92214951 Refrigeration and Air Conditioning

7/28/2019 92214951 Refrigeration and Air Conditioning

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Refrigeration

SEA TRAINING PRESENTATION

MARINE ENGINEERING DEPARTMENT

POLITEKNIK PREMIER UNGKU OMAR

KUNA RAO BULAK NAIDU

01DKP09F2018


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INTRODUCTION

Refrigeration may be defined as the process ofremoving heat from a substance under controlledconditions and reducing and maintaining thetemperature of a body below the temperature of itssurroundings by the aid of external work.

In a Refrigerator, power is to be supplied toremove the heat continuously from the refrigeratorcabinet to keep it cool at a temperature less than

the atmospheric temperature.

A medium called Refrigerant continuously extractsthe heat from the space within the refrigerator whichis to be kept cool at temperatures less than theatmosphere and finally rejects to it to the


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Parts of a Refrigerator

To accomplish the task of producing the

cooling

effect, a refrigerator must consist of thefollowing

main parts,

1.Evaporator

2.Compressor

3.Condenser


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Condenser

Evaporator

High Pressure

Side

Low

Pressure

Side

CompressorExpansion Device

1 2

3

4

Refrigeration cycle


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In the evaporator (heart of the refrigerator)liquid refrigerant is evaporated by the

absorption of heat from the refrigerator cabinet

in which the substances which have to be

cooled are kept. The evaporator consists of simply metal tubing

which surrounds around the freezing and

cooling compartments to produce the cooling

effect required for freezing ice or lowering thetemperature of perishables placed in the

cooling compartment.

Since it produces the cooling effect it is also

sometimes called as cooling coil or freezer coil.

1. Evaporator


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Condenser

Evaporator

High Pressure

Side

Low

Pressure

Side


1 2

3

4

Low pressure liquid refrigerant

in evaporator absorbs heat and

changes to a gas


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The circulating system comprises of the mechanical

devices such as compressors or pumps necessary to

circulate the refrigerant to undergo the refrigeration

cycle.

They increase the pressure and therefore, the

temperature of the refrigerant.

Generally these devices are driven by the electricmotors. The electrical energy input to the motor is the

energy input to the refrigerators.

2. Compressor


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Condenser

Evaporator

High Pressure

Side

Low

Pressure

Side


1 2

3

4

The superheated vapour enters

the compressor where its

pressure is raised


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A condenser is an appliance in which the heat from

the refrigerant is rejected at higher temperature to

another medium, usually the atmospheric air.

In a condenser the refrigerant vapour gives off itslatent heat to the air and consequently condenses

into liquid so that it can be recirculated in the

refrigeration cycle.

The latent heat of the refrigerant that is given off inthe condenser comprises mainly of the heat absorbed

in the refrigerator cabinet and the heat developed due

to compression.

3.

Condenser


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Condenser

Evaporator

High Pressure

Side

Low

Pressure

Side


1 2

3

4

The high pressure

superheated gas is cooled

in several stages in the

condenser


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An expansion valve serves as a device to reduce the

pressure and temperature of the liquid refrigerant

before it passes to the evaporator.

The liquid refrigerant from the condenser is passed

through an expansion valve where it reduces its

pressure and temperature.

4. Expansion Valve


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Condenser

Evaporator

High Pressure

Side

Low

Pressure

Side


1 2

3

4

Liquid passes through expansion

device, which reduces its

pressure and controls the flow

into the evaporator


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Refrigeration Effect and Unit of

Refrigeration

In a refrigeration system, the rate at which the heat is

absorbed in a cycle from the interior space to be cooled

is called refrigeratingeffect.

The capacity of a refrigeration system is expressed in

tons of refrigeration which is the unit 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 0C.

In S.I. System,1 Ton of Refrigeration = 210 kJ/min

= 3.5 kW


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Coefficient of Performance

The performance of a refrigeration system isexpressed by a factor known as the coefficient ofperformance.

The COP of a refrigeration system is defined as

the ratio of heat absorbed in a system to the work supplied.

IfQ = Heat Absorbed or Removed (kW), W = Worksupplied, kW

Then,COP=Q/W


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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.

Some of the fluids like, Ammonia, Freon, Methyl

Chloride, Carbon dioxide are the commonly used

refrigerants.


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Refrigerants Commonly Used in

Practice

The most commonly used refrigerants are : Ammonia - in vapour absorption refrigerator.

Carbon dioxide - in marine refrigerators.

