Refrigeration & air conditioning

Principle and working of vapour compression refrigeration, SYLLABUS Refrigeration. Definition –refrigeration effect-Ton of refrigeration, Ice making capacity -COP, relative COP, Units of refrigeration. Principle and working of vapour compression refrigeration, vapour absorption refrigeration. Principles and applications of air conditioners.

DEFINITIONS REFRIGERATION: It is a method of reducing the temperature of system below that of the surroundings and maintain it at the lower temperature by continuously abstracting the heat from it. AIR CONDITIONING: Providing a cool indoor atmosphere at all times regardless of weather conditions needed either for human comfort or industrial purposes by artificially cooling, humidifying or dehumidifying ,cleaning and recirculation the surrounding air is called air conditioning.

CONCEPTS OF REFRIGERATION Heat transfer is possible from a high temperature region to a low temperature region. Heat transfer is possible from a lower temperature system to higher temperature surroundings by some external means as per the 2nd law of thermodynamics. The working fluid changes from vapour phase to liquid phase after heat rejection and from liquid phase to vapour phase after heat absorption. The change of phase of the working fluid from liquid phase to vapour phase results in cooling effect.

PRINCIPLE OF REFRIGERATION It is based on 2nd law of Thermodynamics. As per Clausius Statement, heat cannot flow from a body at lower temperature to a body at higher temperature unless assisted by some external means. In refrigeration, heat is continuously removed from the system at a lower temperature and same heat is rejected to the surroundings at a higher temperature. This is done by using an external source like a compressor (or) a pump. Heat from a system at lower temperature is abstracted by using a working fluid/medium called refrigerant.

PARTS OF A REFRIGERATOR Condenser Evaporator Expansion device Compressor

To compress and circulate the low temperature and low pressure working fluid into high temperature and high-pressure vapour. They are power absorbing mechanical devices and need input power. An electrical motor supplies power to these drives. 1. COMPRESSOR (OR) PUMP:

2. EVAPORATOR It has cooling coils arranged in form of U – tubes 2. EVAPORATOR It has cooling coils arranged in form of U – tubes. The function of the evaporator is to reduce the temperature of the refrigerator cabinet. The low temperature two phase mixture of refrigerant passing through the evaporator coils absorbs heat from the cabinet and changes into vapour phase. This effect of cooling is also known as refrigerating effect

3. CONDENSER Condenser consists of a series of coils in the form of U – tubes. The high pressure, high temperature refrigerant from the compressor enters condenser Where the refrigerant rejects its heat to the surrounding atmosphere. The latent heat of the refrigerant is given to the surrounding atmosphere, which results in change of phase of the refrigerant.

4. EXPANSION VALVE: The high pressure and temperature liquid refrigerant expands in the expansion valve to low pressure & low temperature two-phase mixture. The temperature of the refrigerant drops in the expansion valve due to partial evaporation

REFRIGERATION EFFECT In a refrigeration system ,the rate at which the heat is absorbed in a cycle from the interior space to be cooled is called refrigerating effect. Capacity of refrigeration system is expressed in ton of refrigeration

TON OF REFRIGERATION A ton of refrigeration is defined as the quantity of heat absorbed in order to form one ton of ice in 24 hours when the initial temperature of the water is 0° 1 ton of refrigeration= 210 KJ/min = 3.5 KW

C O P The performance of a refrigeration system is expressed by a factor known as the co-efficient of performance It is the ratio of heat absorbed in a system to the work supplied Q=Heat absorbed or removed (KW) W=Work supplied (KW) C O P = Q / W

REFRIGERANT In a refrigerator, a medium called refrigerant continuously extracts the heat from the space within the refrigerator which is to be kept cool at temperatures less than the atmosphere and finally rejects to it.

PROPERTIES OF REFRIGERANTS Thermodynamic properties 1. Boiling point – low 2. Freezing point –low 3. Latent heat of evaporation - Very high

PROPERTIES OF REFRIGERANTS Physical properties 1. Specific volume – low 2. Specific heat – low 3. Viscosity – low

PROPERTIES OF REFRIGERANTS Safety working properties 1.Toxity-non toxic 2.Flammability-non flammable 3.Corrosiveness-non corrosive 4. chemical stability - non reactive

PROPERTIES OF REFRIGERANTS Other properties 1. C O P – high 2. Action with lubricating oil - non reactive

LIST OF COMMONLY USED REFRIGERANTS AMMONIA (NH3) -in vapour absorption refrigeration CARBONDIOXIDE(CO2) -In marine refrigeration SULPHUR DIOXIDE(SO2): - In house hold refrigerator METHYL CHLORIDE - In small scale refrigeration and domestic refrigeration FREON-12 -In domestic vapour compression refrigerators FREON-22 - In air conditioners

Types of refrigeration systems 1. Air refrigeration system: Air is used as a refrigerant to absorb heat from the low temperature sink and reject the same into the high temperature source

2. VAPOUR COMPRESSION SYSTEM Refrigerant vapour is compressed in a compressor, condensed in a condenser, expanded in the expansion value and finally evaporated in the evaporator

VAPOUR COMPRESSION REFRIGERATION

1.Evaporator or refrigerated chamber 2.compressor Components 1.Evaporator or refrigerated chamber 2.compressor 3.condenser or cooler 4.Throttle valve or pressure reducing valve.

Vapour Compression refrigeration systems Refrigerant vapour absorbs heat from the cooling cabinet and rejects the same heat in the condenser. Here the refrigerant alternately undergoes a change of phase from vapour to liquid and liquid to vapour during the cycle. The latent heat of vaporization is used for absorbing the heat at low temperature from the refrigerated space and the same heat is rejected during the condensation at high pressure

3. VAPOUR ABSORPTION SYSTEM: A combination of an absorber and a refrigerant is used. The absorber absorbs the refrigerant solution and gives up the vapour refrigerant when heated.

VAPOUR ABSORPTION REFRIGERATOR PRINCIPLE In vapour absorption refrigeration system, we choose a combination of a refrigerant and solvent in such a way that the solvent absorbs large quantities of vapour refrigerant at low temperatures and gives off vapour when heated to higher temperature. Ammonia and water are such a combination.

Comparison between Vapour Compression and Absorption refrigerators Sl no Principle Vapour compression system Vapour absorption system 1 Working method Refrigerant vapour is compressed Refrigerant vapour is absorbed and heated 2 Mechanical energy supplied To run compressor To run pump 3 Cop Cop is higher Cop is lower 4 Noise Noise is more due to the presence of compressor Almost quiet in operation 5 Refrigerant Freon 12 Ammonia

Air-Conditioning: Providing a cool indoor atmosphere at all times regardless of weather conditions needed either for human comfort or industrial purposes by artificially cooling, humidifying or dehumidifying ,cleaning and recirculation the surrounding air is called air conditioning. Principle An air conditioner draws air from an indoor space to be cooled, cools it by the refrigeration principles and discharges back into the same indoor space that needs to be cooled.