Refrigeration & Air-conditioning

Refrigeration

• Refrigeration may be defined as the process of achieving and maintaining a temperature below that of the surroundings.

• To cool some product or space to the required temperature.

Classification of Refrigeration

• Natural Refrigeration

• Artificial Refrigeration

Natural Refrigeration

Use of ice or evaporative cooling. In earlier times, ice was either:• Transported from colder regions.• Harvested in winter and stored in ice houses

for summer use.• Made during night by cooling of water by

radiation to stratosphere.

Natural Refrigeration

• In olden days refrigeration was achieved by natural means such as the use of ice or evaporative cooling.

In earlier times, ice was either:

1. Transported from colder regions.2. Harvested in winter and stored in ice houses for summer use or. 3. Made during night by cooling of water by radiation to stratosphere.

Some interesting facts about refrigeration

• In India Tudor’s ice was cheaper than the locally manufactured ice by nocturnal cooling.

• North America transported to southern states of America and other countries such as Great Britain, Russia, Canada, Norway and France.

• Ice was either transported from colder regions or was harvested in winter and stored in icehouses for use in summer.

• The ice trade reached its peak in 1872 when America alone exported 225000 tonnes of ice to various countries as far as China and Australia.

Some interesting facts about refrigeration

• In Europe, America and Iran icehouses were built to store ice.

• Materials like cork, sawdust or wood shavings as insulating materials in icehouses.

• Ice only for rich people.• In India, the Mogul emperors were very fond of ice

during the harsh summer in Delhi and Agra, ice used was made by nocturnal cooling.

• In 1806, Frederic Tudor (ice king) brought it from Hudson River and ponds of Massachusetts - exported it to various countries including India.

Art of Ice making by Nocturnal or night sky Cooling

• Ice was made by keeping a thin layer of water in a shallow earthen tray and then exposing the tray to the night sky.

• Compacted hay of about 0.3 m thickness was used as insulation.

• The water looses heat by radiation to the stratosphere, which is at around -55°C to -3 °C and by early morning hours the water in the trays freezes to ice.

Evaporative Cooling

Process of reducing the temperature of a system by evaporation of water :

• Used in India for centuries to obtain cold water in summer by storing the water in earthen pots.

• By placing wet straw mats on the windows is also very common in India.

• Straw mat “khus” adds its inherent perfume also to the air.

• Desert coolers are used in hot and dry areas to provide cooling in summer.

Cooling by Salt Solutions

• Common salt, when added to water dissolve in water and absorb its heat of solution from water (endothermic process).

• Reduces the temperature of the solution (water+salt).

• NaCl can yield temperatures up to -20°C and Calcium Chloride (CaCl2) up to - 50°C in properly insulated containers.

• Has limited application, as the dissolved salt has to be recovered from its solution by heating.

Artificial Refrigeration

• Refrigeration as it is known these days is produced by artificial means.

• When the Scottish professor William Cullen made the first refrigerating machine in 1755, which could produce a small quantity of ice in the laboratory.

• Based on the working principle, refrigeration systems can be classified as vapour compression systems, vapour absorption systems, gas cycle systems etc.

Green house effect & Ozone layer and refrigerants

• Hydrochlorofluorocarbons or HCFC-22 (R-22).• Releases of R-22 is a greenhouse gas contribute to

ozone depletion. • Manufacture of R-22 results in a by-product (HFC-23)

that contributes significantly to global warming. • Manufacturing of R-22 is phased out over the coming

years as part of the agreement to end production of HCFCs.

• Manufacturers of residential air conditioning systems are offering equipment that uses ozone-friendly refrigerants.

Refrigeration cycle on T-S and T-h diagram

Heat

Sensible Heat

Latent Heat

Sensible Heat

• When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat.

• Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat.

Heat that causes a change in temperature in an object is called sensible heat. Heating water from normal temperature to boiling temp.

Latent Heat

All pure substances in nature are able to change their state.

Solids can become liquids (ice to water) and liquids can become gases (water to vapor) but changes such as these require the addition or removal of heat.

The heat that causes these changes is called latent heat

Heat that causes a change of state with no change in temperature is called latent heat. Ex water remains at 100°C while boiling.

Latent Heat of Evaporation

Latent heat of vaporization :Amount of energy absorbed or released to change liquid into vapour or vapour into liquid without changing its temperature is known as Latent heat of vaporization.

Latent heat of fusion :Amount of energy absorbed or released to melt solid into liquid or to freeze liquid into solid without changing its temperature is known as Latent heat of fusion.

