MODULE III Refrigeration KTUNOTES.IN Air Conditioning

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Dept of ME, UEC, Vallivattom

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MODULE III

Refrigeration & Air Conditioning

AMAL V K

Asst. Professor

UEC, Vallivattom

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Refrigeration• Refrigeration is the process of

maintaining a system at atemperature below temperatureof its surroundings.

• Equipments employed tomaintain a system at lowertemperature by removing heatfrom system is calledRefrigerating system.

• System which is kept at lowertemperature is known asRefrigerated system.

• Working fluid used inrefrigerating system is known asRefrigerant.

2Refrigeration & Air Conditioning

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Refrigeration

• Rate of heat absorbed from a body or space to becooled is termed as Refrigerating effect.

• Unit of refrigeration is ton of refrigeration.

• One ton of refrigeration is defined as amount ofrefrigerating effect obtained by melting one ton of iceat 00C to water at 00C in 24 hours.

• One ton of refrigeration= 3.52kW = 210kJ/min

• Major application of refrigeration is in Foodprocessing, Preservation & distribution, Chemical &process industries, Medical field, Comfort airconditioning, etc.


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Coefficient Of Performance

• COP is the measure of effectiveness orperformance of refrigeration system.

• It is the ratio of desired refrigeration effect towork done to produce that refrigeration effect.

• COP can be greater than 1.

COP =


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Heat Pump

• Device used tomaintain a system athigher temperature bysupplying heat tosystem is called Heatpump.

• [COP]H.P. = 1 + [COP]ref

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Vapour Compression Refrigeration System


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• Dry saturated vapour refrigerant from evaporator isdrawn into compressor at low pressure P1 &temperature T1

• Refrigerant is compressed isentropically intosuperheated state having high pressure P2 & hightemperature T2

• Refrigerant vapor at this state is then cooled in thecondenser by circulating cold water therebyremoving heat from refrigerant.

• Further removal of latent heat of condensation fromrefrigerant, makes it to condense into lowtemperature, high pressure liquid; P3,T3.


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• High pressure liquid refrigerant is then expanded inexpansion valve to low pressure, where it ispartially vapourised; T4,P4

• Low temperature, low pressure refrigerant passesthrough evaporator & absorbs latent heat ofvapourisation from the products to be cooled & getsvapourised into dry saturated vapour.

• This absorption of heat is utilised for cooling food &water.

• Dry saturated vapour again passes to compressor &cycle is repeated.


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COP = (Heat extracted in evaporator) ÷ (Work done by compressor)


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Study of household refrigerator•Refrigerator maintains atemperature, few degreesabove the freezing point ofwater.•Freezer maintains atemperature below thefreezing point of waterwith a temperature ofabout -18°C.•Further below arecompartments withprogressively highertemperatures (3°C to 5°C).


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Study of household refrigerator

• Bottom-most compartment which is meant forvegetables is the least cold one.

• Cold air being heavier flows down from freezer tobottom of the refrigerator

• Warm air being lighter rises from vegetablecompartment to the freezer, gets cooled and flowsdown again.

• Thus a natural convection current is set up whichmaintains a temperature gradient between top &bottom of refrigerator.

• In a household refrigerator, compressor is placed atthe backside & at the bottom of refrigerator.


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Study of household refrigerator• The coils behind the refrigerator, where heat is

dissipated to the kitchen air, serve as the condenser.

• Expansion of refrigerant takes place at the expansionvalve or capillary tube.

• The tubes in the freezer compartment where heat isabsorbed by the refrigerant serves as the evaporator.

• A fan blows air from the refrigerator or freezercompartment across these tubes & refrigerant absorbsheat & gets vapourised.

• Defroster is also provided which prevents build up offrost

**Working is same as Vapour Compression Refrigeration System12Refrigeration & Air Conditioning

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Energy efficiency rating• Efficiency rating of refrigerators

are given by Bureau of EnergyEfficiency (BEE)

• BEE Star Label is provided onrefrigerator which showsenergy consumption ofappliance per year.

