B.tech i eme u 4 r & ac

Course : B.Tech Mechanical

Subject : Elements of Mechanical Engineering

Unit-4

Introduction

Principle & Application of Refrigeration

Definitions of refrigeration

Refrigerants and refrigerators

Type of refrigeration systems

Air conditioning

Windows and splits air conditions

Refrigeration & Air Conditioning

Refrigeration

The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions.

It also includes the process of reducing heat & maintaining the temp. of a body below the general temp. of its surroundings.

In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temp. of its surroundings.

Refrigeration Systems

The mechanism used for lowering or producing low temp. in a body or a space, whose temp. is already below the temp. of its surrounding, is called the refrigeration system.

Here the heat is being generally pumped from low level to the higher one & is rejected at high temp.

Introduction

How does it work?

High Temperature Reservoir

Low Temperature Reservoir

R Work Input

Heat Absorbed

Heat Rejected

Principle of Refrigeration

In chemical industries, for separating and liquefying the gases.

In manufacturing and storing ice.

For the preservation of perishable food items in cold storages.

For cooling water.

For controlling humidity of air manufacture and heat treatment of steels.

For chilling the oil to remove wax in oil refineries.

For the preservation of tablets and medicines in pharmaceutical industries.

For the preservation of blood tissues etc.,

For comfort air conditioning the hospitals, theatres, etc.,

Refrigerating Effect (N): It is defined as the quantity of heat extracted from a cold body or space to be cooled in a given time.

N= Heat extracted from the cold space

Time taken

Specific Heat of water and ice : It is the quantity of heat required to raise or lower the temperature of one kg of water (or ice), through one kelvin or (10 c) in one second.

Specific heat of water, Cpw = 4.19 kJ/kg K

Specific heat of ice, Cpice = 2.1 kJ/kg K.

Capacity of a Refrigeration Unit : Capacity of a refrigerating machines are expressed

by their cooling capacity. The standard unit used for expressing the capacity

of refrigerating machine is ton of refrigeration. One ton of refrigeration is defined as, “the quantity

of heat abstracted (refrigerating effect) to freeze (into ice) one ton of water in a duration of 24 hours at 0o c”.Heat extracted from at oo c = latent heat of ice Latent heat of ice = 336 kJ/kg

i.e., 336 kJ of heat should be extracted one kg of water at 0o C to convert it into ice.

One ton of refrigeration= 336x1000 kJ/24 hrs.

= 336x1000 kJ/min

24x60One ton of refrigeration = 233.333 kJ/min

= 3.8889 kJ/sec

For calculation purpose,One ton of refrigeration = 12600 kJ/hr

= 210 kJ/min

Ton of refrigeration = 3.5 kJ/s

Performance of Refrigerators (Co efficient of Refrigerators) :

The performance of heat engine is expressed by its thermal efficiency.

The performance of a refrigerator cannot be expressed in terms of efficiency.

In case of a refrigerator the aim is to extract maximum quantity of heat from the sink with minimum of work input.

Hence a new term Co efficient of Performance is brought into use to express the performance of refrigerator.

Co efficient of Performance: It is defined as the ratio of heat extracted in a given time (refrigerating effect) to the work input.

Co efficient of performance = Heat extracted in evaporator

Work Input

Co efficient of performance = Refrigerating Effect

Work Input

Co efficient of performance = N

W

The COP is always greater than 1 and known as theoretical coefficient of performance.

Any substance capable of absorbing heat from another required substance can be used as refrigerant i.e. ice ,water, brine, air etc.

Primary Refrigerant

Refrigerants:

Secondary Refrigerant

Refrigerants

Primary Refrigerant

They take part directly in refrigerating systems.

Freon Ammonia , Carbon dioxide, Sulphurdioxide etc.

Secondary Refrigerant

First they are cooled by primary refrigerants then they are circulated .

Chilled water, brine solutions etc.

Refrigerants

Properties of Refrigerants Low Boiling Point Low Freezing Point. High Latent Heat. Chemically Inert & stable . Non Flammable Non toxic Should not react with lubrication oil of

comp. Should not be corrosive

Refrigerants

Important refrigerants Refrigerant is the working fluid used in refrigerators Commonly used refrigerants are:

1.Ammonia : used in ice plants, large cold storages, skating rings etc.

2.Carbon dioxide : used in marine refrigerators, dry ice making etc.

3.Methyl chloride: used in domestic & industrial refrigerators (now obsolate)

4.Fluorocarbons : Freon -11,12,13,22,113,114, etc.

These are used in domestic & industrial application. These chemicals damage the ozone layer and hence they are being replaced.

