evaporator, a compression device, a condenser and a refrigerant as
the basic components of the system. Instead of mechanical
compression device, the system characteristically employs a steam
ejector or booster to compress the refrigerant to the condenser pressure
level.
3.4 REFRIGERANTS
Any substance that absorbs heat through expansion or
vaporization may be called a refrigerant [67]. Examples are ammonia,
R12, R134a, R22 and hydro carbons etc. A broader definition may
include secondary cooling mediums such as brine solutions and cold
water.
3.4.1 Requirements of a Good Refrigerant:
1) It should be non poisonous and non toxic.
2) It should be non explosive and non-inflammable.
3) It should be non corrosive.
4) One should be able to detect the leak easily.
5) It should have low boiling point
6) Parts moving in the fluid should be easy to lubricate.
7) It should have a well balanced enthalpy of evaporation per unit
mass.
8) It should have a small relative displacement to obtain a certain
refrigerating effect.
10)Minimum pressure difference between the vaporizing and
condensing pressures is desirable.
3.4.2 Classification of Refrigerants:
Refrigerants have been classified by three groups. They are:
1) Group One: These are the safest refrigerants. They do not show
flame propagation when tested in air at 21°C and 1.01325 bar.
(Example: R113, R11, R21, R114, R12, R30, R22, R744, R502, R13,
R14, R500, R134a, etc.)
2) Group Two: These are toxic and somewhat inflammable refrigerants.
These refrigerants have a lower flammability limit of more than
0.10kg/m3 at 210C and 1.01325bar and a heat of combustion of less
than 19kJ/kg. (Example: R1130, R611, R160, R764, R40, R717 etc.)
3) Group Three: These are highly inflammable refrigerants and this
group is defined by a lower flammability limit of less than or equal to
0.10 kg/m3 at 210C and 1.01325 bar or a heat of combustion more
than or equal to 19 kJ/kg. (Example: R600a, R290, R600, R1270, etc.)
3.4.3 Desirable Thermo physical Properties of Refrigerants
1) Evaporator and Condenser Pressures: In order to avoid any leakage
of air and moisture from outside and to be able to detect leakage of
refrigerant from the system, it is preferable that both evaporator and
condenser pressures should be above the atmospheric pressure; but
then these pressures should not be very high because the
construction of compressor, condenser and evaporator will have to
be heavy and consequently initial cost will increase. The
compression ratio should be as small as possible to avoid leakage
across the piston.
2) Critical Temperature and Pressure: If the critical temperature of a
refrigerant is very near to the condensing temperature, the power
requirements are large.
3) Freezing Temperature: A refrigerant is required to have its freezing
temperature much below the operation temperature in the plant.
4) Latent Heat of Vaporization: The more is the latent heat of
vaporization, the more is the refrigeration effect. Thus mass of
refrigerant required for per ton of refrigeration will be reduced. The
area under reduction due to throttling and area under the super
heat horn becomes negligibly small as compared to enthalpy of
vaporization. The COP in such a situation will be close to that of
Carnot value.
5) Specific Volume: The theoretical compressor displacement depends
on the specific volume of the refrigerant vapour at evaporator
temperature, i.e. at suction to compressor and the refrigerating
effect per kg of refrigerant. Small volume of displacement per ton of
refrigeration allows reciprocating compressor to be used, whereas
centrifugal compressors are preferred when volume displacement
per ton of refrigeration is high.
6) Stability and Inertness: An ideal refrigerant should not decompose
at temperature of operation in the cycle and should not get
polymerized. Some refrigerants decompose into gases which do not
condense in the condenser and cause high condenser pressures and
vapour lock.
7) Viscosity: It is desirable that both the liquid and vapour refrigerants
should have low viscosity so that the pressure drops during flow are
small. Heat transfer is also improved in the evaporator and the
condenser due to low viscosity.
8) Thermal Conductivity: High thermal conductivity is desirable for
efficient heat transfer in evaporator and condenser. Moreover, the
surface wetting characteristics also improve heat transfer.
