A Term Paper Report
On
Refrigeration and its Working and Principle
Submitted To
Amity University
Utter Pradesh
In partial fulfilment of the requirements for the award of the degree of
BACHELOR OF TECHNOLOGY
In
MECHANICAL AND AUTOMATION ENGINEERING
By
ABHISHEK KUMAR
A7605415007
Under the guidance of
MR. Mragank Sharma
DEPARTMENT OF MECHANICAL AND AUTOMATION ENGINEERING
AMITY SCHOOL OF ENGINEERING AND TECHNOLOGY
AMITY UNIVERSITY UTTER PRADESH
LUCKNOW (U.P.)
2017
(I)
DECLARATION
I, Abhishek Kumar, student of B.tech, (Mechanical and Automation Engineering), V Semester, hereby declare that the term paper report titled “Refrigeration and its Working and Principle” which is submitted by me to Department of Mechanical and Automation Engineering, Amity School of Engineering and Technology, Amity University Utter Pradesh, Lucknow, in partial fulfilment requirements of the award of the degree of Bachelor of Technology in Mechanical and Automation Engineering, has not been previously formed the basis for the award of any degree, diploma or other similar title or recognition.
Lucknow Signature of Student
Date: Abhishek Kumar
(II)
CERTIFICATE
On the basis of declaration submitted by Abhishek Kumar, student of B.Tech (Mechanical and Automation Engineering), V Semester, hereby certified that the term paper report titled “Refrigeration and its Working and Principle” which is submitted to Department of Mechanical and Automation Engineering, Amity School of Engineering and Technology, Amity University Utter Pradesh, Lucknow, in partial fulfilment of the requirement for the award of the degree of Bachelor of Technology in Mechanical and Automation Engineering is an original contribution with existing knowledge and faithful record of work carried out by them under my guidance and supervision.
To the best of my knowledge this work has not been submitted in part or full for any Degreed or Diploma to this University or elsewhere.
Mr. Mragank Sharma
Assistant Professor
Mechanical and Automation Engineering
ASET,
Amity University Lucknow
(iii)
Acknowledgement
I would like to express my sincere gratitude to all the people who have contributed towards the successful completion of my project.
It is a matter of pleasure and privileges to acknowledgement my profound gratitude to my teachers and classmates who helped me in completing this project.
I would also like to thank my Term Paper mentor, Mr. Mragank Sharma Sir such an important and interesting topic of internal, for allowing me to choose. Study of Refrigeration and its Working and Principle and for providing me with all the necessary guidelines for preparing my reports as this project has project has sharpen my knowledge and help me a lot to know about Refrigeration. I would again like to thanks our HOD, Prof. (Dr) Aryandra Kumar Jouhari, for making us prepare such a report that will help groom our knowledge and command over our subjects.
Thanks
(IV)
CONTENTS
1:-REFRIGERATION
Refrigeration is a process in which work is done to move heat from one location to another.
It is the process of removing heat from an enclosed space or from a substance for the purpose of lowering the temperature. This process keeps an item below room temperature by storing the item in a system or substance designed to cool or freeze.
The process of refrigeration is used for many day-to-day applications. It is used to chill water, make ice and ice cream, keep the food fresh and a host of other purposes. It helps to preserve fruits, vegetables and other food items. The food items get damaged at atmospheric temperature because bacteria can easily survive in these conditions.
The simplest form of refrigeration observed in daily life is the use of ice. Ice absorbs heat from the surroundings and melts. During this process the surrounding air becomes cool due to the loss of heat the capacity of refrigeration is expressed in tonne.
Fig 1:- Refrigerator
(VI)
2:-INTRODUCTION
Refrigeration is a process of removing heat from a low-temperature reservoir and transferring it to a high-temperature reservoir. The work of heat transfer is traditionally driven by mechanical means, but can also be driven by heat, magnetism, electricity, laser, or other means. Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, and air conditioning.
Refrigeration also known the process of removing heat from surface to purpose of lowering the temperature.
Heat Reject
Work Input
Heat Absorb
Fig 2:- How it work
(VII)
3: PRINCIPLE AND CONSTRUCTION OF REFRIGERATION
The refrigeration cycle also known as vapour compression cycle. The cycle operates at two pressures high and lo, to produce a continuous cooling effect.
