refrigeration ppt

DEFINITION:DEFINITION:

IS THE PROCESS OF PRODUCING AND IS THE PROCESS OF PRODUCING AND MAINTAINING A TEMPERATURE OF A SPACE LOWER MAINTAINING A TEMPERATURE OF A SPACE LOWER THAN THAT OF SURROUNDING BY THE REMOVAL OF THAN THAT OF SURROUNDING BY THE REMOVAL OF HEAT.HEAT.

High Temperature Reservoir

Low Temperature Reservoir

R Work Input

Heat Absorbed

Heat Rejected

UNIT OF REFRIGERATIONUNIT OF REFRIGERATION

The unit of refrigeration is Tonnes of The unit of refrigeration is Tonnes of Refrigeration.Refrigeration.

It is the amount of heat removed from 1 ton It is the amount of heat removed from 1 ton (1000 kg) of pure water supplied at 0(1000 kg) of pure water supplied at 000C to form C to form at 0at 000C in 24 hours.C in 24 hours.

This heat is the latent heat of pure water (334.5 This heat is the latent heat of pure water (334.5 X 1000 kJ/day).X 1000 kJ/day).

1 TR = 334.5 X 1000 /24 = 14000 KJ/hr1 TR = 334.5 X 1000 /24 = 14000 KJ/hr = 233 KJ/min = 233 KJ/min

= 3.889 KW= 3.889 KW

Co-efficient of Performance:Co-efficient of Performance:

It is the ratio of net refrigerating effect to It is the ratio of net refrigerating effect to the work required to produce that effect.the work required to produce that effect.

Refrigeration effect (or) TRefrigeration effect (or) T22

COP = COP = ------------------------------------------------------------------------------------------------------------------------------

Work Required (OR) ( TWork Required (OR) ( T11 – T – T22))

Where,Where,

TT22 = Sink Temperature = Sink Temperature

TT11= Source Temperature= Source Temperature

Heat and Work Interactions in refrigeration Heat and Work Interactions in refrigeration systemssystems

Applications of Refrigeration:Applications of Refrigeration:

Food and milk PreservationsFood and milk Preservations

Ice FormationIce Formation

Comfort and industrial air conditioningComfort and industrial air conditioning

Storage of liquid fuels in rocketsStorage of liquid fuels in rockets

Treatment of metals in processing Treatment of metals in processing industries.industries.

Processing of Beverages and Processing of Beverages and TransportationTransportation

of food below freezing.of food below freezing.

Medical and surgical aids especially in Medical and surgical aids especially in preserving human bloods and tissues.preserving human bloods and tissues.

RefrigerantRefrigerant: : is the working substance in Refrigeration systemsis the working substance in Refrigeration systems

Properties of ideal Refrigerant:Properties of ideal Refrigerant:

It should have It should have low boiling pointlow boiling point and and low freezing pointlow freezing point.. It must have It must have low specific heatlow specific heat and and high latent heathigh latent heat It should have It should have high thermal conductivityhigh thermal conductivity to reduce the heat transfer to reduce the heat transfer in evaporator and condenser.in evaporator and condenser. It should have It should have low specific volumelow specific volume to reduce the size of the compressor. to reduce the size of the compressor.It should be It should be non- flammable, non- explosive, non- toxic, and non- non- flammable, non- explosive, non- toxic, and non- corrosivecorrosive..It should have It should have high critical pressure and temperaturehigh critical pressure and temperature to avoid large to avoid large power requirementspower requirements

It should give It should give high COPhigh COP to reduce the running cost of the system. to reduce the running cost of the system.It must be It must be cheapcheap and must be and must be readily availablereadily available..

Refrigerant are Ammonia, Sulpher-di-Oxide, Freon -11, Freon -12, Refrigerant are Ammonia, Sulpher-di-Oxide, Freon -11, Freon -12, Carbon-di-Oxide, etc., Carbon-di-Oxide, etc.,

R12 = CClR12 = CCl22FF2 2 = Dichloro –Diflouro Methane= Dichloro –Diflouro MethaneR22 = CHClR22 = CHCl22FF22 = Dichloro – Diflouro Methane = Dichloro – Diflouro Methane

Types of Refrigeration cycles:Types of Refrigeration cycles:

Vapour Compression Refrigeration Vapour Compression Refrigeration CycleCycle

Vapour absorption Refrigeration cycleVapour absorption Refrigeration cycle

Vapour Compression Refrigeration CycleVapour Compression Refrigeration Cycle

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side

CompressorExpansion Device

1 2

3

4

T – S diagram for Vapour Compression T – S diagram for Vapour Compression

Refrigeration CycleRefrigeration Cycle

Schematic diagram and T-S Cycle Schematic diagram and T-S Cycle

P-H diagram for VCR P-H diagram for VCR systemssystems

Vapour Compression Refrigeration system:

The refrigerant undergoes a change of

phase from vapour to liquid and liquid to

vapour during the cycle.