9) Oil Effect: With non oil miscible refrigerants, due to poor heat
conduction properties of oil, large heat transfer surfaces are
required. Thus miscibility is an advantage both from points of view
of heat transfer and that refrigerant acts as carrier of oil to moving
parts.
The choice of a refrigerant for a VCR is limited by 1) Economy (2)
Equipment type and size and (3) Application
3.4.4 Alternative Refrigerants
One of the major challenges posed to the Montreal Protocol is to
protect the stratospheric ozone layer and also global warming while
ensuring that developing countries are not economically disadvantaged
during their transition to new technologies that do not rely on ozone
depleting substances (ODS). This is particularly applicable to the
refrigeration sector, which accounts for the largest share of ODS
consumption in developing countries and touches virtually every
person‘s life, directly or indirectly.
HFC refrigerants have no Ozone Depletion Potential, but they do
have a high Global Warming Potential [40]. The GWP of HFCs is not as
high as CFCs, but they are significantly higher than the natural
refrigerants such as hydrocarbons and ammonia. The international
agreement, Kyoto Protocol, between developed nations seeks to reduce
emissions of carbon dioxide and five other Green House Gases (GHGs),
of which HFCs are one [8).
HC refrigerants are simple compounds containing carbon and
hydrogen and do not contain any halogens like chlorine, fluorine etc.
These refrigerants are non -toxic but highly flammable and have zero
ODP and negligible GWP. HC refrigerants are completely miscible with
commonly used mineral oils as well as PAG and POE [26].
R134a is becoming widely accepted as the replacement for R12 in
domestic refrigerator/freezer and automotive air-conditioning
applications and also as the replacement for medium pressure chillers.
First, R134a is not compatible with the mineral oils commonly
used for compressor lubrication [30]. There are a number of synthetic
candidates being evaluated for use with R134a, but none have been
proven totally useful. The refrigeration capacity and coefficient of
performance (COP) of alternative refrigerants must also be established.
Several investigations have been conducted to determine the capacity
and performance of alternatives relative to their CFC counter-parts. A
comparison of R134a with R12 in a residential heat pump showed that
approximately the same heating output was achieved with R134a. But
the COP of the system was approximately 15% less with R134a than
with R12 [6]. In another series tests were conducted at ARI (Air-
conditioning and Refrigeration Institute). Heat Pump Rating Conditions
showed that R134a exhibits a 6 to 11% increase in COP for moderate
and warm rating conditions [41], while R134a has a nearly identical
COP to that of R12 for a cold rating condition. In a test conducted for a
household refrigerator/freezer, R134a was shown to consume
approximately 8% more power than R12 and require more run-time,
resulting in energy consumption 45% greater than R12. Even though
R134a has proven as an alternative refrigerant to the CFCs and has
lower GWP of 1300 which is less than R12 (8500) is considerably high
and has to be controlled as per the Kyoto Protocol.
Since last couple of years hydrocarbons are being used as
alternative refrigerants to R12 and R134a due to their excellent thermo
physical properties. But pure hydrocarbons are not suitable for drop in
replacement for existing systems due to mismatch of its saturation
properties. It demands changes in the design, especially compressor
[36]. The use of hydrocarbons was restricted due to its flammable
properties.
The above discussion indicates that a lot of work had been done
already and is still continuing to develop and test CFC pure-refrigerant
alternatives. These alternatives have a lot of potential, but their total
acceptance has not been found out.
3.4.5 Synthetic Mixtures
The synthetic mixtures may be broadly classified as azeotropic,
near-azeotropic, or zeotropic [62].
1. Azeotropes
An azeotrope is defined as a point at which the concentration of
the liquid and the vapour phase is the same for a given temperature
and pressure. An azeotrope, a mixture behaves like a single-constituent
system. Almost all azeotropic refrigerants have a boiling point lower
than either of the constituents (which are known as a minimum
temperature or maximum pressure azeotrope).