Sulphur dioxide - in household refrigerators.

Methyl chloride - in small scale refrigeration and

domestic refrigerators.

Freon -12 - in domestic vapour compression

refrigerators. Freon-22 - in Air Conditioners.


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1. Ammonia

Ammonia as a refrigerant is employed in refrigerators

operating on the absorption principles. Because of its

high latent heat (1300 kJ/kg at -15C) and low

specific volume (0.509mVkg at -15C) it produces

high refrigeration effects even in small refrigerators.

Since ammonia will not harm the ozone, it is

environmental friendly. It is widely used in cold

storage, ice making plants, etc.

Its toxic, flammable, irritating and food destroyingproperties makes it unsuitable for domestic

refrigerators.


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2. Carbon dioxide

The efficiency of the refrigerators using carbon dioxide

refrigerant is low. Therefore it is rarely used in domesticrefrigerators, but is used in dry ice making plants.

It is colourless, odourless, non-toxic, non inflammable andnon-corrosive.

3. Sulphur dioxide Earlier sulphur dioxide was one of the most commonly

used refrigerants in domestic refrigerators. Although it hasbetter thermodynamic properties, it has low refrigeratingeffect and high specific volume, therefore large capacity

high speed compressors are required. Since it combines with water and forms sulfurous and

sulfuric acids which are corrosive to metals, therefrigerators using sulphur dioxide as refrigerant areseldom used.


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4. Methyl Chloride Methyl chloride was used earlier in domestic and small

scale industrial refrigerators. Since it will burn under someconditions and slightly toxic, is not generally used.

5. Freon Freon group of refrigerants is used almost universally in

domestic refrigerators. These refrigerants are colourless, almost odourless, non-

toxic, non-inflammable, non-explosive and non-corrosive,Freon-12 and Freon-22 are the two freon refrigerantscommonly used in domestic refrigerators and air

conditioners. Although these refrigerants are being now used

extensively in the refrigerators and the air conditioners, ithas been found that these refrigerants posing a majorthreat to the global environment through their role in the

destruction of the ozone layer.


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1. Boiling Point

An ideal refrigerant must have low boilingtemperature at atmospheric pressure.

2. Freezing Point

An ideal refrigerant must have a very low freezing

point because the refrigerant should not freeze atlow evaporator temperatures.

3. Evaporator and Condenser Pressure

In order to avoid the leakage of the atmospheric air

and also to enable the detection of the leakage ofthe refrigerant, both the evaporator and condenser

pressures should be slightly above the atmospheric

pressure.


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4. Latent Heat of Evaporation

The latent heat of evaporation must be

very high so that a minimum amount of

refrigerant will accomplish the desired

result, in other words, it increases the

refrigeration effect.5. Specific Volume

The specific volume of the refrigerant

must be very low. The lower specificvolume of the refrigerant at the suction

of the compressor reduces the size of

the compressor.


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6. Specific heat of liquid and vapour

A good refrigerant must have low specific heat when it is

in liquid state and high specific heat when it isvapourised. The low specific heat of the refrigerant

helps in sub-cooling of the liquid an high specific heat of

the vapour helps in decreasing the superheating of the

vapour. Both these desirable properties increase therefrigerating effect.

7. Viscosity

The viscosity of a refrigerant at both the liquid and

vapour states must be very low as improves the heattransfer and reduces the pumping pressure.

8. Non-toxicity refrigerant

A good refrigerant should be non-toxic, because any

leakage of the toxic refrigerant increase suffocation and


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9. Corrosiveness

A good refrigerant should be non-corrosiveto prevent the corrosion of the metallic

parts of the refrigerators.

10. Chemical StabilityAn ideal refrigerant must not decompose

under operating conditions.

11. Coefficient of Performance The coefficient of performance of a

refrigerant must be high so that the energy

spent in refrigeration will be less.


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12. Odour

A good refrigerant must be odourless; otherwise somefoodstuff such as meat, butter, etc. loses their taste.

13. Leakage Tests

The refrigerant must be such that any leakage can be

detected by simple tests.

14. Action with Lubricating Oil

A good refrigerant must not react with the lubricating

oilused in lubricating the parts of the compressor.


