Session – 3

Working Principle of Vapour compression refrigeration

system

Alfred Franklin VCell Number: 9965370082

Course: B.E/B.Tech (First Year)-common to all Branches.Subject: Basic Civil and Mechanical Engineering.University: Anna University Tirunelveli.

Session Objectives

At the end of this session, the learner will be able to:

Define refrigeration.Classify refrigeration systems.Explain the working principle.List the Applications.

Teaching Learning Material

Black Board and ChalkPPTAnimation

Topic Name: Working Principle of vapour compression refrigeration system. Page 2Author Name: Alfred Franklin.V College Name: St. Xavier’s Catholic College of Engineering

Session Plan

Time(in min) Content

Learning Aid and

Methodology

Faculty Approach

Typical Student Activity

Skill andCompetency Developed

10 Recap : Refrigeration

Question and Answer Facilitates Participates

Remembering/Knowledge Intrapersonal

10 Classification & Components PPT Explains Listens

Remembering/KnowledgeUnderstanding/Comprehension

15 Working Principle PPT/Animation Explains Listens

Remembering/KnowledgeUnderstanding/Comprehension

5 Merits& Demerits PPT Explains Listens

Remembering/KnowledgeUnderstanding/Comprehension

5 Applications Question and Answer Conducts Participates

ComprehensionInterpersonal/ intrapersonal

5 Conclusion Pick & speak Conducts & Facilitates Participates

ComprehensionInterpersonal/ intrapersonal

Session Inputs

Recap: Refrigeration

Topic Name: Working Principle of vapour compression refrigeration system. Page 3Author Name: Alfred Franklin.V College Name: St. Xavier’s Catholic College of Engineering

The learners are already familiar with refrigerators. The same can be recapped through a question and answer round before introducing the new topic.

Suggested Activity: Question & answer

The following questions can serve as sample questions for the activity: 1. What is refrigeration?2. What is a refrigerator?3. What is a refrigerant?4. Name some refrigerants.

What is refrigeration? The process of cooling or freezing (e.g., food) for preservative purposes

What is a refrigerator? A refrigerator is a machine that removes heat from a low temperature region to higher temperature region.

What is a refrigerant? Working substance of a refrigerator.

Type of refrigerant• Refrigerant determined by the required cooling

temperature• Chlorinated fluorocarbons (CFCs) or freons: R-11, R-

12, R-21, R-22 and R-502

Types of refrigeration:1. Vapour compression refrigeration system.

(uses mechanical energy)

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2. Vapour absorption refrigeration system. (uses thermal energy)

3. Air refrigeration system.4. Thermoelectric refrigeration.5. Magnetic refrigeration.

Unit of refrigeration

Commercial refrigerators in the US are mostly rated in tons of refrigeration (TR), but elsewhere in kW.

1 tonne of refrigeration is the rate of heat removal required to freeze a metric ton (i.e., 1000 kg) of water at 0 °C in 24 hours.

1 TR = 3.5 kW.

Components of Compression refrigeration system:

Before explaining the working principle of vapour compression refrigeration system we have to explain the components and their functions so that the students can understand the refrigeration system easily and quickly.

The four basic components of the vapour compression refrigeration system are

Compressor Condenser Throttle valve Evaporator.

1. Compressor - The compressor does work on the system increasing the pressure from that existing in the evaporator (drawing in low- pressure, low-

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temperature saturated vapour) and to that existing in the condenser (i.e., delivering high pressure and high temperature vapour to the condenser.

2. Condenser - The high pressure, high temperature (superheatedor saturated) vapour that enters the condenser has heat removed from it and as a results it is condensed back into a liquid phase.

3. Evaporator - Heat is absorbed to boil the liquid at a low temperature, therefore a low pressure must be maintained in this section.

4. Throttle valve - The high pressure liquid from the condenser is expanded through this valve, allowing its pressure to drop to that existing in the evaporator. As already mentioned, Vapor-compression refrigeration systems are the most common type of refrigerator in use today.