2. Near Azeotropes
For a near-azeotropic mixture, the vapour and liquid
concentrations at a given temperature and pressure differ slightly. Most
azeotropic refrigerant mixtures become near-azeotropic when the
pressure or temperature is varied from the azeotrope point. R410A is a
near-azeotropic mixture of 50%R32/50%R125. For standard condenser
pressures and temperatures, the bubble and dew points for this
concentration vary by less than 0.10C.
3. Zeotropes or Non Azeotropes
For a zeotropic mixture, the concentrations of the liquid and the
vapour phase are never equal. This creates a temperature glide during
the phase change. Zeotropic mixtures are the most common type of
refrigerant blend. Use of non azeotropic refrigerant mixture reduces the
irreversibility and increases the COP [50].
3.5 MONTREAL PROTOCOL
The Montreal Protocol on substances that deplete the ozone layer is an
international treaty designed to protect the ozone layer by a scheduled
phasing out of the ozone depleting substances. It is believed that if the
international agreement is adhered to, the ozone layer is anticipated to
recover by 2050. Due to its extensive adoption and implementation it
has been hailed as an example of outstanding international co-
operation. Kofi Annan says that "perhaps the single most successful
international agreement to date has been the Montreal Protocol".
Impact of Montreal Protocol
Since the Montreal Protocol came into effect, the atmospheric
concentrations of the most important CFCs and related chlorinated
hydrocarbons have either leveled off or decreased. Also, the
concentration of the HCFCs increased drastically atleast partly because
HCFCs have been substituted by CFCs in most cases.
On a molecule-for-molecule basis HFC compounds are upto
10,000 times more potent greenhouse gases than carbon dioxide. The
Montreal Protocol currently calls for a complete phase out of HCFCs by
2040 in developing countries like India, but does not place any
restriction on HFCs. Since the HCFCs themselves are as powerful as
greenhouse gases, the mere substitution of HFCs for CFCs does not
significantly increase the rate of anthropogenic global warming, but
over a period of time a steady increase in their use could increase the
danger that human activities may change the climate.
3.6 GLOBAL WARMING POTENTIAL AND KYOTO PROTOCOL
GWP is a measure of how much a given mass of greenhouse gas
is estimated to contribute to global warming. It is a relative scale which
compares the gas in question to that of the same mass of CO2. A global
warming potential is calculated over a specific time interval and the
value of this must be stated whenever a global warming potential is
quoted other wise the value becomes meaningless. Its usage is being
governed by the Kyoto Protocol.
In 1997 the world nations came together in Kyoto in Japan to
discuss Global Warming, the Kyoto Protocol finally came into force. The
very phrase ‗Kyoto Protocol‘ has become synonymous with the idea of
saving the earth from global meltdown.
The Kyoto Protocol aims to tackle global warming by setting
target levels for nations to reduce green house emissions worldwide.
These targets vary between countries and regions, but globally the
initial target is to reduce greenhouse gas emissions to 5.2% below 1990
levels (base levels) during the commitment period, i.e., 2008 – 2012
[56].
The focus of the Kyoto Protocol however, is on the reduction in
the levels of the following six gases: Carbon dioxide (CO2), Methane
(CH4), Nitrous Oxide (N2O), Hydroflurocarbons (HFCs), Perflurocarbons
(PFCs) and Sulphur Hexafluoride (SF6).
3.7 REASONS FOR SELECTION OF R134a AND HYDROCARBON
REFRIGERANTS
R134a has the following advantageous properties.
1) As compared to R12 which has an ODP of 1, R134a has an ODP of 0
due to complete absence of chlorine atoms.
2) When compared to R12, which has a GWP of 8500, R134a has a
GWP which is approximately one tenth of that of R12 i.e. 1300.
(Here GWP of CO2 = 1; time base = 100 years).
3) Unlike the hydrocarbon blends, R134a is non-flammable.
4) R134a has an excellent material compatibility.
5) It is non toxic as compared to refrigerants like ammonia. However it
does require some ventilation to avoid displacement of oxygen.
6) It has a COP pattern and magnitude which is near to that of R12.
Hydrocarbons (HCs) have similar properties to CFCs and HCFCs.