The working principle of refrigeration is same as working principle of air conditioner. Such that consider that a room temperature maintain at constant temperature of 25 degree centigrade. In the air conditioner, the air from the room is drawn by a fan and is made to pass over a cooling coil, the surface of which is maintained, say, at a temp of 10 degree centigrade. After passing over the coil, the air is cooled before being supplied at the room. After picking up the room heat, the air is again returned to the cooling coil at 25 degree centigrade.
Fig: components
The basic principle of refrigeration is simple. You simply pass a colder liquid continuously around the object that is to be cooled. This will take heat from the object. In the example shown, a cold liquid is passed over an apple, which is to be cooled. Due to the temperature difference, the apple loses heat to the refrigerant liquid. The refrigerant in turn is heated due to heat absorption from the apple.
(VIII)
When the gas enters at a temp say 5 degree centigrade and evaporates, thus absorbing this latent heat of vaporization from the room air. This equipment which the refrigerant evaporate is called an evaporator.
After evaporation, the refrigerant becomes vapour. To enable it to condense back and to release the heat—which it absorbed from the room while passing through the evaporator – its pressure is raised by compressor. Following this, the high pressure vapour enter the condenser. In the condenser the outside atmospheric air, say, at a temp of 45 degree centigrade in summer, is circulate by a fan. After picking up the latent heat of condensation from the condensing refrigerant, the air is let out into the environment say temp 55 degree centigrade. the condensation of refrigerant may occur say temp 60 degree centigrade.
After condensation, the high pressure liquid refrigerant is reduced to the low pressure of the evaporator by passing it through a pressure reducing device called the expansion device. Thus the cycle of operation is completed.
4:- COMPONENTS OR TYPES OF REFRIGERATOR
There are four important parts of a refrigerator: compressor, condenser, expansion valve and evaporator. Freon (monochloro-difluoro methane) gas passes through all these parts continuously in the form of a cycle and undergoes various phase and temperature and pressure transformations. Freon acts as the media for transfer of heat.
There are various parts of the refrigerator. Freon gas passes through these parts and undergoes various phase transitions. The flow of this gas through various parts of the refrigeration system is called a refrigeration cycle.
The main four components of refrigeration are given below:-
i) Compressor: Compressor is the heart of the refrigeration system. The power required for transmitting heat from low temperature space to high temperature space is given here. When Freon gas passes through a compressor it gets highly compressed i.e. pressurized and its temperature also becomes very high. As it leaves the compressor, Freon gets converted into the gaseous state.
ii) Condenser: All the heat the Freon has absorbed from the substance at low temperature is thrown out to the atmosphere via the condenser. For household refrigerators, the condenser is usually a coil of copper exposed to the atmosphere. When Freon passes though this coil it gives up its heat partially to the atmospheric or surrounding air and its temperature reduces, but its pressure remains unchanged. In case of the bigger refrigerators water cooled condensers are used.
iii) Expansion valve: When the high pressure and low temperature Freon passes through expansion valve its pressure reduces suddenly and along with it its temperature also reduces suddenly and drastically. In the household refrigerator usually capillary, a thin copper tube is
(XI)
used as the expansion valve. Freon leaving the expansion valve is partially in liquid state and partially in gaseous state.
iv) Evaporator: The freezer section of the refrigerator is the evaporator which is in the form of various coils of copper or aluminium tubing. Here the foods or the substances which are initially at higher temperature are kept for cooling. When the low temperature and low pressure Freon passes through the evaporator, it chills the freezer space and food items kept here. It absorbs the heat from the substances to be chilled and so its temperature rises as does its pressure. It leaves the evaporator or freezer in the vapour state and then enters into the compressor where the cycle is repeated.
5:- Basic Refrigeration System Fundamentals – Matter and Heat Behaviour
A). STATES OF MATTER
All known matter exists in one of three physical forms or states: solid, liquid, or gaseous. There are distinct dissimilarities among these physical states namely:
• Matter in a liquid state will retain its quantity and size but not its shape. The liquid will always conform to the occupying container. If a cubic foot of water in a container measuring 1 foot on each side is transferred to a container of different rectangular dimensions, the quantity and volume of the water will be the same although the dimension will change.
