Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: 3rd Term
Week: 3
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: Refrigerants: Uses of Refrigerants in Cooling Systems.
Topic: Compressor Types of compressor
Subject Matter: Reciprocating compressor, centrifugal compressor, screw compressor, rotary compressor

Specific Objectives

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

Cognitive Domain:

Define a compressor.
Identify different types of compressors used in refrigeration and air conditioning systems.
Describe the basic working principle of each compressor type.

Affective Domain:

Appreciate the importance of different compressor types in various refrigeration and air conditioning applications.
Show interest in learning about the components of refrigeration systems.

Psychomotor Domain:

Differentiate between various compressor models when presented with diagrams or physical examples.
Sketch a simple diagram representing the concept of a reciprocating compressor.

Participate actively in discussions about the characteristics and applications of different compressor types.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum for Technical Vocational Education.
State Unified Scheme of Work for Senior Secondary Schools.
Refrigeration and Air Conditioning Technology by William C. Whitman, Bill Johnson, and John Tomczyk.
Vocational Studies for Senior Secondary Schools.

Instructional Materials

The teacher will teach this lesson with the aid of:

Actual compressor models (if available).
Motor unit.
Cutaway diagrams of various compressor types (reciprocating, rotary, screw, centrifugal).
Whiteboard and markers.

Rationale for the Lesson

Understanding different compressor types is important because the compressor is the “heart” of any refrigeration or air conditioning system. This lesson helps pupils to know how these systems work and to identify the right compressor for different applications, which is a fundamental skill in the trade.

Prerequisite/Previous Knowledge

Pupils are assumed to have basic knowledge of what a compressor is and its general function in a refrigeration cycle from previous lessons.

Lesson Content/Board Summary

Types of Compressors in Refrigeration and Air Conditioning Systems

1. Reciprocating Compressor

A reciprocating compressor uses a piston moving back and forth within a cylinder to compress refrigerant vapour. It operates similarly to an internal combustion engine, converting rotary motion into reciprocating linear motion.

The following are characteristics of reciprocating compressors:

They operate with pistons driven by a crankshaft.
Can be open, semi-hermetic, or hermetic depending on how the motor and compressor are sealed.
Commonly used in household refrigerators, freezers, and small to medium-sized air conditioning units.

2. Rotary Compressor

A rotary compressor uses a rotating element to compress refrigerant vapour. Instead of pistons, it employs vanes, rollers, or scrolls to create compression.

The following are common types of rotary compressors:

Rolling Piston Type: A cylindrical roller rotates eccentrically within a cylinder, trapping and compressing gas.
Vane Type: A rotor with sliding vanes rotates inside a larger cylinder, creating variable volume chambers.
Scroll Type: Two interleaved spiral scrolls, one fixed and one orbiting, trap and compress gas.

Rotary compressors are known for smooth, quiet operation and are widely used in residential and light commercial air conditioners.

3. Screw Compressor

A screw compressor employs two meshing helical rotors (screws) to trap and compress refrigerant vapour as they rotate. The gas is drawn in at one end, compressed as it moves along the screws, and discharged at the other end.

The following are characteristics of screw compressors:

They can handle large volumes of refrigerant.
Used in medium to large commercial and industrial refrigeration and air conditioning systems.
Known for good efficiency, especially at full and part loads.

4. Centrifugal Compressor

A centrifugal compressor uses a high-speed rotating impeller to accelerate refrigerant vapour, converting kinetic energy into pressure. The gas enters the centre of the impeller and is thrown outwards by centrifugal force.

The following are characteristics of centrifugal compressors:

Best suited for very large refrigeration and air conditioning loads.
Handles large volumes of gas at low pressure ratios.
Commonly found in large commercial buildings, industrial plants, and district cooling systems.

Teaching Methods/Instructional Techniques

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

Instructional Procedures

Step 1: Introduction

Time: 4 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and reminds them of the previous lesson on the general function of a compressor in a refrigeration cycle. The teacher then introduces the topic by asking pupils to name types of compressors they might have seen or heard of.
Pupils’ Activity: Pupils respond to the greetings and attempt to answer the questions.
Learning Point: Pupils recall previous knowledge and are prepared for the new topic.