In condenser, the latent heat of

refrigerant is removed which is used for

absorbing the heat from the cold body in

the evaporator, and hence the constant

temperature would be maintained.

Vapour compression Refrigeration System:IT CONSISTS OF 1. Compressor

Compresses the refrigerant to high pressure and temperature from low pressure and temperature.

2. CondenserThe latent heat of refrigerant is removed by circulating water or atmospheric air.

3. Expansion ValveIn the throttle valve where the pressure is reduced at a controlled rate.

4. EvaporatorA liquid-Vapour mixture refrigerant then enters the evaporator at low pressure where the latent heat of evaporation is converted into vapour and the cycle repeats.

Determination of COP:

Assumptions made for drawing T-S and P-h Diagram:

1. The refrigerant leaving the evaporator is dry and saturated.2. The compression of vapour in the evaporator is isentropic.3. There is no sub cooling of the refrigerant in the condenser.4. There is no pressure losses in the system.

Conditions of vapour at the end of compression:

There are three different conditions at which the refrigerant from the compressor.

a. Vapour is dry and saturated.b. Vapour is wet condition.c. Vapour is superheated condition.

a. Vapour is dry and saturated condition:Process 1-2 – Adiabatic compressionProcess 2-3 - Constant pressure condensationProcess 3-4 - ThrottlingProcess 4-1 - Constant pressure evaporation.

Compression Compression workwork

CondenserCondenser

EvaporatorEvaporator

ThrottlinThrottlingg

TT

SS

11

2233

44

Determination of COP:Determination of COP:

COP = RE/WDCOP = RE/WD

Where,Where,

RE = Refrigeration EffectRE = Refrigeration Effect

= h= h11 – h – h44

WD = Work done = hWD = Work done = h11 – h – h22

Therefore,Therefore,

COP = (hCOP = (h11 – h – h44)/(h)/(h11 – h – h22))

= (h= (h11 – h – h33)/(h)/(h11 – h – h22))

b. Vapour being Wet condition:Process 1-2 – Adiabatic compressionProcess 2-3 - Constant pressure condensationProcess 3-4 - ThrottlingProcess 4-1 - Constant pressure evaporation.



Where,Where,


= h= h11 – h – h44



COP = (hCOP = (h11 – h – h44)/(h)/(h11 – h – h22))

= (h= (h11 – h – h33)/(h)/(h11 – h – h22))

Where,Where,

hh22 = h = hf2f2+X+X22 h hfg2 , fg2 , also, also,

hh44 = h = h3 3 (Since throttling (Since throttling process)process)

CondenserCondenser



SS

11

2233

44


TT

c. Vapour being Super heated condition: Process 1-2 – Adiabatic compression

Process 2-3 - Constant pressure condensation Process 3-4 - Throttling Process 4-1 - Constant pressure evaporation.



Where,Where,


= h= h11 – h – h44



COP = (hCOP = (h11 – h – h44)/(h)/(h11 – h – h22))

= (h= (h11 – h – h33)/(h)/(h11 – h – h22))

Where,Where,

hh22 = h = hg2g2 + C+ Cp p ln (Tln (Tsupsup – – TTsatsat) )

hh44 = h = h3 3 (Since throttling (Since throttling process)process)

SS


CondenserCondenser



TT

11

2233

44

Advantages of Vapour Compression Refrigeration System:

1. COP is better due to constant temperature 2. NO expander is present 3. Heat transfer coefficient is high. 4. Same refrigerant can be used again and again.

Sub-cooling effect:When a liquid is cooled below its saturation temperature at the given pressure, the liquid is said to be sub-cooled or under-cooled.

The difference between the saturation temperature and sub-cooling temperature at that pressure is called degree of sub-cooling.

This is done to reduce the mass of vapour during the expansion.

Vapour absorption Refrigeration System:It is a heat operated unit which uses a refrigerant that is alternatively absorbed and liberated from the absorbent.

In this system, the compressor is replaced by an absorber generator and a pump involving the less mechanical work.

Aqua – Ammonia Refrigeration Systems:This is most commonly used cycle.

Ammonia – is used as RefrigerantWater – as Absorbent.

The following components are used in this refrigeration system. Analyzer:

It removes the water vapour partially by passing strong solution into it, otherwise it will lead freezing at the throttle valve.

Rectifier:Rectifier removes the remaining water vapour by providing water cooling.

Absorber:When ammonia dissolves in water, heat is released which reduces the absorption capacity. So, Water is circulated around the absorber by spraying the cooled weak NH3, solution. This external cooling increases the absorption capacity of the weak NH3 Solution.

