lecture 4. refrigerants

7/29/2019 Lecture 4. Refrigerants

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1

Refrigerants

Lecture 4

Refrigeration and Air-conditioning

(MEng 4711)

Technology CollegeMechanical Engineering

Department


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Introduction

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Any body or susbstance that acts as a cooling medium by

extracting heat from another body or substance.

In a closed cycle systems, refrigerant is any working fluid

that undergoes cyclic changes during system operation.

A refrigerant is a substance that picks up latent heat when

the substance evaporates from a liquid to a gas. This is

done at a low temperature and pressure.

A refrigerant expels latent heat when it condenses from a

gas to a liquid at a high pressure and temperature. The

refrigerant cools by absorbing heat in one place anddischarging it in another area.


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Introduction


Properties of an ideal refrigerantA high latent heat of vaporization

Non-toxic, non-flammable and non-corrosive

Compatible with component materials

Not mix with oil since compressors are lubricated.

Operate at moderate pressures to reduce compressor

work and leakage.

Be relatively cheap to produce and store.

Ease of leak detection

Environmental friendly


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Classification of Refrigerants


Fluids suitable for refrigeration purposes can be classifiedinto primary and secondary refrigerants.

Primary refrigerants are those fluids which are useddirectly as working fluids, for example in vaporcompression and vapour absorption refrigeration systems.

When used in compression or absorption systems, thesefluids provide refrigeration by undergoing a phase changeprocess in the evaporator.

Secondary refrigerants are those liquids which are used

for transporting thermal energy from one location toanother. Secondary refrigerants are also known under thename brines or antifreezes.


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If the operating temperatures are above 0oC, then purewater can also be used as secondary refrigerant, for

example in large air conditioning systems.

Antifreezes or brines are used when refrigeration is

required at sub-zero temperatures. Unlike primary refrigerants, the secondary refrigerants do

not undergo phase change as they transport energy from

one location to another.

The commonly used secondary refrigerants are thesolutions of water and ethylene glycol, propylene glycol or

calcium chloride. These solutions are known under the

general name of brines.



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The primary refrigerants are further classified into the following four

groups: Halo-carbon Refrigerants

Azeotropic and Zeotropic Refrigerants

Inorganic Refrigerants and

Hydro-carbon Refrigerants

Halo-carbon refrigerants: The American Society of Heating,

Refrigeration and Air-conditioning Engineers (ASHRAE) identifies 42

halo-carbon compounds as refrigerants, but only a few of them are

commonly used for e.g. R-12,R-22, R-114 etc.

Azeotropic Refrigerants: The term 'azeotrope' refers to a stable

mixture of refrigerants whose vapour and liquid phases retain identical

compositions over a wide range of temperatures. However, these

mixtures usually have properties that differ from either of their

components. The example of Azeotrope Refrigerants are R-500 (73.8%

R-12 & 26.2% R-152), R-502, R-152 etc.



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Azeotropic Refrigerants

A stable mixture of two or several refrigerants whose

vapour and liquid phases retain identical compositions over

a wide range of temperatures.

Examples : R-500 : 73.8% R12 and 26.2% R152

R-502 : 8.8% R22 and 51.2% R115

R-503 : 40.1% R23 and 59.9% R13


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Zeotropic Refrigerants

A zeotropic mixture is one whose composition in liquidphase differs to that in vapour phase. Zeotropic refrigerants

therefore do not boil at constant temperatures unlike

azeotropic refrigerants.

Examples: R404a : R125/143a/134a (44%,52%,4%)R407c : R32/125/134a (23%, 25%, 52%)

R410a : R32/125 (50%, 50%)

R413a : R600a/218/134a (3%, 9%, 88%)


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Inorganic Refrigerants: The inorganic refrigerants wereexclusively used before the introduction of halocarbon

refrigerants. These refrigerants are still in use due to their

inherent thermodynamic and physical properties; for

example Ammonia R-717, Air R-729, CO2 R-744 etc. Hydro-carbon refrigerants: Most of the hydro-carbon

refrigerants are successfully used in industrial and

commercial installations. They possess satisfactory

thermodynamic properties but are highly flammable and

explosive e.g. Ethane R-170,Propane R-290,Propylene R-

1270 etc.



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CFC Refrigerants


The first halogen-based refrigerants (fluorinated

hydrocarbons) were developed over sixty years ago

These refrigerants are composed of chlorine, fluorine, and

carbon, and are called chlorofluorocarbons (CFCs).

These refrigerants are low in toxicity, noncorrosive, andcompatible with other materials. They are not flammable or

explosive with, but sizable quantities must not be released

where there is a flame or electric heating element. They are

particularly harmful to the respiratory system.

Common CFC refrigerants include R-11, R -12, R -113,R -

114,R -115, R-500, R-502, and R-503.


