Lecture 1 of Unit 1

8/3/2019 Lecture 1 of Unit 1

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Unit I

Definition:

What is Refrigeration?

According to ASRE (American Society ofRefrigeration Engineers): The science ofproviding and maintaining temperaturebelow that of the surroundingatmosphere.


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Methods of Refrigeration

1. Ice Refrigeration

2. Evaporative Refrigeration

3. Refrigeration by Expansion of Air4. Refrigeration by the Throttling of the gas

5. Vapor Compression Refrigeration

System6. Vapor Absorption Refrigeration System


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1. Ice Refrigeration

It is the oldest method. Natural ice wasstored and utilized for refrigerationpurposes. The temperature usually

between 5C to 10C was easily attainableby this methods.

To achieve temperature below 0C ice +

salt mixture was used.


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Direct Contact Method

In the direct contact method ofrefrigeration cool air directly passes overthe ice and food storage area, but in,

indirect contact method, the ice isisolated from the storage space andcooling being carried out by means of a

medium known as brine. Find out yourselves various types of brines


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Fig.1: Indirect Refrigeration usingice as refrigerant


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2. Evaporative Refrigeration

Heat is absorbed when a liquidevaporates. Evaporation of water is anexample. Evaporation of moisture from the

skin surface of a man helps to keep himcool.

Another example is, Desert Bag used tokeep drinking water cool. It consists of atightly woven fabric. The bag is not waterproof.


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2. Evaporative Refrigeration(Continued)

Consequently, some water sweepsthrough and surface of the bag remainsmoist. Under desert conditions which are

usually both hot and dry, moisture on thesurface of the bag evaporates rapidly.


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3. Refrigeration by Expansionof Air

By first law of thermodynamics, for closedsystems, we have

Q =U + W

For a reversible adiabatic process, Q= 0,

Then, Wad = -U

This means that the temperature of a gas

can be reduced by the adiabaticexpansion of the gas. This principle wasused by Bell-Coleman in


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3. Refrigeration by Expansion ofAir (Continued)

the air-refrigeration system. Numerically,let us assume that Tatm

=27C = 300 K,

And it is compressed to 5 times in thecompressor. Then, final temperature of airleaving the compressor is given by

T2 = T1(p2/p1)(-1)/

= (300)(5)0.286 = 475 K


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This high-temperature, high-pressure air iscooled in the heat-exchanger, then finaltemperature of the air (under effectiveness

of heat-exchanger = 1) will be 27C.

Now, if this air at 27C expands,

isentropically, thenT3= T1(p1/p2)

(-1)/

= (300)(1/5)0.286 = 189.5 K


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Which is below atmospheric temperature!!!


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4.Refrigeration by throttlingof the gas

It is known that when the gas passesthrough porous plug, adiabatic throttlingprocess occurs, which is a constant-

enthalpy process. As enthalpy is a functionof temperature only for ideal gases,temperature also remains constant. But

with actual gases temperature of the gascan either be increase, decrease, orremain constant.


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Fig.3


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It is known that Joule-Thomson coefficient

is given by

h

JTp

T


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In the throttling process as pressure fallsfrom point ato b, we observe thattemperature rises. If the throttling occurs

from point bto cor from point ato c,temperature drops.


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5. Vapor Refrigeration Systems

In vapor refrigeration system, instead ofair, vapor like ammonia (NH3), carbondioxide (CO2), sulpher dioxide (SO2),

etc. are used as working fluid.

Vapor refrigeration system is furtherdivided in two parts: (a) Vapor

compression refrigeration system and (b)Vapor absorption refrigeration system.


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5(a).Vapor CompressionRefrigeration Systems(Fig.4)


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5(a).Vapor CompressionRefrigeration Systems

The difference between vapor absorptionsystem and vapor compression system isin the manner in which external heat is

added to the vaporized refrigerant toincrease its thermal level aboveatmosphere which is necessary to reject

heat in the condenser. In case of vaporcompression system, the refrigerant vapor


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is socked into the compressor andcompressed, adding the energy in theform of work to increase its thermal level

above atmosphere.

In case of absorption refrigerationsystem, working fluid (refrigerant) is taken

which has high affinity to dissolve in water.

The refrigerant which is commonly used in


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absorption refrigeration system isammonia. The liquid strong in ammonia isheated further by the application of

external heat to generate vapor and itstemperature is increased aboveatmospheric temperature.


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The distinct advantage of absorptionsystem over the compression system isreduced noise due to absence of

compressor.

Lecture 1 of Unit 1

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