The HCs with better properties as refrigerant are isobutane (R600a),
propane (R290) and their mixtures. These substances fulfil thermo
physics, ecological, physiologic and economic requirements to be
located among the best options to substitute CFC or HFCs. The most
important characteristics are:
1) The thermodynamic properties of hydrocarbons are similar to that of
R134a. HCs have low viscosity and high thermal conductivity that
guarantee a good performance of the system. These superior
transport properties are believed to contribute to the higher energy
efficiency of hydrocarbons.
2) As shown in Table 3.1 the global warming potential (GWP) of
hydrocarbons is much lower than that of synthetic refrigerants.
3) The Table 3.1 also shows that the ozone depletion potential (ODP) of
hydrocarbons is zero.
4) Another advantage of hydrocarbons is their solubility in most of oils
like mineral oil and POE which is traditionally used as lubricants in
the hermetic compressors.
5) They are compatible with the materials used with traditional
refrigerant, metal components and oils.
6) High latent heat in the boiling process
7) As the density is less than lower than CFCs/HFCs, inspite of its
flammability, the refrigerant mass requirements are low.
8) The ecological advantages include zero ozone depletion potential,
non toxic substances and negligible global warming potential.
Table 3.1 Properties of various refrigerants
Refrig
erant
Code
Molecu
lar
weight
Boiling Point
0C
(1.01325bar)
Critical
Tempera
ture 0C
Latent
heat
kJ/kg
Explosive
Limits in
air, % by
volume
ODP GWP
R12 120.93 -29.8 112 165.24 Non-
flammable 0.82 8100
R134a 102.03 -26.1 101.1 216.87 Non-
flammable 0 1300
R290 44.1 -42.04 96.7 423.33 2.3-7.3 0 20
R600a 58.13 -11.73 134.7 364.25 1.8-8.4 0 20
3.8 CRITERIA FOR SELECTION OF CHOSEN MIXTURES
The present experiment was mainly performed to test a ternary
mixture of R134a/R600a/R290. Though, less than R12‘s GWP; R134a
still has a pretty high GWP. This value could be reduced if it is mixed
with hydrocarbons which have very low GWP values. Mixing of HFC
and HC refrigerants allows the adjustment of the undesirable
properties of the individual components such as flammability of HC
refrigerants which can be reduced by adding non-flammable
components like HFCs [19, 62]. Unless major changes in the
compressor are made, the saturation properties of the alternative
refrigerant should match closely with the base refrigerant [10]. Safe
limit of hydrocarbons in refrigerators is 150g as specified by Prof.
R.S.Agarwal. In any refrigerator, if the charge of the refrigerant is less
than or equal to 150 grams no need to take any safety precautions with
flammability concern [60]. In the proposed ternary mixture of
R134a/R600a/R290, the total quantity of the hydrocarbons is less
than 150 grams so the mixture is a non-flammable refrigerant.
3.9 THEORETICAL ANALYSIS FOR THE SELECTION OF
OPTIMUM COMPOSITION OF REFRIGERANT MIXTURES
A majority of refrigeration systems in the India are using R134a
as their refrigerant. In a refrigeration system, the most expensive
component is the compressor. Thus if a surrogate to R134a is achieved
which could be used without the replacement of the compressor, the
substitute would be highly economical.
Thus the most important performance parameter that is
considered for selecting a specific composition from a large number of
compositions was the matching of the saturation properties. The
saturation properties of the HC mixture (50%R600a/50%R290) match
closely the saturation properties of R134a [10]. Therefore, any mixture
of R134a/R600a/R290 at any mole fractions can have saturation
properties very close to R134a. The proposed alternative ternary
mixture can be considered as drop in replacement for R134a
refrigerators.
COP was considered as a secondary performance parameter and
calculations were done for each composition. As a tertiary performance
character, we considered the pressure ratio, to ensure that the
operating pressures were in attainable limits for the compressor of the
domestic refrigerator.
For the selection of the best alternative mixture, all the
calculations were performed at -200C and 400C of evaporator and
condenser temperatures respectively. All percentages are in terms of
percentage by mass. In order to compare various refrigerants as
working fluids in domestic refrigerator the thermodynamic properties
were taken from the software REFPROP 6.0 [58].