• Matter in solid state will retain its quantity, shape, and physical dimensions. A cubic foot of wood will retain its weight, size, and shape even if moved from place to place.
• Matter in gaseous state does not have a tendency to retain either its size or its shape. If a one foot cylinder containing steam or some other gas is connected to a 2-cubic foot cylinder on which a vacuum has been drawn, the vapour will expand to occupy the volume of the large cylinder. Although these specific differences exist in the three states of matter, quite frequently, under changing conditions of pressure and temperature, the same substance may exist in any one of the three states, such as a solid, a liquid, or vapour (ice, water, and steam, for example). Solids always have some definitive shape, whereas liquids and gases have no definitive shape of their own, but will conform to the shape of their containers.
B). MOLECULAR MOVEMENT
All matter is composed of small particles known as molecules, for the present we will concern ourselves only with the molecule, the smallest particle into which any matter or substance can be broken down and still retain its identity. Molecules vary in shape, size, and weight. In physics we learn that molecules have a tendency to cling together. When heat energy is applied to a substance it increases the internal energy of the molecules, which increase their
(X)
motion or velocity of movement. With this increase in the movement of the molecules, there is also increase in the temperature of the substance. When heat is removed from a substance, it follows that the velocity of the molecular movement will decrease and also that there will be a decrease of the internal temperature of the substance.
C). CHANGE OF STATE
Solidification_:- The temperature at which this change of state in a substance takes place is called its melting point. Let us assume that a container of water at 70 deg F, in which a thermometer has been placed, is left in the freezer for hours. When it is take from the freezer, it has become a block of ice or ice cubes- solidification has taken place.
Liquefaction: – If it is allowed to stand at room temperature, heat from the room air will be absorbed by the ice until the thermometer indicates a temperature of 32 deg F, when some of the ice will begin to change into water or become water. With heat continuing to transfer from the room air to the ice, more ice will change back into the water; but the thermometer will continue to indicate a temperature a temperature of 32 deg F until when all the ice has been melted. Liquefaction has now taken place.
Vaporization : -As mentioned, when all the ice is melted, the thermometer will indicate a temperature of 32ºF, but the temperature of the water will continue to rise until it reaches or equals room temperature. If sufficient or more heat is added to the container of water through outside means such as a burner or any other, the temperature of the water will increase until it reaches 212 degree F, at this temperature, and under "standard" atmospheric pressure, another change will take place – vaporization.
D:-TEMPERATURE CONVERSION
Most frequently a conversion from one temperature scale to other is made by the use of a conversion table, but if one is not available, the conversion can be done easily by a formula using these equations:
• Deg. F = 1.8 ºC + 32
Deg. F = 5/9 ºC + 32
• Deg. C = (ºF – 32)/1.8
Deg. C = 5/9(ºF – 32)
E:-SECOND LAW OF THERMODYNAMICS According to second law of thermodynamic, the second law of thermodynamics, states that heat transfer in only one direction –downward; and this takes place through one of the three basic methods of heat transfer.
(XI)
A. Conduction,
B. Convection,
C. Radiation.
A: – Conduction: – Conduction is described as the transfer of heat between closely-packed molecules of the substance, or between substances that are contact with one another. When the transfer of heat occurs in a single substance, such as a metal rod with one end in a flame, movement of heat continues until there are is a temperature balance throughout the length of the rod.
B: – Convection: – Another means of heat transfer is by motion of the heated material itself and is limited to liquid or gas. When a material is heated, convection currents are set up within it, and the warmer portions of it rise, since heat brings about the decrease of a fluid's density and an increase of its specific volume.
Air within a refrigerator and water being heated in a pan are prime examples of the result of convection currents. The air in contact with the cooling coil on a refrigerator becomes cool and therefore denser, and begins to fall to the bottom of the refrigerator. In doing so, it absorbs heat from the product and the walls of the refrigerator, which, through conduction, has picked up heat from the room.
C: -Radiation: – Al low temperatures there is only a small amount of radiation, and only minor temperature differences are noticed; therefore radiation has very little effect in actual process of refrigeration itself. But results of radiation from direct solar rays can cause an increased refrigeration load in a building air conditioning system. Radiant heat is readily absorbed by dark or dull materials or substances, whereas light-colured surfaces or materials will reflect radiant heat waves, just as they do light rays.