Refrigerants

Properties of Ideal Refrigerants An ideal refrigerant should satisfy the

following thermodynamic , chemical & physical requirements:

Thermodynamic properties. 1. low boiling point 2. high critical temperature & low critical

pressure. 3.Freezing point should be lower than the

refrigeration system temperature to avoid blocking of pipes.

Refrigerants

Properties of Ideal Refrigerants

Thermo physical properties.

1. low viscosity.

2. high thermal conductivity.

3.high latent heat of vaporization.

4.low specific volume.

5.low specific heat.

Refrigerants

Chemical requirements of Ideal Refrigerants

1. Should be non inflammable.

2. It should not be toxic.

3. Should be non corrosive to metals.

4.Should be capable of mixing well with lubricating oil.

5.Electric resistance should be high.

6.Leakage should be detected easily.

7.Should be odourless.

Refrigerants

Ice Refrigerators : Ice is kept in the cabinet of refrigerators and this acts as the refrigerating means.

Air Refrigerators : Air is used as working agent in these types of refrigerators.

E.g., Bell Coleman Cycle.

Vapour Refrigerators: The working agents employed in this type of refrigerators are ammonia, CO2, SO2, freons etc.,

Types of Refrigeration

• Vapour Compression Refrigeration (VCR): uses mechanical energy

• Vapour Absorption Refrigeration (VAR): uses thermal energy

Refrigeration systems


Vapour Compression Refrigeration

• Highly compressed fluids tend to get colder when allowed to expand

• If pressure high enough

•Compressed air hotter than source of cooling

•Expanded gas cooler than desired cold temperature



Two advantages

• Lot of heat can be removed (lot of thermal energy to change liquid to vapour)

• Heat transfer rate remains high (temperature of working fluid much lower than what is being cooled)



Refrigeration cycle

1



Type of refrigerant

• Refrigerant determined by the required cooling temperature

• Chlorinated fluorocarbons (CFCs) or freons: R-11, R-12, R-21, R-22 and R-502



Choice of compressor, design of condenser, evaporator determined by

• Refrigerant

• Required cooling

• Load

• Ease of maintenance

• Physical space requirements

• Availability of utilities (water, power)


Vapour Absorption Refrigeration

Refrigeration cycle

2

In this system compression process of vapour compression cycle is eliminated. Instead of that the following three processes are carried out.

1. Absorbing ammonia vapour into water.

2. Pumping this solution to a high pressure cycle

3. Producing ammonia vapours from ammonia solution by heating.

Vapour Absorption Refrigeration

Construction:

The vapour absorption system consists of a condenser, an expansion valve and an evaporator.

They perform the same as they do in vapour compression method.

In addition to these, this system has an absorber, a heat exchanger, an analyser and a rectifier.

Working:

1. Dry ammonia vapour at low pressure passes in to the absorber from the evaporator.

2. In the absorber the dry ammonia vapour is dissolved in cold water and strong solution of ammonia is formed.

3. Heat evolved during the absorption of ammonia is removed by circulating cold water through the coils kept in the absorber.

4. The highly concentrated ammonia (known as Aqua Ammonia) is then pumped by a pump to generator through a heat exchanger.

Working:6. In the heat exchanger the strong ammonia

solution is heated by the hot weak solution returning from the generator to the absorber.

7. In the generator the warm solution is further heated by steam coils, gas or electricity and the ammonia vapour is driven out of solution.

8. The boiling point of ammonia is less than that of water.

9. Hence the vapours leaving the generator are mainly of ammonia.

Working:

9. The weak ammonia solution is left in the generator is called weak aqua.

10. This weak solution is returned to the absorber through the heat exchanger.

11. Ammonia vapours leaving the generator may contain some water vapour.

12. If this water vapour is allowed to the condenser and expansion valve, it may freeze resulting in chocked flow.

13. Analyser and rectifiers are incorporated in the system before condenser.

Working:14. The ammonia vapour from the generator

passes through a series of trays in the analyser and ammonia is separated from water vapour.

15. The separated water vapour returned to generator.

16. Then the ammonia vapour passes through a rectifier.

17. The rectifier resembles a condenser and water vapour still present in ammonia vapour condenses and the condensate is returned to analyser.

18. The virtually pure ammonia vapour then passes through the condenser.

Working:

19. The latent heat of ammonia vapour is rejected to the cooling water circulated through the condenser and the ammonia vapour is condensed to liquid ammonia.

20. The high pressure liquid ammonia is throttled by an expansion valve or throttle valve.

21. This reduces the high temperature of the liquid ammonia to a low value and liquid ammonia partly evaporates.

22. Then this is led to the evaporator.

23. In the evaporator the liquid fully vaporizes.

Working:

24. The latent heat of evaporation is obtained from the brine or other body which is being cooled.

25. The low pressure ammonia vapour leaving the evaporator again enters the absorber and the cycle is completed.

26. This cycle is repeated again to provide the refrigerating effect.

S.No.