Working Principle of Vapour compression Refrigeration system

After explaining the components of vapour compression refrigeration system, the students can understand the working principle easily. The following figures and P-H and T-S diagrams are used for explaining the working principle.

Vapor-Compression Cycle The Vapor Compression Cycle uses energy input to drive a compressor that increases the pressure and

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pressure of the refrigerant which is in the vapor state. The refrigerant is then exposed to the hot section (termed the condenser) of the system, its temperature being higher than the temperature of this section. As a result, heat is transferred from the refrigerant to the hot section (i.e. heat is removed from the refrigerant) causing it to condense i.e. for its state to change from the vapor phase to the liquid phase (hence the term condenser). The refrigerant then passes through the expansion valve across which its pressure and temperature drop considerably. The refrigerant temperature is now below that existing in the cold or refrigerated section (termed the evaporator) of the system, its temperature being lower than the temperature in this section. As a result, heat is transferred from the refrigerated section to the refrigerant (i.e. heat is absorbed by the refrigerant) causing it to pass from the liquid or near-liquid state to the vapor state again (hence the term evaporator). The refrigerant then again passes to the compressor in which its pressure is again increased and the whole cycle is repeated.

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Processes

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• Process 4 – 1(Evaporation/Vaporization). Low-pressure liquid refrigerant in the evaporator absorbs heat from its surroundings, usually air, water or some other process liquid. During this process it changes its state from a liquid to a gas, and at the evaporator exit is slightly superheated.

• Process 1 – 2 (Compression). The superheated vapour enters the compressor where its pressure is raised. The temperature will also increase, because a proportion of the energy put into the compression process is transferred to the refrigerant.

• Process 2 – 3 (Condensation). The high pressure superheated gas passes from the compressor into the condenser, where it condensed into high pressure liquid.

• Process 3 – 4 (Expansion/Throttling). The refrigerant liquid is further throttled in the expansion device, to lower the pressure and temperature. In this process the enthalpy is constant.

Coefficient of Performance-It is a measure of the efficiency of the device.

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Merits & Demerits

The advantages and disadvantages of Vapour compression Refrigeration system is to be listed compared to other systems.

Merits1. High COP.2. Running cost is 20% of air refrigeration.3. Compact.

Demerits1. High initial cost.2. Inflammability & toxity of refrigerants.

Applications

Finally the applications of Vapour compression Refrigeration system are to be explained.

1. The refrigeration of foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in our kitchens for the storage of fruits and vegetables has allowed us to add fresh salads to our diets year round, and to store fish and meats safely for long periods.

2. Refrigeration is used to liquify gases like oxygen, nitrogen, propane and methane for example. In compressed air purification, it is used to condense water vapor from compressed air to reduce its moisture content.

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3.In oil refineries, chemical plants, and petrochemical plants, refrigeration is used to maintain certain processes at their required low temperatures (for example, in the alkylation of butanes and butane to produce a high octane gasoline component). Metal workers use refrigeration to temper steel and cutlery.

4. In transporting temperature-sensitive materials by trucks, trains, airplanes and sea-going vessels, refrigeration is a necessity.

5. Dairy products are constantly in need of refrigeration, and it was only discovered in the past few decades that eggs needed to be refrigerated during shipment rather than waiting to be refrigerated after arrival at the grocery store.

Conclusion:

To conclude this session, we can do a pick and speak activity

Suggested Activity: Pick & Speak The keywords from this topic are written on some chit and put it on a bowl. Then the learners are asked to take it one by one and they have to talk about the key word that they took.

The key words are: 1.Compressor 2. Condensation 3. Throttling 4.COP 5. Evaporation 6. P-h diagram

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Summary In this session, we learnt to:

Define refrigeration.Classify refrigeration systems.Explain the working principle.List the merits, demerits & Applications.

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References

Book1. Thermal engineering by R.K.Rajput.

Websites

www.energyefficiencyasia.orgwww.images.google.com

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