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R-500, R-502, and R-503 are azeotropic mixtures but are

classified as CFCS by the American Society of Heating,

Refrigeration, and Air Conditioning Engineers (ASHRAE).

CFCs are thought to be one of the major causes of ozone

depletion. By international agreement, they have not been

manufactured since 1995. However, they are still widely

used in existing residential units

CFC Refrigerants


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HCFC Refrigerants


Hydrochlorofluorocarbons (HCFCS) are moleculescomposed of methane or ethane in combination with a

halogen. This makes up a new molecule that is considered

to be partially halogenated.

The HCFCs have shorter lives and cause less ozonedepletion than the fully halogenated CFCS. Therefore,

they have reduced potential for global warming.

HCFCS such as R-22 and R-123 are considered to be

interim refrigerants. They will be used until suitable

replacements are available.

The EPA requires the phase out of HCFCs by the year

2030.


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HFC Refrigerants


Hydrofluorocarbons (HFCs) include suchrefrigerants as R-134a and R-23.

They are different from chlorofluorocarbons-they

contain one or more hydrogen atoms and no

chlorine atoms.

HFCS are considered to have zero potential for

ozone depletion.

They have only a slight effect on global warming.


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Refrigerant Blends(Azeotropic- Zeotropic)


Another more recent category is that of refrigerant blends,commonly referred to as "azeotopic" and "zeotropic."

The use of refrigerant blends is increasing. Azeotropic

blends do not change or separate in composition when

used in Refrigeration systems. Zeotropic refrigerants are also blends composed of various

refrigerants. when used in a refrigeration system, their

volumetric composition and saturation temperature do not

change.


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Refrigerant Selection Criteria


In order to select the a correct refrigerant, it isnecessary that it should satisfy those properties

which make it ideal to be used for a particular

application. The considered properties are:

Thermodynamic properties,

Chemical properties,

Physical properties

Environmental and safety properties/concerns &Cost


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Refrigerant development


Refrigerant development can be devided into three

phases:1. Early refrigerants (prior to CFCs)

2. Chloro-fluoro-carbon (CFC) based refrigerants

3. Refrigerants afterMontreal protocol

1. Early Refrigerants

Ethyl etheris the first refrigerant to be used by Jacob Perkinsin 1835.

Ether is liquid at ambient conditions

Ether boiling point is 34C0 at 1atm: for low temperatueconditions, the evaporatorshould work at vacuum.

Possibility forleakage and explosive mix Ether is toxic

Alexander Twinning proposed the use of ammonia and CO2 in1850.

Ammonia is important refrigerant in todays applications

It has good thermal properties, easily available,inexpensive, easy to handle


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Charles Tellier used dimethyl ether as refrigerant in 1864.

it is toxic Raoul Pictet used SO2as refrigerant in 1874.

It is not flammable: it is flame detergent.

Acts as auto-lubricant!

In the presence of water, it forms acid which is toxic and corrosive to

the system.

Linde builed a system based on NH3as refrigerant in 1877.

Imporant land mark in refrigerant development!

Excellent thermodynmic and thermophysical property!

But is toxic, strong smell and slightly flammable

Windhausen used CO2as refrigerant in 1885.

Non toxic and non flamable thus safe!

Imporant application in marine refrigeration and air conditioning.

It needs high operating pressure.




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Problems with Early Refrigerants Toxicity (eg. Ethers, Ammonia)

Flammability (eg. Hydrocarbons)

Material compatibility (eg. Ammonia)

Chemical stability (eg. SO2)

Operating Pressure (eg. CO2)

Limited temperature range (eg. All)

Developments of refrigerants which are non-toxic,

non-flamable, material compatible, chemicallystable!!

Chloro-Flouro-Carbon (CFCs) refrigerants invented!!!Thursday, March 14, 201318



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Chloro-fluoro-carbon (CFC) based refrigerants

Thomas midgley and his associates take up the task of

developing refrigerants that are free from problems associated

with early refrigerants in 1928 in USA.

CFCs are synthesized by replacement

hydrocarbons!!

Hydrocarbon

A series of refrigerant family are formed!!!Thursday, March 14, 201319



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Chloro-fluoro-carbon (CFC) based refrigerants

CFCs in the trade name ofFREONS enter themarket in 1930s!!

Freon-12 (CCl2F2) is introduced in 1931 and widelyused in domestic refrigeration!!!

Freon-11 (CCl3F) is introduced in 1932 and widelyused in air conditioning in large plants!!!

Freon-22 (CHClF3) and other CFCs aredeveloped!!!

A numbering system is developed for identifyingthe different drived refrigerants!




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Early refrigerants vs. CFCs

Compared to the early refrigeratnts, CFCs havesolved the previous problems!!!

non-toxic

Non-flammable

Chemically stable

Compatable with common materials

Avaialble for a wide refrigeration temperature

Becuase of the favorable properties, CFC refrigeratntsbecame popular and get extensive application!!