Ternary Mixture: In the ternary mixture, the compounds selected for
simulative testing were R134a, Isobutane (R600a) and Propane (R290)
the calculation procedure explained by the Philippe F.Launay [49] was
followed
(a) The percentage of R134a was varied in steps of 5% by keeping the
remnant percentage of mass shared equally between isobutane and
propane. The results have been given tabulated in Table 3.2. The
following were the inferences drawn from results.
As the percentage of R134a in the ternary mixture was increased,
corresponding saturation pressures are matching closely with the
pure R134a as shown in Figure 3.2 which reflects the proposed
alternative mixture can be used as drop in replacement for R134a.
The COP started increasing with the increasing percentage R134a
and reach a maximum value at 25% of R134a (Mixture-3) and then
started decreasing with a further increase in R134a mass fraction.
Table 3.2 Performance comparison of selected alternative refrigerants with the base refrigerants with same operating conditions
Refrigerant Refrigeration Effect(kJ/kg)
Specific Work
(kJ/kg)
COP Specific Volume
(m3/kg)
R134a 130.2 56.4 2.3 0.1474
HC mixture (50%R290/50%R600a)
266.7 97 2.74 0.269
Mixture-1 (5%R134a/47.5%R290/
47.5%R600a) 261.7 93.4 2.8 0.250
Mixture-2
(15%R134a/42.5%R290/
42.5%R600a)
249.1 85.6 2.91 0.2152
Mixture-3 (25%R134a/37.5%R290
/ 37.5%R600a)
233.4 78.7 2.96 0.1853
Mixture-4 (35%R134a/32.5%R290
/ 32.5%R600a)
215.4 73.2 2.94 0.1611
Mixture-5
(45%R134a/27.5%R290/
27.5%R600a)
195.6 68.6 2.84 0.1422
3.10 THEORETICAL ANALYSIS OF PROPERTIES OF THE
SELECTED ALTERNATIVE REFRIGERANT MIXTURES
Pure R290 or R600a is not suitable as a direct drop in
replacement for R134a. However, the saturation properties of the
mixture, developed by Rauolt's rule, of HC mixture (50% R600a and
50% R290), matches closely the saturation properties of R134a.
Therefore, any mixture of R134a/R600a/R290 at any mole fraction can
have saturation properties very close to R134a. The deviation of vapour
pressure with saturation temperature of the chosen alternative
refrigerants, R134a and HC mixture are plotted in Figure 3.2. It shows
that the alternative mixtures, viz, mixture-1, mixture-2, mixture-3,
mixture-4 and mixture-5 have close values of vapour pressure with
R134a. In this work, different masses of R134a/R600a/R290 mixtures
were studied to find the best mass of this mixture to replace R134a in
domestic refrigerators.
To decide the charge quantity density of the liquid refrigerant is an
important parameter. Refrigerant charge is a key parameter in a
Vapour compression refrigeration system that influences the
performance of the system. The deviation of liquid density with
saturation temperature of the chosen alternative refrigerants and
R134a are plotted in Figure 3.3. It shows that the alternative mixtures
mixture-1, mixture-2, mixture-3, mixture-4 and mixture-5 have lower
liquid density than R134a. For the alternative mixtures the density is
45% to 56% lower than R134a for the considered operating range. Due
to this it can be inferred that less mass of alternative refrigerant
mixtures is needed when compared with R134a in an existing system
[13, 11].