When radiant heat or energy (since all heat is energy) is absorbed by a material or substance it is converted into sensible heat – that which can be felt or measured. Everybody or substance absorbs radiant energy to some extent, depending upon the temperature difference between the specific body or substance and other body or substances. Every substance will radiate energy as long as its temperature is above absolute zero and another substance within its proximity is at a lower temperature.
(XII)
F: – REFRIGERATION EFFECT – "TON"
A common term that has been used in refrigeration work to define and measure capacity or refrigeration effect is called a ton of refrigeration. It is the amount of heat absorbed in melting a tone of ice (2,000 lb) over a 24-hour period.
The ton of refrigeration is equal to 288,000 Btu. This may be calculated by multiplying the weight of ice (2,000 lb) by the latent heat of fusion (melting) of ice (144 Btu/lb).
Thus
2.000 lb x 144 Btu/lb = 288,000 Btu
In 24 hours or 12,000 Btu per hour (288,000 / 24). Therefore, one ton of refrigeration = 12,000 Btu/hr.
Fluid is "any substance that can flow, liquid or gas." Refrigerant may be classified as fluid, since, so we can say that within the refrigeration cycle, it exists both as a liquid and as a vapour or gas.
6: -FLUID PRESSURE
Fluid pressure is the force per unit area that is exerted by gas or a liquid. It is usually expressed in terms of Psi. It varies directly with the density and the depth of the liquid, and, at the same depth below the surface, the pressure is equal in all directions. Force means the total weight of the substance; pressure means the unit force or pressure per square inch.
Pressure = Force/Area or P = F/A
7: – PASCAL'S PRINCIPLE
Pascal’s Law says that if you apply pressure to a fluid (liquid or gas) in a confined container, the fluid will apply the same pressure in all directions. This is why hydraulic systems work. Refrigeration systems also operate on this principle. It is also state that: –
“Pascal's law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container”.
Fig: – Example of Pascal Principle
(XIII)
• DENSITY
Density is the weight per unit volume of a substance, and it may be expressed in any convenient combination of units of weight and volume used, such as pounds per cubic inch. The equation is given below:
D = W/V
Where: D = density; W = weight; V = volume
• SPECIFIC VOLUME
In thermodynamics, the specific volume of a substance is the ratio of the substance's volume to its mass. It is the reciprocal of density and an intrinsic property of matter as well. Specific volume is defined as the number of cubic meters occupied by one kilogram of a particular substance.
• Atmospheric Pressure
The pressure exerted by the weight of the atmosphere, which at sea level has a mean value of 101,325 Pascal (roughly 14.6959 pounds per square inch).
• PRESSURE OF GAS
The volume of gas is affected by a change in either the pressure or temperature, or both. There are laws that govern the mathematical calculation in computing these variables.
Boyle's Law : -states that volume of a gas varies inversely to its pressure if the temperature of the gas remains constant. This means that the product of the pressure times the volume remains constant, or that if the pressure of the gas doubles the new volume will be one-half of the original volume. Or it may be considered that, if the volume is doubled, the absolute pressure will be reduced by one-half.
This concept may be expressed as:
p1V1 = p2V2
Where: p1 = original pressure; V1 = original volume; p2 = new pressure; V2 = new volume.
It must be remembered that p1 and p2 have to be expressed in the absolute pressure terms for the above equation to be used correctly.
EXPANSION OF GAS: –
Most gases will expand in volume at practically the same rate with an increase in temperature, providing that the pressure does not change. And, if the gas is confined so that its volume will remain the same, the pressure in the container will increase at about the same rate as an increase in temperature.
(XIV)
Charles' Law: –
According to Charles law or it states that the volume of gas is in direct proportion to its absolute temperature, providing that the pressure is kept constant; and the absolute pressure of gas is in direct proportion to its absolute temperature, providing the volume is kept constant.
As the temperature increase the volume increase because the faster molecules collide harder and push each other farther apart. That is:-
V1/V2 = T1/T2
And
P1/P2 = T1/T2
Where: T = absolute temperature; P = absolute pressure
Or these may be expressed also as: V1T2 = V2T1 and P1T2 = P2T1
8: – The Compression Cycle
THE COMPRESSOR
The compressor does exactly as its name says: it compresses the refrigerant. The compressor receives low pressure gas from the evaporator and converts it to high pressure gas. As mentioned earlier, as the gas is compressed, the temperature rises. The hot refrigerant gas then flows to the condenser.