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IMPACT OF CFCs

All early refrigerants except ammonia are replacedby various CFCs!

Rapid growth in refrigeration due to the widespreaduse of CFCs!

Refrigeration enters Households in the form ofdomestic refrigerators and air conditioners, thanks

to CFCs! Production and consumption of CFCs increased

exponentially!!




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Ozone layer depletion

Ozone is imporant to protect life on earth from UV radiation.

UV is harmful to life on earth!

Ozone layer depletion consequences

For human beings, increased instances of :

Skin cancer

Weakened immune systems

Damages DNA structure

cataracts

Adverse effect on crop yield

Adverse effects on terrestrial and aquatic ecosystem




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Ozone layer depletion

In 1974, Rowland and Molina propose thetheory ofOzone Layer Depletion due to CFCs

released into atmosphere!

Subsequent studies show Ozone depletingpotential (ODPs) of CFCs and related

substances!

o R-11 (CFCl3) as a reference! =1ODP

o Example: R-12 = 1 ODPo R-114 = 0.8 ODP

o R-22 = 0.055 ODP




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CFCs and Global Warming

CFCs act as greenhouse gases and hence contributesignificantly to Global warming!

Atmoshperic blanket keeps the earth warm and this is essential

for life on earth!

The atmoshperic blancket consists of several gases

(greenhouse gases) such as CO2 N2, H2O!!




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Solar radiation mainly consists of long and shortwave radiation.

Atmoshperic blanket with greenhouse gases allows

short wave radiation to pass and reach the earth,

while absorbing the long wave radiations. The earth surface absorbs these shortwave

radiations and keep warm!

The earth also emmits long wave radiations and

these are absorbed at the atmoshpere (by thegreenhouse gases)!!

absorb longwave radiation and transmit short wave

radiations!!

The earth is kept warm!! Without GHG: T = -Thursday, March 14, 201327



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The temperature condition in the atmoshpere with

out any additives is balanced for this situation!

With the presence of large scale CFCs, the balance

of global warming is disturbed!

CFCs are capable of absorbing much more long

wave radiations compared to the conventional

greenhouse gases such as CO2!!

Global temperature increase!!




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Refrigerants after Montreal protocol

(3rd generation of refrigerants)

Replacements for CFCs should be:

Non-ozone depleting

Have as low GWP as possible

Several Synthetic and natural refrigerants are found toreplace CFCs

Some of the early refrigerants such as CO2 make a

comeback!

CFCs and Replacements



Application CFCs

Domestic refrigerators CFC 12 HFC 134a, HCs

Air conditioning CFC 11, CFC 12, HCFC22

HFC 134a, HCs, CO2, and mixtures

Cold storages HCFC 22, R 502 Ammonia, HFC mixtures


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Designation of refrigerants


Before the invention of chlorofluorocarbons (CFCs),

refrigerants were called by their chemical names. Because of the complexity of these names, especially the

CFCs the fully halogenated CFCs, and

hydrochlorofluorocarbons (HCFCs), the not fully

halogenated HCFCs, a numbering system was developed forhydrocarbons and halocarbons and is used widely in the

refrigeration industry.

From the number one can get some useful information about

the type of refrigerant, its chemical composition, molecularweight etc.

The refrigerants are internationally designated as 'R'

followed by certain numbers such as R-11, R-12, R-114 etc.


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Inorganic refrigerants

These are designated by number 7 followed by the

molecular weight of the refrigerant (rounded-off).

Examples:

Ammonia: Molecular weight is 17, the designation is R 717 Carbon dioxide: Molecular weight is 44, the designation is

R744

Water: Molecular weight is 18, the designation is R 718



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Mixtures

Azeotropic mixtures are designated by 500 series, whereaszeotropic refrigerants (e.g. non-azeotropic mixtures) are

designated by 400 series.

Examples

R500, R502, R503, R404A, R407A


Hydrocarbons

Propane (C3H8) : R290

n-butane (C4H10) : R600

iso-butane (C4H10) : R600a

Unsaturated Hydrocarbons: R1150 (C2H4)

R1270 (C3H6)


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Seminar Assignment


Major Equipment of Vapor-Compression RefrigerationSystems:

Group 1: Compressors

Group 2: Condensers

Group 3: EvaporatorsGroup 4: Expansion Devices (Flow Control Devices)

Assessment (10%)

Report (5%)

Presentation (5%)

Report and presentation should include Types, Principle of

operation, Performance Analysis.


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Grouping

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Group 1: (Ecaporator)

1. Yoftahe

2. Redia

3. Tadesse

Group 2: (condeser)

1. Acham

2. Dereje

3. G/Medhin

Group 3: (Compressor)1. Asehafi

2. Andualem

3. Chalachew

Group 4: (Exapansion

Device)

1. Lijalem

2. Zinabu3. Mengestab

lecture 4. refrigerants

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