0
4
8
12
16
20
-20 -10 0 10 20 30 40 50
Temperature in 0C
Pre
ssu
re in
bar
R134a Mix-1 Mix-2 Mix-3 Mix-4 Mix-5
Figure 3.2 Variation of vapour pressure with saturation temperature
400
800
1200
1600
-20 -10 0 10 20 30 40 50
Temperature in 0C
Liq
uid
den
sit
y in
kg/m
3
R134a Mix-1 Mix-2Mix-3 Mix-4 Mix-5
Figure 3.3 Variation of liquid density with saturation temperature
In the present study the system considered is working with
vapour compression refrigeration system principle, it is necessary to
study the vapour density of the selected refrigerants. The vapour
density of considered alternative refrigerants and R134a is plotted in
Figure 3.4. It shows that vapour density of the mixture-1 53.7% to
61.3% and mixture-5 24.57% to 36.68% is lower than that of the
R134a and the range of other mixtures falls in between mixture-1 and
mixture-5. Hence the considered mixtures charge quantity by mass as
compared to R134a will be lesser, when R134a compressors are used.
0
20
40
60
80
-20 -10 0 10 20 30 40 50
Temperature in 0C
Vapou
r D
en
sit
y in
kg/m
3
R134a Mix-1 Mix-2
Mix-3 Mix-4 Mix-5
Figure 3.4 Variation of vapour density with saturation temperature
The important thermodynamic property that plays a dominating
role while deciding the refrigeration effect is the latent heat. For a given
compressor it can handle a particular volume flow rate, from the above
discussion it has been found that the mass flow rate of alternative
mixtures is less than that of R134a due to its lower density. The latent
heat of vaporization of the mixtures doesn‘t match with that of R134a,
results in decreasing cooling capacity. The latent heat of considered
alternative refrigerants and R134a is plotted in Figure 3.5. It shows
that the alternative mixtures mixture-1, mixture-2, mixture-3, mixture-
4 and mixture-5 have 34% to 76% higher latent heat than R134a .From
the graph it was observed that latent heat of vaporisation of the
alternative mixtures decreases from mixture-1 to mixture-5, which is
due to increase in the mass quantity of R134a .Thus there is a scope
for the lower mass of alternative mixtures to have the same or better
cooling effect compared with R134a.
Viscosity is one of the important thermo physical properties of
the refrigerant which influences the flow-ability of refrigerant through
the system. It influences the flow characteristics of the refrigerants
inside the capillary tube. Pressure loss increases with the increase of
viscosity. The viscosity of the considered alternative refrigerants and
R134a in liquid and vapour form is plotted separately in Figure 3.6 and
Figure 3.7 respectively. As the viscosity values of the alternative
refrigerant mixtures have more or less same values, mixture-3 was
chosen in comparison with R134a.
125
225
325
425
-20 -10 0 10 20 30 40 50
Temperature in 0C
Late
nt
heat
in k
J/kg
R134a Mix-1 Mix-2 Mix-3
Mix-4 Mix-5 HC
Figure 3.5 Variation of latent heat with saturation temperature
50
150
250
350
450
-20 -10 0 10 20 30 40 50
Temperature in 0C
Liq
uid
vis
cosit
y
R134a Mix-3
Figure 3.6 Variation of liquid viscosity with saturation temperature
6
8
10
12
14
-20 -10 0 10 20 30 40 50
Temperature in 0C
Vapou
r vis
cosit
y
R134a Mix-3
Figure 3.7 Variation of vapour viscosity with saturation temperature
Figures 3.6 and 3.7 show that viscosities of the selected mixtures
are 40% to 47% and 23 to 26% which is lower than R134a in liquid and
vapour phase respectively. Hence for alternative refrigerants, due to
their lesser viscosity higher capillary lengths are required for the same
pressure drop as compared with R134a[53, 24].
0.06
0.07
0.08
0.09
0.1
0.11
0.12
-20 -10 0 10 20 30 40 50
Temperature in 0C
Th
erm
al con
du
cti
vit
y in
W/m
/K
R134a Mix-3
Figure 3.8 Variation of liquid thermal conductivity with saturation temperature
0
0.005
0.01
0.015
0.02
0.025
-20 -10 0 10 20 30 40 50
Temperature in 0C
Th
erm
al con
du
cti
vit
y in
W/m
/K
R134a Mix-3
Figure 3.9 Variation of vapour thermal conductivity with saturation
temperature
The thermal conductivity of the selected alternative refrigerants
and R134a in liquid and vapour form is plotted in Figure 3.8 and
Figure 3.9 respectively. From the Figures 3.18 and 3.19, it is inferred
that thermal conductivity of the selected mixtures is higher than
R134a.For example Micture-3 is showing 6.8 to 7.7% and 23.2 to
24.5% higher than R134a in liquid and vapour phases respectively.