The compressor is the mechanical heart of a refrigeration system. It causes refrigerant to flow and is where energy is applied to perform the work of removing heat in the evaporator. The compressor serves two functions. First, it regulates pressure in the evaporator by withdrawing refrigerant vapours when pressure (or temperature) is higher than desired. By regulating pressure, the evaporating temperature is fixed. Second, it compresses the gas and in doing so, adds energy or heat content to the gas.
Heat can flow from refrigerant to condensing medium only if there is a reasonable difference in temperature. The compressor's function is to provide that higher temperature. At the same time, an increase in gas density reduces the volume that needs to be handled in the condenser.
Compression of gas is nearly adiabatic or isentropic. This means that essentially all mechanical energy applied is converted into energy that is retained by the gas. In other words, mechanical energy is changed into heat energy, which results partly in an increase in temperature and partly in a change in molecular velocities. Both are reflected by an increase in pressure and decrease in volume. In an actual compressor, the gas is not insulated from its surroundings and some heat of compression is lost by conduction through metal walls of the cylinder, while at the same time heat is added from the friction of piston rings, bearings and other sources.
(XV)
9: – What is a chiller? The Principles of Basic Refrigeration
A chiller is simply a device that used to remove heat from something. For industrial purposes, chillers can be thought of as a component within a complex mechanical system that is used to remove heat from a process or substance. To really understand what a chiller is, a fundamental knowledge of the principles of basic refrigeration is required. Welcome to Berg's School of Cool.
Before getting into the fundamentals of refrigeration, a few basic definitions should be considered:
A). Heat is a form of energy transferred by virtue of a difference in temperature. Heat exists everywhere to a greater or lesser degree. As a form of energy it can be neither created or destroyed, although other forms of energy may be converted into heat, and vice versa. It is important to remember that heat energy travels in only one direction; from a warmer to a cooler object, substance, or area.
B). Cold is a relative term referring to the lack of heat in an object, substance, or area. Another definition describes it as the absence of heat, no process yet has been devised of achieving "absolute zero," the state in which all heat has been removed from any object, substance, or area. Theoretically this zero point would be 459.69 degrees below zero on the Fahrenheit thermometer scale, or 273.16 degrees below zero on the Celsius thermometer scale.
C). Refrigeration, or cooling process, is the removal of unwanted heat from a selected object, substance, or space and its transfer to another object, substance, or space. Removal of heat lowers the temperature and may be accomplished by use of ice, snow, chilled water or mechanical refrigeration.
D). Mechanical refrigeration, is the utilization of mechanical components arranged in a "refrigeration system" for the purpose of transferring heat.
E). Refrigerants, are chemical compounds that are alternately compressed and condensed into a liquid and then permitted to expand into a vapour or gas as they are pumped through the mechanical refrigeration system to cycle.
The refrigeration cycle is based on the long known physical principle that a liquid expanding into a gas extracts heat from the surrounding substance or area. (You can test this principle by simply wetting your finger and holding it up. It immediately begins to feel cooler than the others, particularly if exposed to some air movement. That's because the liquid in which you dipped it is evaporating, and as it does, it extracts heat from the skin of the finger and air around it).
(XVI)
10: – METHODS OF REFGRIGERATION
Methods of refrigeration can be classified as non-cyclic, cyclic, thermoelectric and magnetic.
• Non-cyclic refrigeration
This refrigeration method cools a contained area by melting ice, or by sublimating dry ice. Perhaps the simplest example of this is a portable cooler, where items are put in it, and then ice is poured over the top. Regular ice can maintain temperatures near, but not below the freezing point, unless salt is used to cool the ice down further (as in a traditional ice-cream maker). Dry ice can reliably bring the temperature well below freezing.
• 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, and its inverse, the thermodynamic power cycle. In the power cycle, heat is supplied from a high-temperature source to the engine, part of the heat being used to produce work and the rest being rejected to a low-temperature sink. This satisfies the second law of thermodynamics.
