Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: 3rd Term
Week: 8
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: Evaporators: Types of Evaporators.
Topic: Evaporators: Construction of Evaporators
Subject Matter: Direct expansion construction, coil arrangement, refrigerant flow

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 chart showing a DX coil construction.
Samples of copper tubing and fins.
A mounting frame for an evaporator.
Whiteboard and markers.

Rationale for the Lesson

This lesson helps pupils understand how evaporators are built and operate, which is important for understanding how cooling systems work. Knowing the construction details helps pupils identify and explain problems in refrigeration and air conditioning units they may encounter in daily life or future careers.

Prerequisite/Previous Knowledge

Pupils should have a basic understanding of the refrigeration cycle and the general components of a refrigeration system.

Lesson Content/Board Summary

Evaporators: Construction of Evaporators

Definition of an Evaporator

An evaporator is a heat exchanger in a refrigeration system where the refrigerant absorbs heat from the surrounding space or substance, causing the refrigerant to change from a low-pressure liquid to a low-pressure vapor. This process cools the area or substance.

Direct Expansion (DX) Evaporator Construction

Direct expansion (DX) evaporators are commonly used in many refrigeration and air conditioning systems. In a DX evaporator, the refrigerant directly cools the air or product.

The main components of a DX evaporator are:

Coil (Heat Exchanger): Made of copper or aluminum tubing, through which the refrigerant flows.
Fins: Thin metal sheets (aluminum or copper) attached to the coils to increase the surface area for heat transfer from the air to the refrigerant.
Manifold/Distributor: Connects the expansion device to multiple evaporator circuits, ensuring even distribution of refrigerant.
Mounting Frame: Provides structural support for the coil and fins, allowing for installation within an air handling unit or refrigerated space.
Drain Pan: Collects condensate (water) that forms on the cold coil surfaces.

Coil Arrangement in Evaporators

The way the coils are arranged impacts the evaporator’s efficiency and capacity. Common arrangements include:

Serpentine Coil: A single continuous tube that winds back and forth, providing a long path for the refrigerant.
Multi-Circuit Coil: Several parallel circuits connected to a common manifold, allowing for better refrigerant distribution and reduced pressure drop.
Finned Tube Coil: Tubes with fins attached, which is the most common type for air cooling applications.
Plate Evaporators: Constructed from two metal plates pressed together with channels for refrigerant flow, often used for liquid chilling or contact freezing.

Refrigerant Flow in Evaporators

Refrigerant enters the evaporator as a low-pressure, low-temperature liquid/vapor mixture and exits as a low-pressure, low-temperature vapor. The process involves:

Expansion: The refrigerant, after passing through an expansion device, enters the evaporator at a low pressure and temperature.
Heat Absorption: As the refrigerant flows through the evaporator coils, it absorbs heat from the warmer air or substance surrounding the coils.
Phase Change (Evaporation): The absorbed heat causes the liquid refrigerant to boil and change into a vapor. This phase change is where most of the cooling effect occurs.
Superheating: After all the liquid has evaporated, the refrigerant vapor continues to absorb heat, becoming slightly superheated before exiting the evaporator. This ensures no liquid refrigerant returns 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 briefly, and then introduces the topic by asking pupils where the cold air comes from in a refrigerator or air conditioner. The teacher will then state the lesson topic and objectives.
Pupils’ Activity: Pupils respond to questions, listen attentively, and note the topic and objectives.
Learning Point: Pupils are prepared for the new lesson and connect it to their prior knowledge.

Step 2: Direct Expansion (DX) Evaporator Construction

Time: 7 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher explains what an evaporator is and describes the construction of a direct expansion (DX) evaporator, identifying its key components such as coils, fins, manifold, and mounting frame, using diagrams or actual samples if available.
Pupils’ Activity: Pupils listen, observe the diagrams/samples, and ask questions for clarification.
Learning Point: Pupils understand the physical structure and components of a DX evaporator.

Step 3: Coil Arrangement in Evaporators

Time: 7 minutes
Teaching Skill: Illustration/Discussion
Teacher’s Activity: The teacher discusses various coil arrangements used in evaporators, such as serpentine and multi-circuit coils, explaining how each arrangement affects heat transfer and refrigerant distribution. The teacher will also mention finned tube and plate evaporators.
Pupils’ Activity: Pupils listen, take notes, and participate in discussions about the advantages of different arrangements.
Learning Point: Pupils learn about different coil designs and their implications for evaporator performance.

Step 4: Refrigerant Flow in Evaporators

Time: 7 minutes
Teaching Skill: Elucidation/Diagramming
Teacher’s Activity: The teacher explains the path and state changes of the refrigerant as it flows through the evaporator, from entering as a liquid/vapor mixture to exiting as a superheated vapor, highlighting the role of heat absorption and phase change.
Pupils’ Activity: Pupils follow the explanation, trace the refrigerant flow on diagrams, and note the different stages.
Learning Point: Pupils understand the thermodynamic process of refrigerant within the evaporator.

Step 5: Visualisation and Component Identification

Time: 5 minutes
Teaching Skill: Practical Application/Identification
Teacher’s Activity: The teacher displays the instructional materials (DX coil, copper tubing, fins, mounting frame) and guides pupils to identify each component, relating them to the construction details discussed. The teacher may ask pupils to describe the function of each part.
Pupils’ Activity: Pupils actively observe, identify components, and explain their functions.
Learning Point: Pupils connect theoretical knowledge with practical components.

Step 6: Class Discussion and Recap

Time: 5 minutes
Teaching Skill: Facilitation/Questioning
Teacher’s Activity: The teacher facilitates a brief class discussion, inviting pupils to summarize the main points of the lesson. The teacher asks quick questions to check for general understanding.
Pupils’ Activity: Pupils volunteer answers, contribute to the discussion, and listen to peer responses.
Learning Point: Pupils reinforce their understanding through active recall and peer interaction.

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.
Mention three main components used in the construction of a direct expansion evaporator.
Describe one common coil arrangement in evaporators.
Explain what happens to the refrigerant as it flows through the evaporator.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 4 minutes
Teaching Skill: Summarization/Assignment
Teacher’s Activity: The teacher summarizes the key learning points of the lesson, emphasizing the importance of evaporators in refrigeration. The teacher then gives homework:
1. Draw and label a simple diagram of a finned tube evaporator.
2. Research two different materials used for evaporator fins and coils.
Pupils’ Activity: Pupils listen to the summary and copy the homework assignment.
Learning Point: Pupils consolidate their learning and are given tasks for further study.

Lesson Keywords

Evaporator – A heat exchanger where refrigerant absorbs heat and changes from liquid to vapor.
Direct Expansion (DX) – A type of evaporator where refrigerant directly cools the air or product.
Coil – The tubing within an evaporator through which refrigerant flows.
Fins – Thin metal sheets attached to coils to increase heat transfer surface area.
Refrigerant Flow – The path and state changes of the refrigerant within the evaporator.
Phase Change – The process where liquid refrigerant turns into vapor by absorbing heat.

Differentiation

For pupils who grasp concepts quickly, the teacher can encourage them to research different types of evaporators (e.g., flooded, dry expansion) and their specific applications. For pupils needing more support, the teacher will provide simplified diagrams and offer one-on-one guidance to identify components and trace refrigerant flow.

Note for teachers using this lesson plan

Ensure that diagrams or actual components are clearly visible to all pupils during the lesson. Encourage pupils to ask questions and relate the concepts to everyday appliances like refrigerators and air conditioners. Emphasize safety precautions if handling actual refrigeration components.