Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: 3rd Term
Week: 10
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: Evaporators: Construction of Flooded Expansion Evaporators.
Topic: Evaporators
Subject Matter: Evaporation process, heat absorption, cooling effect

Specific Objectives

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

Cognitive Domain:

Define an evaporator.
Explain the working principle of an evaporator.
Describe the evaporation process within an evaporator.
State how heat is absorbed by the refrigerant in an evaporator.

Affective Domain:

Appreciate the importance of evaporators in refrigeration and air conditioning systems.
Show interest in learning more about the components of refrigeration systems.

Psychomotor Domain:

Draw a simple diagram illustrating the flow of refrigerant through an evaporator.
Identify the key parts of an evaporator on a given diagram.

Participate actively in class discussions about evaporators.
Collaborate with peers to understand complex concepts related to evaporation.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum (Junior Secondary School).
State Unified Scheme of Work for Senior Secondary Schools.
Refrigeration and Air Conditioning Technology by William C. Whitman et al.

Instructional Materials

The teacher will teach this lesson with the aid of:

A diagram of an evaporator.
Airflow arrows to show movement.
A thermometer.
Whiteboard and markers/chalk.

Rationale for the Lesson

This lesson helps pupils understand how cooling is achieved in everyday appliances like refrigerators and air conditioners. Understanding evaporators is important for comprehending the basic principles of refrigeration and its applications in daily life and various industries.

Prerequisite/Previous Knowledge

Pupils have a basic understanding of heat, temperature, and the states of matter (solid, liquid, gas).

Lesson Content/Board Summary

Evaporators: Working Principles

1. Definition of an Evaporator

An evaporator is a heat exchanger in a refrigeration system where the liquid refrigerant absorbs heat from the surrounding area and changes into a gas (vapor). This process results in a cooling effect.

2. Working Principle of an Evaporator

The evaporator functions by allowing the low-pressure, low-temperature liquid refrigerant to boil and vaporize. As the refrigerant changes state from liquid to gas, it absorbs a significant amount of heat from the air or substance passing over or around the evaporator coil.

3. The Evaporation Process

The evaporation process in an evaporator involves:

Refrigerant Entry: Low-pressure, low-temperature liquid refrigerant enters the evaporator coil.
Heat Absorption: Heat from the warmer environment (e.g., air in a refrigerator) transfers to the colder refrigerant through the coil walls.
Phase Change: As the refrigerant absorbs this heat, it boils and evaporates, changing from a liquid to a gas (vapor). This is an endothermic process, meaning it takes heat from its surroundings.
Refrigerant Exit: The refrigerant leaves the evaporator as a low-pressure, low-temperature gas, ready to proceed to the compressor.

4. Heat Absorption and Cooling Effect

The cooling effect produced by an evaporator is a direct result of the refrigerant absorbing latent heat of vaporization. Latent heat is the energy absorbed or released during a phase change without a change in temperature.

The following points explain heat absorption and cooling effect:

When the liquid refrigerant evaporates, it draws heat from the space it is intended to cool.
This removal of heat causes the temperature of the surrounding air or medium to drop, creating the desired cooling effect.
The amount of heat absorbed depends on the type of refrigerant, the surface area of the evaporator, and the temperature difference between the refrigerant and the space being cooled.

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 know about how a refrigerator or air conditioner cools a space. The teacher introduces the topic “Evaporators” as a key component in achieving this cooling.
Pupils’ Activity: Pupils respond to the teacher’s questions and listen attentively to the introduction.
Learning Point: Pupils connect prior knowledge to the new topic and understand the relevance of evaporators.

Step 2: Definition of Evaporator

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines an evaporator, explaining its primary function in a refrigeration system using the board summary. The teacher may draw a simple sketch to illustrate its position.
Pupils’ Activity: Pupils listen, take notes, and ask questions for clarification.
Learning Point: Pupils understand the definition and basic role of an evaporator.

Step 3: Working Principle of an Evaporator

Time: 10 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher explains the working principle of an evaporator, referring to the provided diagram. The teacher describes how the refrigerant flows through the coil and absorbs heat.
Pupils’ Activity: Pupils observe the diagram, listen to the explanation, and identify the key processes.
Learning Point: Pupils grasp the fundamental mechanism by which an evaporator operates.

Step 4: The Evaporation Process

Time: 8 minutes
Teaching Skill: Detailed Explanation
Teacher’s Activity: The teacher elaborates on the evaporation process, explaining the change of state from liquid to gas and the role of latent heat absorption. The teacher emphasizes that this phase change is what causes cooling.
Pupils’ Activity: Pupils pay attention, take notes, and ask questions about the phase change and heat absorption.
Learning Point: Pupils understand the specific steps involved in the refrigerant’s evaporation within the coil.

Step 5: Heat Absorption and Cooling Effect

Time: 7 minutes
Teaching Skill: Concept Reinforcement
Teacher’s Activity: The teacher further explains how the absorbed heat leads to a cooling effect in the surrounding environment. The teacher may use the thermometer to demonstrate temperature changes if a simple setup is available.
Pupils’ Activity: Pupils listen and relate the process to the observable cooling effect in real-world applications.
Learning Point: Pupils clearly link heat absorption by the refrigerant to the resulting cooling of the space.

Step 6: Class Activity

Time: 3 minutes
Teaching Skill: Application
Teacher’s Activity: The teacher asks pupils to draw a simple sketch of an evaporator coil and use arrows to show the direction of heat absorption and refrigerant flow.
Pupils’ Activity: Pupils attempt to draw the sketch based on the lesson, demonstrating their understanding.
Learning Point: Pupils apply their knowledge to a practical representation.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

What is an evaporator?
Briefly explain the working principle of an evaporator.
Describe the evaporation process that occurs inside an evaporator.
How does the refrigerant’s absorption of heat lead to a cooling effect?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the importance of evaporators in refrigeration and air conditioning. The teacher assigns homework: “List three common applications of evaporators you observe in your daily life.”
Pupils’ Activity: Pupils listen to the summary and copy down the homework assignment.
Learning Point: Pupils consolidate their understanding of the topic and prepare for future lessons.

Lesson Keywords

Evaporator – A component in a refrigeration system where liquid refrigerant absorbs heat and changes into a gas.
Evaporation – The process of a liquid changing into a gas, typically by absorbing heat.
Refrigerant – A substance used in refrigeration systems that absorbs and releases heat as it changes state.
Latent Heat – The heat absorbed or released during a phase change (e.g., liquid to gas) without a change in temperature.
Cooling Effect – The reduction in temperature of a space or substance, achieved by removing heat.
Heat Absorption – The process by which a substance takes in heat energy from its surroundings.

Differentiation

For pupils who grasp concepts quickly, the teacher can ask them to research different types of evaporators (e.g., finned, plate, bare tube). For those needing more support, the teacher can provide simplified diagrams and allow peer-to-peer explanation or one-on-one clarification.

Note for teachers using this lesson plan

Ensure that the diagram of the evaporator is clear and large enough for all pupils to see. Emphasize the concept of latent heat as it is central to understanding the cooling process. Encourage pupils to relate the lesson to real-world examples they encounter daily.