Class: Senior Secondary School 1 (SS1 / SSS1)
Term: 3rd Term
Week: 7
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: Compressors: Working Principles of Compressors.
Topic: Evaporators
Subject Matter: Direct expansion evaporator, flooded evaporator, air cooling coils

Specific Objectives

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

Cognitive Domain:

Define an evaporator.
State the functions of an evaporator in a refrigeration system.
List different types of evaporators.
Differentiate between direct expansion and flooded evaporators.

Affective Domain:

Appreciate the importance of evaporators in cooling systems.
Show interest in learning about various refrigeration components.

Psychomotor Domain:

Identify different types of evaporators from diagrams or physical models.
Draw a simple diagram illustrating the working principle of an evaporator.

Collaborate with peers to discuss evaporator types.
Participate actively in class discussions about refrigeration components.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum for Basic Technology (JSS 1-3) and Senior Secondary Education Curriculum for Trade Subjects.
State Unified Scheme of Work for Senior Secondary Schools.
Refrigeration and Air Conditioning Technology by Billy C. Langley.
Modern Refrigeration and Air Conditioning by Althouse, Turnquist, Bracciano.

Instructional Materials

The teacher will teach this lesson with the aid of:

Charts showing different types of evaporators.
Diagrams of refrigeration cycles highlighting the evaporator.
Physical models of evaporator coils, cooling fins.
An air blower to demonstrate air movement over coils.
Marker board and markers.

Rationale for the Lesson

Understanding evaporators helps pupils grasp how cooling is achieved in refrigerators, air conditioners, and other cooling systems they use daily. This knowledge is important for those pursuing careers in refrigeration and air conditioning maintenance and repair, as it forms a core part of the trade.

Prerequisite/Previous Knowledge

Pupils should have basic knowledge of the refrigeration cycle and the main components of a refrigeration system, including the compressor and condenser.

Lesson Content/Board Summary

Evaporators

Definition of 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 low-pressure, low-temperature vapour. This process causes cooling of the space or substance around the evaporator.

Functions of an Evaporator

The main functions of an evaporator are:

To absorb heat from the refrigerated space or substance.
To provide a surface for the liquid refrigerant to evaporate.
To convert the low-pressure liquid refrigerant into a low-pressure vapour.

Types of Evaporators

Evaporators are generally classified based on how the refrigerant is fed and how heat is transferred. The main types include:

Direct Expansion Evaporator (Dry Expansion)
Flooded Evaporator
Air Cooling Coils (a form of Direct Expansion)

Direct Expansion Evaporator (Dry Expansion Evaporator)

This type of evaporator is designed so that the refrigerant almost completely evaporates by the time it reaches the outlet. Only a small amount of liquid refrigerant is present at the inlet, and it gradually evaporates as it flows through the coil.

Characteristics of Direct Expansion Evaporators:

Refrigerant enters as a mixture of liquid and vapour.
The majority of the coil contains superheated vapour.
Uses a thermostatic expansion valve (TXV) or capillary tube to control refrigerant flow.
Commonly used in household refrigerators and small air conditioning units.

Flooded Evaporator

In a flooded evaporator, the coils are kept completely filled with liquid refrigerant. The vapour formed during evaporation rises to the top of a liquid separator (accumulator), from where it is drawn by the compressor. The liquid level is maintained by a float valve.

Characteristics of Flooded Evaporators:

The heat transfer surface is always wet with liquid refrigerant.
Provides a high rate of heat transfer due to complete wetting.
Requires a larger refrigerant charge.
Uses a float valve to maintain the liquid level.
Commonly used in large industrial and commercial refrigeration systems.

Air Cooling Coils (Dry Expansion Evaporators)

Air cooling coils are a common form of direct expansion evaporators used to cool air. Air is blown over the surface of the finned coils, and heat is transferred from the air to the refrigerant inside the coils, causing the air to cool down.

Characteristics of Air Cooling Coils:

Usually finned to increase the surface area for heat transfer to air.
Air is circulated over the coils by fans or blowers.
Used in air conditioners, cold rooms, and display cases.
Refrigerant expands directly into the coils, absorbing heat from the passing air.