A refrigeration cycle describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a refrigerator. It is also applied to heating, ventilation, and air conditioning HVACR work, when describing the "process" of refrigerant flow through an HVACR unit, whether it is a packaged or split system.
Heat naturally flows from hot to cold. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. Insulation is used to reduce the work and energy needed to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by Sadi Carnot in 1824 as a heat engine.
The most common types of refrigeration systems use the reverse-Rankin vapour-compression refrigeration cycle, although absorption heat pumps are used in a minority of applications.
Cyclic refrigeration can be classified as:
1. Vapour cycle, and
2. Gas cycle
Vapour cycle refrigeration can further be classified as:
1. Vapour-compression refrigeration
2. Vapour-absorption refrigeration
(XVII)
S.no Aspect Vapour Absorption System Vapour Compression System
1 Quality of the Energy Input Low grade energy sources are more than capable of running a vapour absorption system. These sources can be waste heat from furnaces, exhaust steam etc. Solar power can also be used for running it.
Vapour compression system needs high grade energy. It needs electrical or mechanical energy for operating compressor which is an essential part of VC refrigeration system.
2 Moving part in the system The only moving part of Vapour absorption refrigeration system is the pump. In Vapour compression the moving part is the compressor which operated by electric motor or engine .
3 Effect of Evaporator pressure Very little effect is seen in the refrigeration capacity with the lowering evaporator pressure. The refrigerating effect or refrigeration capacity decreases with the lowering evaporator pressure.
4 Evaporator exit In vapour absorption system, if the liquid refrigerant leaves the evaporator, the refrigerating effect is reduced but the system functions well without any problem. Liquid refrigerant entering compressor is not desirable in Vapour compression system. It could damage the compressor. So the refrigerant is superheated before leaving the evaporator.
5 Lowest temperature When water is used as refrigeration the temperature attained is above 0 degree Celsius -150 degree Celsius or even lower can be achieved with the cascading system.
6 Coefficient of Performance The COP of absorption refrigeration system is poor. The COP of Vapour compression system is very good.
7 Capacity Capacity above 1000 TR is easily achievable. It is difficult to capacity above 1000 TR with single compression system.
8 Refrigerant Ammonia or water can be used as refrigerant with a proper absorber. Hydrocarbons, Chlorofluorocarbons and Hydro chlorofluorocarbons are used as refrigeration’s.
11: – Comparison between vapour absorption and vapour compression refrigeration systems
(XVIII)
12: – Selection Criteria of Refrigerants:
There is no rule of selecting a refrigerant. However, there a basic five criteria should be taken while selecting refrigerant: –
• Thermophysical properties.
• Technological issues.
• Economic aspects.
• Safety.
• Environmental factors.
In addition to these criteria, other considerations such as; local regulations and standards, maintainability and capability (such as having staff with skills to support the units). User training requirements should be taken into account.
The desirable characteristics of ‘ideal’ refrigerants are considered to be as follows:
• Normal boiling point below 0°C.
• Non-flammable.
• Non-toxic.
• Easily detectable in case of leakage.
• Stable under operating conditions.
• Easy to recycle after use.
• Relatively large area for heat evaporation.
• Relatively inexpensive to produce.
• Low environmental impacts in case of accidental venting.
• Low gas flow rate per unit of cooling at compressor.
Evaluation Of Refrigerant Alternatives: –
Established by the Air-Conditioning and Refrigeration Institute (ARI), the Alternative Refrigerants Evaluation Program (AREP) was directed by an executive committee comprised of senior executives from ARI member companies and was focused primarily on identifying possible alternatives to R-22 and R-502 refrigerants. As part of the program, tests were conducted with 19 refrigerants identified as potential replacements for R-22. Individual test reports issued included compressor calorimeter, system drop-in, heat transfer, and soft-optimized system tests for most of these refrigerants.
AREP also tested many types of compressors, including reciprocating, rotary, screw, and scroll compressors. In addition, system performance was evaluated across a range of applications, including split-system heat pumps, both air- and water-cooled packaged heat pumps, window units, and condensing units. More than 180 AREP reports were approved and released to the public when the committee completed its testing in 1997.
(XIX)
Types of Available Refrigerant: –
Commercial organisations have traditionally used an R12, CFC, R502, or CFC/HCFC blend. To enable the government targets to be achieved most manufacturers have adopted R404A, an HFC blend, or R134a. However, these are potent greenhouse gases.