Boiling Point & Pressure

• The Boiling point of a substance changes according to the pressure to which the substance is subjected.

• Boiling point increases when pressure increases and vice versa.

Unit of Refrigeration

• It is defined as heat-extraction capacity of refrigeration and air conditioning equipment.

• Defined as the heat of fusion absorbed by melting 1 ton of pure ice at 0 °C (32 °F) in 24 hours.

• A refrigeration ton is approximately equivalent to 12,000 BTU/h or 3.517 kW

Latent Heat of Evaporation

Classification of Refrigeration systems

Based on the working principle, refrigeration systems can be classified as :

• Vapour compression systems. • Vapour absorption systems. • Gas cycle systems etc.

Vapour Compression Refrigeration Systems

Refrigerants• A special substance used in refrigeration system for absorbing heat

by changing its state is known as refrigerant.

• Ammonia (R717)• Sulphur dioxide ( R764)• Methyl Cholride( R40)• Dichloro-difluro methane (Freon12)• Monochoro—difluro methane ( Freon22)• Carbon dioxide ( R744)

Dry ice, sometimes referred to as "cardice" is the solid form of carbon dioxide. It is used primarily as a cooling agent

Refrigerants

Properties of refrigerant

• Low Boiling point.• Condensing pressure must be low.• Specific volume in the gaseous state should be low

( it reduces the size of the compressor , evaporator).• Latent heat of vaporization should be high.• Critical temperature of the refrigerant should be

high. ( it is the temperature to which the boiling of the liquid can be increased by increasing pressure.

• Non Corrosive and non solvent.• Stability- Should not break.

Parts in Vapor compression cycle

• Evaporator • Compressor• Condenser• Expansion Valve

Evaporator

• It is cooling unit of the cycle.• It is placed where heat is to be removed from

the product.• The refrigerant absorbs heat within the

evaporator.• Refrigerant is in a liquid state and it is under

reduced pressure.• As refrigerant absorbs heat it boils changing to

gas.

Compressor

• It receives gaseous refrigerant from the evaporator.

• It compresses the gas which increases the pressure.

• It also adds small amount of heat to the refrigerant gas which increases its temperature.

• Increase in pressure increases refrigerants temp.

Condenser

• Gas refrigerant is cooled and condensed back to liquid.

• Its place where the heat which was absorbed at the evaporator is rejected.

• It is maintained a temperature below the condensing temperature of the high pressure gas leaving the compressor.

• Can be air cooled by natural or forced convection.• Can b water cooled also.

Expansion valve

• It reduces the pressure on the liquid refrigerant before the refrigerant is allowed to flow back to evaporator.

• Reducing the pressure also reduces the boiling point of the liquid refrigerant.

Vapour Compression Refrigeration Systems

Condenser

Evaporator

High Pressure Side

Low Pressure

Side

CompressorExpansion Device

1 2

3

4

35

Low pressure liquid refrigerant in evaporator absorbs heat and changes to a gas

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side

CompressorExpansion Device

1 2

3

4

36

The superheated vapour enters the compressor where its pressure is raised

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side

CompressorExpansion Device

1 2

3

4

37

The high pressure superheated gas is cooled in several stages in the condenser

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side

CompressorExpansion Device

12

3

4

38

Liquid passes through expansion device, which reduces its pressure and controls the flow into the evaporator

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side

CompressorExpansion Device

1 2

3

4

Refrigerator

Capacity of refrigerator?

• The volume of the space available for the things to be stored in the refrigerator ( inside ) to be cooled is know an litres capacity.

• It is the length, breadth and height calculated in cm and divided by 1000 gives the volume in litres.

DefrostingDefrosting (or thawing) is a procedure, performed periodically on refrigerators and freezers to maintain their operating efficiency. Removal excess ice !!!!

Applications of refrigeration

• Preservation of perishable food products by storing them at low temperatures.

• Providing thermal comfort to human beings by means of air conditioning.

Ice Cream

• In the Persian people would pour grape-juice, rose water , Vermicelli (sevanyia), saffron fruits, concentrate over snow, in bowl, and eat this as a treat.

• Ancient civilizations have served ice for cold foods for thousands of years.

• Frozen mixture of milk and rice was used in China around 200 BC.

Ice Cream • The Roman Emperor Nero (37–68

AD) had ice brought from the mountains and combined it with fruit toppings.

• Arabs used milk as a major ingredient in the production of ice cream and sweetened it with sugar rather than fruit juices.

• It was flavoured with rosewater, dried fruits and nuts.

Any Queries Please !

Thank You

A Tushar Kulkarni

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