• Greater the number of stars onthe label, higher will be energyefficiency of appliance & loweris its electricity consumption


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Energy efficiency rating


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BEE Star Label for Air Conditioner• Efficiency rating of air

conditioners are given byBureau of EnergyEfficiency (BEE)

• BEE Star Label isprovided on AC, whichindicates EnergyEfficiency Ratio, which isthe ratio of cooling outputto power input ofappliance per year.

• Greater the number ofstars on the label, higherwill be energy efficiency ofappliance & lower is itselectricity consumption


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Psychrometry

• Properties of moist air are known asPsychrometric properties

• Subject which deals with the behavior ofmoist air is called as Psychrometry.

• Moist air is a mixture of dry air & water vapor.


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Terms used in Psychrometry

• Dry air: Mixture of oxygen, nitrogen, carbon-dioxide,hydrogen, argon, neon, helium, etc., & that contains nowater vapour.

• Moist air: Ordinary air which is a mixture of dry air &water vapour.

• Saturated air: Air which contains maximum amount ofwater vapour which the air can hold at a giventemperature & pressure.

• Relative Humidity (RH): Ratio of mass of water vapourin a given volume of moist air at a given temperature tothe mass of water vapour contained in the same volumeof saturated moist air at the same temperature.

• Dry Bulb Temperature (DBT): Temperature of airmeasured by an ordinary thermometer.


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Terms used in Psychrometry

• Wet Bulb Temperature (WBT): Temperature recordedby a thermometer, when its bulb is covered by a wet cloth& is exposed to a current of moving air.

• Dew point temperature: Temperature at which thecondensation of moisture begins when the air is cooled atconstant pressure.

• Sensible heat of air: Enthalpy of dry air, which can becalculated by measuring its dry bulb temperature.

• Total heat: Sum of sensible heat of dry air and sensibleplus latent heat of water vapour present in it.

• Humidity ratio or specific humidity: Ratio of the mass ofwater vapour to the mass of dry air. It can also be definedas mass of water vapour present in per kg of dry air.


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Psychrometric processes

Sensible heating or sensible cooling

• Heating or cooling without phase change i.e., heating orcooling of air without increase or decrease of moisturecontent.

• For sensible heating the air is passed over heating coils whilefor sensible cooling the air is passed over cooling coils.

Cooling with dehumidification

• Reduction of air temperature along with removal of watervapour from air.

• Removal of moisture content can be done by reducing thetemperature of air below dew point temperature so as toallow condensation of water vapour.


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Psychrometric processes

Humidification• Process of adding moisture to the air.• Required during air conditioning when air being circulated

have only little moisture in it.• Injects steam or spray liquid water into air.Evaporator cooling• This type of cooling is needed in hot & dry climates.• This method has arrangement for spraying Liquid water into

air or passing air through a pad soaked with water.• Due to less humidity of air it shall evaporate some amount of

water in its contact and thus reduce its temperature becauseof heat extracted for evaporation of water.

• Thus air leaving evaporative cooler shall have temperatureless than inlet air temperature & also due to moisture beingpicked up, the humidity ratio gets increased.


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Psychrometric chart


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Psychrometric chart

• Psychrometric chart gives graphical representation ofdifferent important properties of moist air.

• Charts are usually available for 1 atm pressure of air.

• Various properties of air that can be read from thepsychrometric chart are dry bulb temperature, wetbulb temperature, moisture content, relative humidity,specific enthalpy and specific volume.

• If any two properties of air are known then the otherfour can be found from the psychrometric chart.

• Psychrometric charts are used extensively in air-conditioning applications.


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Psychrometric chart

• X-axis of chart gives the dry bulb temperature (°C)

• Y-axis has humidity ratio 'w' in kg or gram of watervapour per kg of dry air.

• Constant relative humidity(RH) ɸ% curves shown ashyperbolic lines are shown in intervals of 10%.

• 100% RH is called as the saturation curve, while dry airis at 0% RH.

• Specific volume is represented in the psychrometricchart by a family of equally spaced straight lines thatare nearly parallel.


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Psychrometric chart

• Enthalpy per unit mass of dry air are shown asoblique lines drawn diagonally downward fromleft to right across the chart that are parallel toeach other.