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Types of Refrigeration Systems

The mechanical refrigeratorsystems are

classified as :

1. Air Refrigerator

2. Vapour Compression Refrigerator

3. Vapour Absorption Refrigerator


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1. Vapour compression

refrigerator


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Vapour compression refrigerator

In a vapour compression refrigerator, vapour is

used as the refrigerant. It is circulated through the system in which it

alternately evaporates and condenses thus

undergoing a change of phase from vapour to

liquid and again liquid to vapour. During evaporation it absorbs the latent heat from

the refrigerated space and subsequently gives off

heat while condensing.

A vapour compression system makes use of

mechanical energy supplied to the compressor to

run the refrigerator.


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It consists of an evaporator made of coiled tubesinstalled in the freezing compartment of the

refrigerator and connected to the suction side of thecompressorand a throttle valve.

The delivery side of the compressor is connected to acondenser which in turn is connected to a throttlevalve.

The object of including a compressor in this system isto draw the vapours from the evaporator andcompress them to higher pressures so that thesaturation temperature corresponding to thesepressures is higher than that of the cooling mediumflowing into the condenser, so that the high pressurevapour can reject heat into the condenser and beready to expand to the evaporator pressure again.

In order to maintain the interior of the refrigerator

within the desired temperature range, the motordrivin the com ressor is controlled b a thermostat


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The refrigerant at low pressure and temperature

passing in the evaporator coiled tubes absorbs the

heat from the contents in the freezing compartmentand evaporates.

This in turn lowers the temperature in the freezing

compartment. The evaporated refrigerant at low

pressure from the evaporator, is drawn by acompressor which compresses it to higher pressures

so that the saturation temperature of the refrigerant

corresponding to the increased pressure is higher

than the temperature of the cooling medium(atmospheric air) in the condenser, so that the high

pressure and high temperature vapours can reject

heat in the condenser and be ready to expand in the

throttle valve to the lower evaporator pressures


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The high pressure-high-temperature refrigerant

vapourfrom the compressor flows to the condenserwhere it gives off its latent heat to the atmospheric

air. As a result of the loss of latent heat in the

condenser, the refrigerant condenses.

The high pressure condensed liquid refrigerantapproximately at room temperature now flows to the

throttle valve in which it expands to a low pressure

and then passes to the evaporator coils for

recirculation once again. The throttling expansion ofthe refrigerant lowers its pressure and temperature

and at the same time causes it to partly evaporate.


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Hence the refrigerant coming out of the

expansion valve will be a very wet vapour and ata very low temperature which will be around -

10C. This wet vapour now passes to the

evaporator coils where it absorbs its latent heat

and then recirculated to repeat the cycle

continuously.

Thus, heat is continuously extracted by the

contents of the refrigerator in the evaporator andrejected in the condenser to the atmospheric air.


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This will keep the contents of the refrigerator at

the required lower temperature. The required lowtemperature is maintained in the refrigerator by a

thermostat switch which switches on and off the

compressor motor by a relay as and when the

temperature either falls below, or rises above therequired temperature.

One of the most commonly used refrigerants in

the vapour compression refrigerator is

dichlorodifluoromethane, popularly known asFreon 12, or R12. This refrigerant vapourises at -

6.7C in the evaporator under a pressure of 246.2

kPa and after compression to 909.2 kPa would

condense at 37.8 C in the condenser.

2 Vapour absorption refrigerator


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2. Vapour absorption refrigerator


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Vapour absorption refrigerator

A vapour absorption system makes use ofthe ability of a substance, called absorbent

to absorb large volumes of the vapour of a

refrigerant even when cold and reduce it toa liquid, and subsequently give off its

vapours when heated.

Water which has this ability is the mostly

used absorbent, and since ammonia

readily dissolves in water and vapourises

when its solution is heated is the

commonly used refrigerant in the vapour


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In this refrigerator, the ammonia liquid

vapourises in the evaporator coils

absorbing the latent heat from thefreezing compartment thus keeping it

cool and subsequently gives off heat

when it condenses in a condenser. Thenammonia liquid from the condenser is

heated in a heater to vapourise it.

Thus the absorption system makes useof heat energy to change the state of the

refrigerant required in the cycle. A pump

is used to circulate the refrigerant in the


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It consists of an absorber, a circulation pump, heat

exchanger, heater cum separator, condenser,

expansion valve and evaporating coiled tubes.

Dry ammonia vapour is dissolved in the cold water

contained in the absorber, which will produce a

strong ammonia solution.

A circulation pump, draws the strong ammonia

solution from the absorber and pumps it to the heat

exchanger, where it is warmed by the warm weak

ammonia solution which is flowing back from theheater-separator.

