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