Step 2: Reciprocating Compressor

Time: 8 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher explains what a reciprocating compressor is, using diagrams or a cutaway model to show the piston and cylinder mechanism. The teacher describes its working principle and common applications.
Pupils’ Activity: Pupils listen attentively, observe the diagrams/models, and ask questions for clarification. They note down key points.
Learning Point: Pupils understand the definition, working, and applications of reciprocating compressors.

Step 3: Rotary Compressor

Time: 8 minutes
Teaching Skill: Explanation/Visualisation
Teacher’s Activity: The teacher introduces the concept of rotary compressors, focusing on rolling piston, vane, and scroll types. The teacher uses diagrams to illustrate the rotation and compression process for each type, highlighting their characteristics and common uses.
Pupils’ Activity: Pupils observe the diagrams, listen to the explanations, and compare the rotary types with the reciprocating type. They participate in a brief discussion.
Learning Point: Pupils learn about the different types of rotary compressors and their operational principles.

Step 4: Screw Compressor

Time: 7 minutes
Teaching Skill: Description/Illustration
Teacher’s Activity: The teacher explains the design and operation of screw compressors, emphasizing the role of the two meshing helical rotors. The teacher discusses their capacity for large volumes and typical industrial applications.
Pupils’ Activity: Pupils listen and try to visualize the screw mechanism. They ask questions about its efficiency and usage.
Learning Point: Pupils grasp the concept and application of screw compressors.

Step 5: Centrifugal Compressor

Time: 7 minutes
Teaching Skill: Explanation/Comparison
Teacher’s Activity: The teacher describes the centrifugal compressor, focusing on how the high-speed impeller works to compress refrigerant vapour. The teacher highlights that these compressors are used for very large systems and explains why.
Pupils’ Activity: Pupils listen, compare centrifugal compressors to other types in terms of size and application, and take notes.
Learning Point: Pupils understand the unique characteristics and large-scale applications of centrifugal compressors.

Step 6: Recap and Discussion

Time: 4 minutes
Teaching Skill: Questioning/Review
Teacher’s Activity: The teacher quickly recaps all four types of compressors discussed, prompting pupils to recall key features or applications for each. The teacher encourages a brief open discussion on where different compressors might be preferred.
Pupils’ Activity: Pupils actively participate in the recap, answering questions, and contributing to the discussion.
Learning Point: Pupils consolidate their understanding of the various compressor types and their distinguishing features.

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 a compressor?
Mention three types of compressors used in refrigeration and air conditioning systems.
Describe the basic working principle of a reciprocating compressor.
State one key application for a centrifugal compressor.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Closure
Teacher’s Activity: The teacher summarizes the lesson by emphasizing that different compressors are designed for specific applications based on their operating principles and capacity. The teacher assigns homework: research specific examples of appliances or systems that use each type of compressor discussed.
Pupils’ Activity: Pupils listen to the summary, copy the homework, and prepare for the next lesson.
Learning Point: Pupils understand the practical relevance of the lesson and are guided for further study.

Lesson Keywords

Compressor – A mechanical device that increases the pressure of a gas by reducing its volume.
Reciprocating – Involving a back-and-forth motion.
Rotary – Involving a rotating motion.
Scroll compressor – A type of rotary compressor using two interleaved spirals.
Screw compressor – A type of compressor using two meshing helical rotors.
Centrifugal compressor – A type of compressor using a high-speed rotating impeller.
Refrigerant – A substance used in a heat pump or refrigeration cycle.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide additional diagrams of compressor internal components to analyze or challenge them to explain why certain compressors are better for specific applications. For pupils needing more support, the teacher will use more simplified diagrams, provide direct examples of appliances that use each compressor type, and offer one-on-one guidance during activity time.

Note for teachers using this lesson plan

It is important to use clear visuals, such as cutaway diagrams or actual models if available, to help pupils understand the internal workings of each compressor type. Encourage pupils to ask questions and relate the concepts to everyday appliances they might encounter. Emphasize safety precautions when discussing or showing actual equipment.