From the heat exchanger, the warm high pressure

strong ammonia solution is passed to the heater-

cum-separator provided with the heating coils.


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The heating coils in the heater-separator

heats the strong ammonia solution. Heating of

the high pressure strong ammonia solutionwill drive out the ammonia vapour from it and

consequently the solution in the heater-

separator becomes weak which in turn flows

back to the heat exchanger where it warms up

the strong ammonia solution passing through

it.

The high pressure ammonia vapour from theheater-separator now passes to a condenser,

where it is condensed. The high pressure

ammonia liquid is now expanded to a low

pressure and low temperature in the throttle


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The low pressure condensed ammonia

liquid at low temperature is passed onto

the evaporator coils provided in thefreezing compartment, where it absorbs

the heat and evaporates.

The low pressure ammonia vapour fromthe freezing compartment is passed again

to the absorber where it is reabsorbed by

dissolving in water.

The strong low pressure ammonia

solution from the absorber is again

recirculated to repeat the cycle

continuousl .


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Ai C diti i (A/C)


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Air - Conditioning (A/C)


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wall

Room Air-conditioning


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It mainly consists ofan evaporator, condenser,compressor, two fans one each for the

evaporator and condenser units usually drivenby the single motor, capillary, etc.

It is generally mounted on a window sill such that

the evaporator unit is inside the room and the

condenser part projecting outside thebuilding.

The high-pressure, low-temperature liquidrefrigerant from the condenser is passed to the

evaporator coils through the capillary tubewhere it undergoes expansion.

The low-pressure, low temperature liquidrefrigerant passes through the evaporator coils.


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The evaporator-fan continuous draws the air from

the interior space with in the room through an air

filter by forcing it to pass over the evaporator

coils.

The air from the interior passing over the

evaporator coils is cooled by the refrigerant whichconsequently evaporates by absorbing the heat

from the air.

The high-temperature evaporated refrigerant

from the evaporator is drawn by the suction of thecompressor which compresses it and delivers it to

the condenser.

The high-pressure, high-temperature

refrigerant vapour now flows through the


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The condenser-fan draws the atmospheric air

from the exposed side-portions of the airconditioner which is projecting outside the

building into the space behind it and discharges

to pass through the centre section of the

condenser unit over the condenser coils.The high-pressure, high-temperature

refrigerant passing inside the condenser coilscondenses by giving off the heat to the

atmospheric air. The cooled high-pressure refrigerant from the

condenser passes through the capillary tube

where it undergoes expansion and is again re-

circulated to repeat the cycle continuously.


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Humidity and its control in Air Conditioning

Humidity is defined as the moisture content

present in the atmosphere. The atmosphere always contains some moisture

in the form of water vapour. The maximum

amount depends on the atmospheric conditions.

The amount of vapour that will saturate the airincreases with a rise temperature.

For example, at 4C, 1000 kg of moist air

contains a maximum of 4.4 kg of water vapour. At

38C, the same amount of moist air contains a

maximum of 18 kg of water vapour. As is evident

that when the atmosphere is saturated with water,

the level of discomfort is high because the

evaporation of perspiration.


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Humidity can be specified in three different ways.

The absolute humidity is defined as the,weight of water vapour contained in a givenvolume of air. It is expressed in grams of watervapour per cubic metre of air.

The specific humidity is defined as the ratio ofweight of water vapour to the total weight ofair. It is expressed in grams of water vapour perkilogram of air.

The relative humidity is defined as the ratio ofthe actual vapour content of the air to thevapour content of air at the same, temperaturewhen saturated with water vapour.


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The temperature-humidity index (THI), also called

discomfort index, expresses in numerical valuesthe relationship between comfort or discomfort

temperature and humidity. It provides an apparenttemperature, or how hot the air feels.

For example, an air temperature of 38C and relative

humidity of 60 percent produces an apparent very

hot temperature or THI, or 54C. It is felt that THI

index of 20C provides a comfortable atmosphere.


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Central Air Conditioning

Centralised air conditioning systems, widely

employed in theatres, offices, stores, restaurants,public buildings, etc, provide the controlled

atmosphere by heating, cooling and ventilation.

The centralised air conditioning systems include

refrigerating units, blowers, air ducts and aplenum chamber in which the air from the interior

of the building is mixed with outside air.

In such installations, cooling and dehumidifying

are done during summer months and regular

heating systems are used during winter.

92214951 Refrigeration and Air Conditioning

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