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, reviews the previous lesson on refrigeration cycle components, and then asks pupils to identify where cooling occurs in a refrigerator or air conditioner. The teacher then introduces the topic: Evaporators.
Pupils’ Activity: Pupils respond to questions and listen attentively.
Learning Point: Pupils recall previous knowledge and are prepared for the new topic.

Step 2: Presentation of Lesson Objectives

Time: 3 minutes
Teaching Skill: Orienting
Teacher’s Activity: The teacher clearly states the specific objectives for the lesson, ensuring pupils know what they are expected to learn by the end of the period.
Pupils’ Activity: Pupils listen and note the objectives.
Learning Point: Pupils understand the learning goals for the lesson.

Step 3: Explanation of Evaporator and its Functions

Time: 7 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher defines what an evaporator is, explaining its role as a heat exchanger in the refrigeration cycle. Using a diagram, the teacher explains how liquid refrigerant absorbs heat and changes state. The teacher then lists and explains the main functions of an evaporator.
Pupils’ Activity: Pupils listen, observe the diagrams, and ask questions for clarification. They take notes.
Learning Point: Pupils understand the definition and basic functions of an evaporator.

Step 4: Introduction of Evaporator Types

Time: 8 minutes
Teaching Skill: Classification/Categorization
Teacher’s Activity: The teacher introduces the main classifications of evaporators, specifically focusing on direct expansion, flooded, and air cooling coils. The teacher shows charts or images of these different types.
Pupils’ Activity: Pupils observe the charts and listen to the introduction of each type. They note down the names of the types.
Learning Point: Pupils can name the different types of evaporators.

Step 5: Detailed Explanation of Direct Expansion Evaporator

Time: 7 minutes
Teaching Skill: Elucidation
Teacher’s Activity: The teacher explains the working principle and characteristics of a direct expansion evaporator. The teacher discusses how refrigerant flow is controlled and gives examples of where this type is commonly used, using visual aids.
Pupils’ Activity: Pupils listen, ask questions, and take notes on the direct expansion evaporator.
Learning Point: Pupils understand the characteristics and application of direct expansion evaporators.

Step 6: Detailed Explanation of Flooded Evaporator and Air Cooling Coils

Time: 8 minutes
Teaching Skill: Comparison/Explanation
Teacher’s Activity: The teacher explains the working principle and characteristics of a flooded evaporator, highlighting its differences from the direct expansion type. The teacher also explains air cooling coils as a specific application of dry expansion evaporators, demonstrating with an air blower and cooling fins.
Pupils’ Activity: Pupils listen, compare the types, and make notes. They observe the demonstration.
Learning Point: Pupils can differentiate between flooded and direct expansion evaporators and understand air cooling coils.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

Define an evaporator.
State two functions of an evaporator.
List three types of evaporators.
Differentiate between a direct expansion evaporator and a flooded evaporator.

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 definition, functions, and types of evaporators. The teacher then gives homework, asking pupils to research other types of evaporators not discussed in class.
Pupils’ Activity: Pupils listen to the summary and note down the homework assignment.
Learning Point: Pupils consolidate their learning and are given a task for further study.

Lesson Keywords

Evaporator – A component in a refrigeration system where liquid refrigerant absorbs heat and changes into vapour.
Direct Expansion – An evaporator type where refrigerant evaporates gradually as it flows through the coil.
Flooded Evaporator – An evaporator type where the coils are completely filled with liquid refrigerant.
Air Cooling Coils – Finned evaporators designed to cool air as it passes over their surface.
Refrigerant – A substance used in refrigeration cycles that absorbs and releases heat.

Differentiation

For pupils who grasp concepts quickly, the teacher can assign additional research on specific applications of each evaporator type or ask them to draw detailed diagrams. For pupils who require more support, the teacher can provide simplified notes, spend extra time explaining difficult concepts, and use more visual aids during instruction.

Note for teachers using this lesson plan

Teachers should ensure they have clear diagrams or actual models of evaporators to aid understanding. Encouraging pupils to relate the concepts to everyday appliances like refrigerators and air conditioners will enhance their engagement. Practical demonstration of an air blower over a finned coil can help illustrate the principle of air cooling coils.