Hence higher heat transfer coefficients can be expected for the selected
refrigerants in the evaporator and condenser. This results in better heat
transfer rates.
Specific volume of the refrigerant plays an important role in
influencing the work of compression. The specific volume of the
considered alternative refrigerants, R134a and HC mixture is plotted in
Figure 3.10. It shows that specific volume of the alternative mixtures
increases from mixture-5 to mixture-1 and it is highest for HC mixture,
lowest for R134a. Even though the specific volume of the selected
refrigerants is higher than that of R134a, since the mass flow rates of
selected refrigerants is lesser it would not result in higher compressor
displacement rates. Hence for the alternative mixtures there is no need
to change the compressor, the same compressor used for R134a can be
used.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
-20 -10 0 10 20 30 40 50
Temperature in 0C
Specific
volu
me in
m3/kg
R134a Mix-1 Mix-2 mix-3
Mix-4 Mix-5 HC
Figure 3.10 Variation of specific Volume with saturation temperature
Vapour specific heat is one of the influencing parameter that
decides the super heat of the refrigerant at inlet to the compressor. For
the same heat transfer the refrigerant which is having high specific
heat leads to decrease in degree of super heat which decreases the
work of compression. The specific heat of the considered alternative
refrigerant, R134a is plotted in Figure 3.11. It shows that specific heat
of the alternative mixture is 60 to 66% which is more than that of the
R134a.Hence the degree of super heating would be lesser for selected
refrigerants as compared to R134a, thereby better performance can be
expected from the alternative mixtures.
0.7
1.1
1.5
1.9
-20 -10 0 10 20 30 40 50
Temperature in 0C
Specific
heat
in k
J/kg/K
R134a Mix-3
Figure 3.11 Variation of Vapour specific heat with saturation temperature
From the above discussion it can observed that with the
increasing R134a quantity in the ternary mixture (R134/R290/R600a)
from mixture-1 to mixture-5 both the specific volume and latent heat
decrease in comparison with HC mixture which is presently used as
alternative refrigerant in the place of R134a. This is because pure
R134a has low specific volume and low latent heat values and HC
mixture has high specific volumes and high latent heat values. When
R134a and HC mixture are mixed together the final mixture results in
lower specific volumes than HC mixture, better latent heat of
vaporisation values than R134a, by taking this advantage the ternary
mixture is expected to perform well when compared with the existing
refrigerants R134a and HC mixture. From Table 3.2 it can be observed
that mixture-1 to mixture-3 compared to decrease of refrigeration
effect, decrease of work of compression is more, which is due to more
decrease of specific volume. Hence it leads to better COP values than
R134a and HC mixture at Mixture-3. From mixture-3 to mixture-5
decrease of refrigeration effect will be more than the decrease of work of
compression which leads to lower COP values. Hence for the selected
mixtures mixture-3 will result in maximum COP values.
3.11 HANDLING HC CYLINDERS
Cylinders containing HC refrigerants should be clearly labelled to
show the type of refrigerant and that it is flammable. The guidelines
given below are recommended as good practices when handling HC
cylinders which are very similar to the guidelines for any refrigerant
cylinder [15]
The valve cap should be fitted when the cylinder is not being
used;
The cylinder should not be heated. Refrigerant cylinders can
usually withstand temperatures up to 45 to 500C. If a cylinder
needs to be heated (e.g. to remove refrigerant more easily), it
should be placed in a container of water not hotter than 45 to
500C.
The cylinder and its valve should not be modified.
The cylinder should not be refilled unless it is designed for
recovered refrigerant.
It should be noted that the weight of the same volume of HC
refrigerant is only 40% to 44% of the weight of R12/R134a refrigerant.