An alternative, and one of the future solutions might be 'natural' refrigerants, but this may require some design changes to air conditioning and refrigeration equipment. The most common natural refrigerants and their characteristics are shown below:
Fig:-
Selection of refrigerant for a particular application is based on the following requirements:
i. Thermodynamic and thermo-physical properties
ii. ii. Environmental and safety properties, and
iii. Economics
(XX)
Thermodynamic and thermo-physical properties:
a) Suction pressure: At a given evaporator temperature, the saturation pressure should be above atmospheric for prevention of air or moisture ingress into the system and ease of leak detection. Higher suction pressure is better as it leads to smaller compressor displacement
b) Discharge pressure: At a given condenser temperature, the discharge pressure should be as small as possible to allow light-weight construction of compressor, condenser etc.
c) Pressure ratio: Should be as small as possible for high volumetric efficiency and low power consumption.
d) Liquid specific heat: Should be small so that degree of sub cooling will be large leading to smaller amount of flash gas at evaporator inlet
e) Vapour specific heat: Should be large so that the degree of superheating will be small
Environmental and safety properties:
a) Ozone Depletion Potential (ODP): According to the Montreal protocol, the ODP of refrigerants should be zero, i.e., they should be non-ozone depleting substances. Refrigerants having non-zero ODP have either already been phased-out (e.g. R 11, R 12) or will be phased-out in near-future (e.g. R22). Since ODP depends mainly on the presence of chlorine or bromine in the molecules, refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be used under the new regulations
b) Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming. Refrigerants with zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future.
c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming. Naturally, refrigerants with as a low a value of TEWI are preferable from global warming point of view.
(XXI)
d) Chemical stability: The refrigerants should be chemically stable as long as they are inside the refrigeration system.
e) Compatibility with common materials of construction (both metals and non-metals)
Economic properties:
The refrigerant used should preferably be inexpensive and easily available.
(XXII)
13:- Transitional Refrigerants (HCFCs)
R-22 has been successfully applied in refrigeration systems of all sizes and temperatures; however, R-22 is an HCFC that is currently being phased out as part of the Montreal Protocol. Phase-out dates for the production of R-22 vary by country, but in the U.S. and Canada, new equipment can no longer be manufactured using R-22 after 2010.
14: – Chlorine-Free Refrigerants (HFCs)
The selection and approval of acceptable long-term refrigerants is a complex and time-consuming task. Many factors must be taken into consideration. The ever-shifting legislative environment, the phase-out of CFCs and HCFCs, the availability of alternate refrigerants, and numerous other factors are just a few of the issues that must be taken into account.
HFCs, or hydro fluorocarbons, are chemicals used in air conditioning and refrigeration applications. They are non-flammable, recyclable, highly effective, energy-efficient refrigerants of low toxicity that are being used safely throughout the world. HFCs were developed by the chemical industry as alternatives to ozone-depleting CFCs, which are being phased out under the Montreal Protocol, a landmark environmental agreement.
15: – Chlorofluorocarbons (CFCs): Refrigerants that Cause Ozone Layer Depletion
Chlorofluorocarbons (CFCs) have been found to cause the depletion of the ozone layer. The ozone layer prevents ultraviolet radiations of the sun from coming on the earth. This article describes the process of damaging effects of CFCs.
Chlorofluorocarbons (CFCs) have been used extensively in last five or six decades as refrigerants in the vapour compression cycle to produce refrigerating and air-conditioning effects. In recent years it has been found that CFCs are most destructive to the environment. It has been proved that CFCs are a major cause of depletion of the earth’s stratospheric ozone layer and contribute to the greenhouse effect (global warming).
How CFCs Destroy Ozone Layer
When the CFC refrigerants are leaked from refrigeration or air-conditioning systems, they drift around the lower layers of the atmosphere. Slowly they start infiltrating into the upper layers of the atmosphere and soon reach the ozone rich stratosphere, where they undergo major chemical changes.
(XXIII)
16: – Applications of Refrigeration
a) Food processing, preservation and distribution
b) Chemical and process industries
c) Special Applications such as cold treatment of metals, medical, construction, ice skating etc.
d) Comfort air-conditioning
(XXIX)