• Lines of wet bulb temperature lie very nearlyparallel to the lines of constant enthalpy lines.Therefore, constant wet-bulb temperature linesare used as constant-enthalpy lines in somecharts.

• Graduations on the saturation curve show thevalues of wet bulb temperature.


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Window air conditioner


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• Consist of two units

– Outdoor unit: Compressor, Condenser, Fan

– Indoor unit: Evaporator, fan

• Capillary tube is provided b/w condenser &evaporator

• Outdoor portion remains outside window

• Dampers are provided at front of indoor portion forchanging direction of air flow.


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• Low pressure vapour refrigerant from theevaporator is sucked by compressor & iscompressed to a high pressure & is delivered to thecondenser.

• In the condenser, the refrigerant vapour iscondensed to liquid by releasing latent heat ofcondensation to the surrounding air.

• Hot air formed is driven out using a fan.

• High pressure liquid refrigerant enters thecapillary tube where the pressure is reduced.


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• This low pressure liquid-vapour refrigerant entersthe evaporator.

• Liquid refrigerant evaporates by absorbing latentheat of vaporization from the surrounding air.

• This cold air is delivered to the room using a fan.

• Direction of air flow can be changed using adamper.

• Low pressure vapour refrigerant is again sucked bycompressor.

• Thus one cycle of operation is completed.30Refrigeration & Air Conditioning

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Split air conditioner

• In window air conditioner, the indoor & outdoorparts are kept in a single casing & the casing isdivided into two parts.

• In the split air conditioner, the unit consisting ofevaporator & fan is located inside the room & theother unit consisting of compressor, condenser &the fan can be kept anywhere outside the room.

• The indoor & outdoor units are connected byextended suction & liquid pipelines for transfer ofrefrigerants.

• System works on vapour compression cycle.


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Advantages of Split Air Conditioner


• It occupy only very smallspace in the room.

• Since the compressor iskept out side the room,the noise inside the roomdue to working ofcompressor is less

• Wide opening of wall orwindow is not required.

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Properties of Ideal Refrigerant

• Both condensing & evaporating pressure of therefrigerant should be above atmospheric pressure toavoid leakage of air into the system.

• Freezing temperature of the refrigerant should bemuch below the operating temperature to prevent thesolidification of the flow.

• Specific heat of the refrigerant liquid should be low

• Latent heat of vapourisation of the refrigerant shouldbe high

• Specific volume of the refrigerant vapour should be low


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Properties of Ideal Refrigerant

• Low viscosity

• High thermal conductivity

• Chemically stable

• Non-inflammable

• Non-corrosive

• Non-toxic

• Cheap & easily available.


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Impact of Refrigerants on environment

• Most commonly used are CFC refrigerants.

• Fully halogenated refrigerants with chlorine atomin their molecules are referred to as chloro-fluoro-carbon (CFC) refrigerants. Eg. R-11, R-12, R-13,R113, R114.

• Refrigerants which contain hydrogen atoms in theirmolecule along with chlorine & fluorine atoms arereferred to as hydro-chloro-fluro-carbon [HCFC]refrigerants. Eg. R-22, R-123 are HCFC refrigerants.


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• Refrigerants which does not contain chlorine atomin their molecules are referred to as hydro-fluorocarbon [HFC] refrigerants. Eg. R134a, R152a

• Refrigerants which do not contain chlorine &fluorine atoms in their molecules are referred to ashydro carbon (HC) refrigerants. Eg. R-290, R600a.

• Once CFC reaches the stratosphere, UV rays formthe sun break down compound, releasing chlorine.

• Chlorine deplete the ozone layer.


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• Relative ability of a substance to deplete ozone layeris known as ozone depletion potential (ODP).

• ODP of refrigerants F - 11 & F - 12 is one.

• HCFC refrigerators have a relatively low ODP.

• ODP of R-22 is 0.05 & that of HFCs are zero.

• So HFC refrigerants dont cause any ozone depletion.

• Another impact on environment is the globalwarming.


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• Ability of a substance to contribute to globalwarming is measured by the global warmingpotential (GWP).

• The GWP of R22 is 100.

• Refrigerants with high GWP is likely to bebanned in future.


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