A cylinder, which can safely contain 10kg of R12/R134a, will only be
able to contain 4 to 4.4 kg of HC. The volume of the liquid refrigerant in
the cylinder should never exceed 80% of the total cylinder volume or
the weight of refrigerant filled should be 80% or less of the maximum
permitted fill weight
3.12 PREPARATION OF REFRIGERANT MIXTURE
The proposed ternary mixture of HFC(R134a)/HC (R600a/R290)
in the present study are zeotrope in nature. Hence mixing of the
refrigerants, handling and charging should be done carefully. Many
guidelines have been reported in the literature regarding procedure and
characteristic of the zeotrope mixtures. The five mixtures mixture-1,
mixture-2, mixture-3, mixture-4 and mixture-5 were prepared in
separate cylinders before they were charged into the system. To control
the concentration shifts, the minimum liquid level of the charge
quantity in the refrigerant mixture cylinder should not be less than
10% volume while charging the system. Hence the mixture quantity has
been prepared sufficiently to maintain the 10% level. To have an
accurate quantity the weight of the mixtures were prepared in small
cylinders of 1kg capacity.
The following are the steps that have been followed by the
researcher for preparing ternary mixture
Initially cylinders were cleaned and flushed with R134a twice.
Evacuate the cylinder by vacuum pump up to 0.1mbar.
Cylinders were kept at a low temperature bath while filling to
avoid cross contamination and quick transfer of refrigerant.
Initially cylinders were filled with required quantity of HC, as HC
has a lower vapour pressure than R134a [54].
Later the required quantity of R134a is filled in to the cylinder.
Each cylinder was properly labeled to indicate the name and
quantity of filled refrigerant mixture
While charging the refrigerant into the system it was ensure that
only liquid has to enter into the system which is done by placing the
cylinder in upright down position. The photographic view of the
charging procedure is as shown in Figure 3.12.
3.13 Charging
The charging of refrigeration systems with hydrocarbon
refrigerants is similar to those using halocarbon refrigerants. As with
all blend refrigerants, hydrocarbon refrigerant blends should also be
charged in the liquid phase in order to maintain the correct
composition of the blend [15]. The following additional requirements
should be adhered to:-
• Ensure that contamination of different refrigerants does not occur
when using charging equipment. Hoses or lines are to be as short as
possible to minimize the amount of refrigerant contained in them.
Figure 3.12 Photographic views of the preparation of the ternary
mixture
a) vacuum process b) charging of the refrigerant
c) weighing scale d) charging kit and low temperature
bath
• It is recommended that cylinders be kept upright and refrigerant is
charged in the liquid phase.
• Ensure that the refrigeration system is earthed prior to charging the
system with refrigerant.
• Label the system when charging is complete. The label should state
that hydrocarbon refrigerants have been charged into the system and
that it is flammable. Position the label in a prominent position on the
equipment.
• Extreme care should be taken as to not to overfill the refrigeration
system. (Note that hydrocarbon charge sizes are typically 40% to 50%
of CFC, HCFC and HFC charge sizes).
3.14 EQUIVALENT CHARGE QUANTITY OF THE MIXTURES
The density difference is important when charging the systems.
When charging hydrocarbons by weight, only 43% of the R134a charge
is used. When charging hydrocarbons by volume, the same volume as
for the halocarbon is used. The system should always be charged with
liquid refrigerant in case of blends. It is essential that the system
should be filled with an exact charge for better performance. HFC/HC
refrigerant is zeotropic blends therefore, while charging with mixture,
make sure that the refrigerant drawn from the cylinder is in the liquid
form. It is recommended that charging should be done by weight using
an electronic weighing scale along with charging equipment.
For the given volume of the visi cooler considering the
instrumentation of the system the manufacturer specified quantity of
R134a is 240 grams. The Table 3.3 shows the equivalent quantity of
considered HFC/HC mixtures in comparison with R134a.
Table 3.3 Equivalent mass of selected alternative refrigerants and HC blend
Refrigerant Equivalent Charge to 240 grams of R134a in grams
Mixture-1 106
Mixture-2 113
Mixture-3 120
Mixture-4 129
Mixture-5 139
HC mixture 104