Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: 3rd Term
Week: 9
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: Evaporators: Construction of Direct Expansion Evaporators.
Topic: Evaporators Construction of evaporators
Subject Matter: Flooded expansion construction, refrigerant circulation, heat absorption

Specific Objectives

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

Cognitive Domain:

Instructional Materials

The teacher will teach this lesson with the aid of:

A diagram or model of a flooded evaporator.
Sections of piping and a control valve (float valve).
Whiteboard and markers.

Rationale for the Lesson

Understanding the construction of evaporators helps pupils grasp how cooling systems effectively remove heat from a space. This knowledge is important for troubleshooting and maintaining refrigeration units, which are common in homes and industries.

Prerequisite/Previous Knowledge

Pupils should have a basic understanding of the refrigeration cycle and the general function of an evaporator as a heat exchanger.

Lesson Content/Board Summary

Evaporators and their Construction

Flooded Expansion Evaporators

A flooded expansion evaporator is a type of evaporator where the refrigerant completely fills the evaporator coils or shell, ensuring full contact with the heat transfer surface. This design aims to achieve maximum heat exchange efficiency.

The construction of a flooded expansion evaporator often includes:

A shell or large header that holds the liquid refrigerant.
Heat exchange tubes submerged in the liquid refrigerant.
A liquid level control mechanism, typically a float valve, to maintain the refrigerant level.
A vapor separator or accumulator at the top to separate liquid droplets from the suction vapor.

Refrigerant Circulation

In a flooded expansion evaporator, the refrigerant circulates by natural convection or with the aid of a pump.

The process of refrigerant circulation involves:

Liquid refrigerant enters the evaporator and fills the shell or headers.
As heat is absorbed, the liquid refrigerant boils and turns into vapor.
The vapor rises, creating a circulation path where colder, denser liquid flows down to replace the warmer liquid.
Some designs use a refrigerant pump to force circulation, ensuring a higher flow rate and improved heat transfer.

Heat Absorption

Heat absorption is the primary function of an evaporator. In a flooded type, this process is highly efficient due to the complete wetting of the heat transfer surfaces.

The principles of heat absorption include:

Heat from the refrigerated space or medium (e.g., air, water) transfers to the outer surface of the evaporator tubes.
This heat then conducts through the tube material to the liquid refrigerant inside.
The absorbed heat provides the latent heat of vaporization, causing the liquid refrigerant to boil and change phase into vapor at a constant low temperature and pressure.
The resulting cold vapor is then drawn to the compressor.

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 the general function of evaporators, and introduces the topic of “Flooded Expansion Evaporators: Construction, Refrigerant Circulation, and Heat Absorption” by asking pupils to recall different types of heat exchangers.
Pupils’ Activity: Pupils respond to questions and listen attentively.
Learning Point: Pupils recall prior knowledge and are introduced to the new topic.

Step 2: Explanation of Flooded Expansion Evaporator Construction

Time: 10 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher defines a flooded expansion evaporator and explains its typical construction using diagrams or a model. The teacher highlights key components like the shell, tubes, and liquid level control.
Pupils’ Activity: Pupils observe the diagrams/model, listen to the explanation, and ask questions for clarification.
Learning Point: Pupils understand the definition and structural components of a flooded expansion evaporator.

Step 3: Description of Refrigerant Circulation

Time: 10 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains how refrigerant circulates within a flooded evaporator, detailing the role of boiling, vapor separation, and liquid level maintenance. The teacher may draw a simple flow diagram.
Pupils’ Activity: Pupils follow the explanation and the flow diagram, noting the path of the refrigerant.
Learning Point: Pupils comprehend the mechanism of refrigerant movement within the evaporator.

Step 4: Principles of Heat Absorption

Time: 7 minutes
Teaching Skill: Explanation/Analysis
Teacher’s Activity: The teacher describes the process of heat transfer from the refrigerated space to the refrigerant, emphasizing the phase change (boiling) and its role in heat absorption.
Pupils’ Activity: Pupils listen and relate the explanation to the cooling effect.
Learning Point: Pupils understand how heat is effectively removed by the refrigerant.

Step 5: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

Define a flooded expansion evaporator.
Mention two key components found in the construction of a flooded expansion evaporator.
Explain briefly how refrigerant circulates in a flooded evaporator.
Describe the process of heat absorption in this type of evaporator.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 3 minutes
Teaching Skill: Summarization/Consolidation
Teacher’s Activity: The teacher summarizes the main points of the lesson, reiterating the importance of flooded evaporators in achieving efficient cooling. The teacher assigns homework: “Draw and label a simple diagram of a flooded expansion evaporator.”
Pupils’ Activity: Pupils listen to the summary and copy the homework.
Learning Point: Pupils consolidate their understanding and receive an assignment.

Lesson Keywords

Evaporator – A component in a refrigeration system where liquid refrigerant absorbs heat and changes into vapor.
Flooded Expansion Evaporator – An evaporator design where the heat exchange surface is completely submerged in liquid refrigerant for maximum heat transfer.
Refrigerant Circulation – The movement of refrigerant through the evaporator, driven by boiling and density differences or pumps.
Heat Absorption – The process by which the refrigerant takes in heat from the surrounding environment, causing it to vaporize.
Float Valve – A control device used in flooded evaporators to maintain a constant liquid refrigerant level.

Differentiation

For pupils who grasp concepts quickly, the teacher can ask them to research the advantages and disadvantages of flooded evaporators compared to other types. For those needing more support, the teacher can provide simplified diagrams and allow peer-to-peer explanation.

Note for teachers using this lesson plan

Emphasize the practical application of this knowledge in industrial refrigeration systems. Use clear, simple diagrams and possibly a physical model if available to demonstrate the construction and working principle. Encourage pupils to ask questions and relate the concepts to everyday cooling devices.