Vapour Compression System Vapour Absorption System

1 This system has more wear and tear and produces more noise due to the moving parts of the compressor.

Only moving part in this system is an aqua pump. Hence the quieter in operation and less wear and tear

2. Electric power is needed to drive the system

Waste of exhaust steam may be used. No need of electric power

3. Capacity of the system drops rapidly with lowered evaporator pressure

Capacity of the system decreases with the lowered evaporative pressure, by increasing the steam pressure in generator.

4. At partial loads performance is poor.

At partial loads performance is not affected.

5. Mechanical energy is supplied through compressor

Heat energy is utilised

6. Energy supplied is ¼ to ½ of the refrigerating effect

Energy supplied is about one and half times the refrigerating

S.No.

Vapour Compression System Vapour Absorption System

7. Charging of the refrigerating to the system is easy

Charging of refrigerant is difficult

8. Preventive measure is needed, since liquid refrigerant accumulated in the cylinder may damage to the cylinder

Liquid refrigerant has no bad effect on the system.

Layout:

3

Air Conditioning

• It is the mechanism to maintain the atmosphere of an enclosed space at a required temp, humidity and purity

• Air conditioning is the science which deals with the supply and maintaining desirable internal atmospheric condition irrespective of external condition.

• Refrigeration system is at heart of AC system

Types Used:

•Self-contained

•Refrigerant circulating

•Chill water circulating

Classification of Air Conditioning

A) According to purpose. i) comfort air conditioning system ii) Industrial air conditioning

B) According season of year. i) Winter air conditioning ii) Summer air conditioning iii) Year round (All weather air conditioning)

Classification of Air Conditioning

C) According to equipment arrangement i) central air conditioning ii) unitary air conditioning D) According to working substance used. i) All air system ii) chilled water system iii) Air water systemE) According to general market requirements i) Window air conditioning ii) Split air conditioning

4

This is also called room air conditioner.

This unit consists of the following.

1. A cooling system to cool and dehumidify the air involves a condenser, a compressor and a refrigerant coil.

2. A filter to any impurities in the air. The filter is made of mesh, glass wool or fibre.

3. A fan and adjustable grills to circulate the air.

4. Controls to regulate the equipment operation.

The low pressure refrigerant vapour is drawn from the evaporator to the hermetic compressor through suction pipe.

It is compressed from low pressure to the high pressure and supplied to the condenser.

It is condensed in the condenser by passing the outdoor air over the condenser coil by a fan.

The liquid refrigerant is passed through the capillary into the evaporator.

5. In the evaporator the liquid refrigerant picks up the heat from the refrigerator surface and gets vaporized.

6. A motor driven fan draws air from the room through the air filter and this air is cooled by losing its heat to the low temperature refrigerant and cold air is circulated back into the room.

7. The vapour refrigerant from the evaporator goes to the compressor from evaporator and the cycle is repeated.

8. Thus the room is air conditioned

9. The quantity of air circulated can be controlled by the dampers.

10. The moisture in the air passing over the evaporator coil is dehumidified and drips into the trays.

11. This water evaporator to certain extent and thus helps in cooling the compressor and condenser.

12. The unit automatically stops when the required temperature is reached in the room. This is accomplished by the thermostat and control panel.

5

Construction

◦ A compressor

◦ An expansion valve

◦ A hot coil (on the outside)

◦ A chilled coil (on the inside)

◦ Two fans

◦ A control unit

Working of this system will be same as window type a.c. system

Image Refrences

1- https://sp.yimg.com/ib/th?id=HN.608049691713736118&pid=15.1&P=0

2-http://engin1000.pbworks.com/f/1299970370/absorption_refrigeration_diagram.jpg

3- https://sp.yimg.com/ib/th?id=HN.607992044664326121&pid=15.1&P=0

4-http://www.hometips.com/wp-content/uploads/2012/04/window-air-conditioner-diagram-11.gif

5-https://www.growershouse.com/wordpress/wpcontent/uploads/2013/05/Mini_split_explanation_diagram.jpeg

Content References

– Elements of Mechanical Engineering by H.G. Katariya, J.P Hadiya, S.M.Bhatt , Books India Publication.

-Elements of Mechanical Engineering by V.K.Manglik, PHI

-Elements of Mechanical Engineering by R.K Rajput.

-Elements of Mechanical Engineering by P.S.Desai & S.B.Soni

https://sp.yimg.com/ib/th?id=HN.608049691713736118&pid=15.1&P=0

https://sp.yimg.com/ib/th?id=HN.607992044664326121&pid=15.1&P=0

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