Class: Senior Secondary School 1 (SS1 / SSS1)
Term: 3rd Term
Week: 1
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: .
Topic: Refrigerants Properties of Refrigerants
Subject Matter: Thermal properties, safety properties, chemical stability, efficiency

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define a refrigerant.
List and explain the thermal properties of refrigerants.
State and explain the safety properties of refrigerants.
Describe the chemical stability of refrigerants.
Explain the concept of efficiency as it relates to refrigerants.

Affective Domain:

Appreciate the importance of understanding refrigerant properties for safe and effective use.
Show interest in learning about different types of refrigerants and their applications.

Psychomotor Domain:

Identify different properties of refrigerants from a property chart.
Handle a safety data sheet to extract relevant information about a refrigerant.

Discuss with peers the implications of using refrigerants with poor safety properties.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum for Basic Technology (JSS 1-3) / Senior Secondary Education Curriculum for Technical Vocational Education (SSS 1-3)
State Unified Scheme of Work for Refrigeration And Air Conditioning
Basic Refrigeration and Air Conditioning by Tomczyk, John.
https://www.engineeringtoolbox.com/refrigerants-properties-d_119.html
https://www.sciencedirect.com/topics/engineering/refrigerant-properties

Instructional Materials

The teacher will teach this lesson with the aid of:

A property chart showing different refrigerant characteristics.
A sample refrigerant cylinder (empty or for demonstration).
A safety data sheet (SDS) for a common refrigerant.
Whiteboard and markers.

Rationale for the Lesson

Understanding the properties of refrigerants is important for pupils to safely and effectively work with refrigeration and air conditioning systems. This knowledge helps them select the correct refrigerant for different applications and handle them responsibly to prevent accidents and protect the environment.

Prerequisite/Previous Knowledge

Pupils should have a basic understanding of heat transfer and the concept of cooling from their general science lessons.

Lesson Content/Board Summary

Refrigerants and their Properties

Definition of Refrigerant

A refrigerant is a substance used in a refrigeration cycle, which typically undergoes phase changes (liquid to gas and back) to absorb and release heat. It is the working fluid in air conditioning and refrigeration systems.

Thermal Properties of Refrigerants

Thermal properties determine how effectively a refrigerant can transfer heat. The following are key thermal properties:

Boiling Point: The temperature at which a liquid refrigerant changes into a gas (vaporizes) at a given pressure. A low boiling point is desirable for refrigeration.
Latent Heat of Vaporization: The amount of heat absorbed by the refrigerant when it changes from a liquid to a gas without a change in temperature. High latent heat means more cooling capacity per unit mass.
Specific Heat: The amount of heat required to raise the temperature of a unit mass of the refrigerant by one degree.
Critical Temperature and Pressure: The temperature and pressure above which a substance cannot exist as a liquid, regardless of pressure.

Safety Properties of Refrigerants

Safety properties relate to the potential hazards a refrigerant poses to humans and the environment. The following are important safety properties:

Toxicity: The degree to which a substance can harm living organisms. Refrigerants should have low toxicity.
Flammability: The ability of a substance to burn or ignite, causing fire or combustion. Non-flammable or low-flammability refrigerants are preferred.
Ozone Depletion Potential (ODP): A measure of a substance’s ability to destroy the ozone layer. Refrigerants with zero ODP are environmentally preferred.
Global Warming Potential (GWP): A measure of how much a given mass of a substance contributes to global warming over a given period compared to the same mass of carbon dioxide. Low GWP refrigerants are desirable.

Chemical Stability of Refrigerants

Chemical stability refers to the refrigerant’s ability to maintain its chemical composition and not break down or react with materials within the refrigeration system over its operational life. A stable refrigerant prevents system corrosion, sludge formation, and loss of cooling capacity.

Efficiency of Refrigerants

The efficiency of a refrigerant is its ability to provide maximum cooling effect with minimum energy input. A highly efficient refrigerant contributes to lower operational costs and reduced environmental impact. Factors contributing to efficiency include high latent heat of vaporization, suitable operating pressures, and good heat transfer characteristics.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and asks them to recall what they understand by “refrigeration” and “air conditioning.” The teacher then introduces the topic “Refrigerants and their Properties” by explaining that a refrigerant is the key substance that makes cooling possible.
Pupils’ Activity: Pupils respond to the teacher’s questions and listen attentively to the introduction.
Learning Point: Pupils recall previous knowledge and are introduced to the lesson’s main topic.

Step 2: Definition of Refrigerant

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines a refrigerant and explains its role in refrigeration and air conditioning cycles. The teacher writes the definition on the board.
Pupils’ Activity: Pupils listen, note key points, and copy the definition into their notebooks.
Learning Point: Pupils learn the definition and basic function of a refrigerant.

Step 3: Thermal Properties of Refrigerants

Time: 10 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher explains the thermal properties of refrigerants (boiling point, latent heat of vaporization, specific heat, critical temperature and pressure) using the property chart as a visual aid. The teacher gives examples for each property.
Pupils’ Activity: Pupils observe the chart, listen to the explanations, and ask questions for clarification. They note the properties.
Learning Point: Pupils understand how thermal properties affect a refrigerant’s cooling capacity.

Step 4: Safety Properties of Refrigerants

Time: 8 minutes
Teaching Skill: Explanation/Discussion
Teacher’s Activity: The teacher explains the safety properties (toxicity, flammability, ODP, GWP) and emphasizes their importance for safe handling and environmental protection. The teacher demonstrates with the safety data sheet, pointing out where these properties are listed.
Pupils’ Activity: Pupils listen, observe the safety data sheet, and discuss the implications of poor safety properties. They note the properties.
Learning Point: Pupils learn about the hazards and environmental impact of refrigerants and the need for safe handling.

Step 5: Chemical Stability and Efficiency

Time: 5 minutes
Teaching Skill: Explanation
Teacher’s Activity: The teacher explains chemical stability and why it is important for the longevity and performance of refrigeration systems. The teacher also explains what makes a refrigerant efficient.
Pupils’ Activity: Pupils listen and note key points about chemical stability and efficiency.
Learning Point: Pupils understand factors contributing to system reliability and operational cost.

Step 6: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define a refrigerant.
Mention three thermal properties of refrigerants.
State two important safety properties that influence refrigerant choice.
Explain why chemical stability is important for a refrigerant.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 7: Conclusion

Time: 5 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the definition of refrigerants and the importance of their thermal, safety, chemical stability, and efficiency properties. The teacher gives homework: “Research and write down three common refrigerants and one key property for each.”
Pupils’ Activity: Pupils listen to the summary and copy the homework assignment.
Learning Point: Pupils consolidate their understanding of the lesson.

Lesson Keywords

Refrigerant – A substance used in a refrigeration cycle to absorb and release heat.
Thermal Properties – Characteristics of a substance related to heat transfer, such as boiling point and latent heat.
Safety Properties – Characteristics indicating the potential hazards of a substance, such as toxicity and flammability.
Chemical Stability – The ability of a substance to maintain its chemical composition without breaking down or reacting.
Efficiency – The ability to achieve desired cooling with minimal energy input.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide additional questions on how different properties might interact or challenge them to suggest ideal properties for a new refrigerant. For pupils needing more support, the teacher will use simpler language, provide more examples, and offer one-on-one assistance during note-taking and question-answering sessions.

Note for teachers using this lesson plan

Ensure the property chart and safety data sheet are clearly visible and explained. Emphasize the practical implications of each property to help pupils connect the theory to real-world applications. Encourage pupils to ask questions, especially regarding safety, to foster a deeper understanding and responsible attitude.