Lithium Bromide – Water systems:Refrigerant – Water ; Absorbent – Lithium Bromide

Lithium bromide has strong affinity for water vapour due its low vapour pressure.

It absorbs the water vapour as fast as it is released in the evaporator.

Advantages of Vapour Absorption Refrigeration Systems:

1. Less noise 2. Low maintenance cost due to very little wear. 3. System does not depend upon electric power.

4. Suitable for above 1000 TR.

Vapour Absorption Refrigeration Systems

1. Moving part is pump only. Hence

less wear and operation is quiet.2. More equipments like Absorber,

rectifier, analyzer, pump, generator, evaporator, condenser, heat exchanger are required.

3. It doesn’t depend on electricity. It can be operated using solar system also.

4. Maintenance cost is low.5. The liquid traces from the

evaporator constitute no danger.6. Automatic operation for

controlling the capacity is easy.7. Less floor space is required.8. It is not affected by load

variation.9. It can be built in capacities

above 1000 tons.

Vapour Compression Refrigeration Systems

1. Moving parts are in compressor makes more noise, wear and tear.

2. Less components like Compressor, condenser, expansion valve, and evaporator are required.

3. The compressor consumes more power.

4. Maintenance cost is high.5. The liquid traces from the

evaporator to the compressor may damage the compressor.

6. Automatic operation for controlling the capacity is difficult.

7. More space is required.8. The system has poor

performance at partial load.9. Suitable for only small capacities

only.

DIFFERENT TYPES OF REFRIGERATION

INTRODUCTION:

It is the study of the properties of air – water mixture under prevailing conditions.

It is mostly used in heating and ventilating of building, ships, air crafts, and cold storages., etc.,

Terminology used in pschrometry: a. Dry air:

is the mixture of gases like N2, O2, CO2, Ar, Ne, He etc.,

major gases are 21% of O2, and 79% of N2 by volume.major gases are 23% of O2, and 77% of N2 by mass.

The molecular mass of dry air is 28.966 kg and gas constant (Ra) is equal to 0.287 kJ/kgK.

b. Moist Air:It is the mixture of dry air and water vapour.

c. Saturated air:It is the mixture of dry air and water vapour, when air contains the maximum amount of water vapour.

The vapour pressure is equal to the saturation pressure of water at the same temperature of the mixture,

Unsaturated airUnsaturated air:is the mixture of unsaturated air and water vapour.PVapour < PSaturation of water

Relative humidity (Ø): is the ratio between partial pressure of water

vapour in a mixture to the saturation pressure of pure water at the same temperature of mixture.

RH (Ø) = (Pw /Psat)

Specific Humidity (W) or Humidity Ratio: is the ratio of mass of water vapour or

moisture to the mass of the dry air in a mixture of water vapour and air.

W = mW/ma

Degree of saturation (µ):is the ratio of the actual humidity and the saturated specific humidity at the same temperature and of the mixture.

it varies from 0 to 1. W = Pw (P-Ps)/Ps (P-Pw) Dew point Temperature: (Tdp)

The temperature at which the water vapour starts condensing is called dew point temperature.

is a measure of specific humidity.

is equal to saturation temperature at the partial pressure of water vapour in the mixture.

Dry bulb Temperature: (DBT) The temperature recorded by the

thermometer with a dry bulb. is the measure of sensible heat. The DBT cannot affected by the

moisture present in the air.Wet Bulb Temperature: (WBT) The temperature recorded by the

thermometer when its bulb is covered by cotton wick (wet) saturated with water.

is measure of enthalpy of air. is the lowest temperature recorded

by the moistened bulb.For saturated conditions of air,

the DBT, WBT, DPT of air are equal.

Dew point Depression:

is the difference between dry bulb temperature and dew point temperature of the air mixture.

DPD = DBT - DPT

PSYCHROMETRIC PROCESSES:

a. Sensible heating or cooling Process

b. Cooling and dehumidification process

c. Heating and humidification process

DBTDBT

WW

aaaa11

bb

CC

Satu

ratio

n lin

e

Satu

ratio

n lin

e

Air conditioning systems:

is defined as the assembly of different parts of a system which is meant for producing a specified conditions of air within a required space or building.

An ideal air - conditioning system should maintain correct temperature, humidity, air purity, air movement and noise level.

AIR CONDITIONING SYSTEMS

Types of air conditioning systems: Human comfort conditions as 240C and 60% RH.

a. Summer air conditioning systems

In summer, the atmospheric air is high temperature at low relative humidity, hence it has to be cooled and humidified it to achieve require condition.

b. Winter air conditioning systemsIn summer, the atmospheric air is low temperature at high relative humidity, hence it has to be heated and dehumidified it to achieve require condition.


refrigeration ppt

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