Part of Unit: HVAC (Heating, Ventilation and Air Conditioning)
Lesson Plan Overview / Details
This is an introductory lesson into the operation and reading of pressures of the common automobile air conditioning system.
Prerequisite: I advise my students to review the two web sites, listed in the resources below, before coming to class
- 2 class sessions
- 60 Minutes
- 1 Lab session
- 180 Minutes
California Career and Technical Education Standards
- T.C.C1.5 Use appropriate personal protective equipment and safety practices.
- T.C.C2.1 Understand and use appropriate tools and equipment, such as wrenches, sockets, a...
- T.C.C2.2 Use tools, equipment, and machines to safely measure, test, diagnose, and analyz...
- T.C.C2.5 Use measurement scales, devices, and systems, such as dial indicators, micromete...
- T.C.C3.2 Understand the function and principles of air conditioning and heating systems.
- T.FS.10.5 Operate, maintain, and troubleshoot equipment.
- T.FS.4.2 Understand the use of technological resources to access, manipulate, and produce...
- T.FS.6.3 Use tools, equipment, and machinery safely and appropriately.
California Academic Content Standards (Reinforced)
- ELA.9-10.R.CAGT.2.3 Generate relevant questions about readings on issues that can be researched.2
- ELA.9-10.R.CAGT.2.5 Extend ideas presented in primary or secondary sources through original analysis...3
- ELA.9-10.R.CAGT.2.6 Demonstrate use of sophisticated learning tools by following technical direction...2
Objectives and Goals
- Students, by use of analogies, will understand latent heat.
- Students will be introduced to the relationship of heat and pressure.
- Students will be introduced to each component, its operation and its relationship to each other in the A/C system. They will then be able to demonstrate the same.
Activities in this Lesson
- Opening questions - Hooks / Set
At the start of class I draw the student’s attention to a question on the board. The question seems very simple: "Is it cold today?" As the students share their answers, I point out their contrasting opinion and then tell them that cold is a relative term. Is it cold here compared to Antarctica in the winter? No, it's rather warm in comparison. Is it cold here compared to the Sahara desert in the summer? Yes.
I point out that heat is measured. Fahrenheit, Celsius, BTU etc... I ask them if they have ever spilled rubbing alcohol or gasoline on their hand. I then ask them if it felt cold. When they agree, I tell them “that’s latent heat” and that the liquid is evaporating and in essence, boiling. Then I propose that if we live on a planet that had a climate of 300 degrees Fahreneit and atmospheric pressure of 14.7 psi we could use water to cool us because it boils at 212F degrees at that pressure and 212F degrees is significantly “LESS HOT” than 300F degrees.
I then ask: "Why doesn’t it evaporate when it’s in the bottle with the cap on?" After several unsuccessful guesses, I point out that some of the alcohol in the sealed bottle has evaporated and because the bottle is sealed it has raised the pressure to a point the liquid can no longer expand in to a vapor. This is the basic concept used in an A/C system.
- Build a virtual system - Group Work
I then ask them what we would need if we were to build a virtual A/C system. Over the next few minutes I help them with coming up with the idea of a compressor, a restriction and two radiators and draw them on the white board.
From there we discuss the process. The compressor will pump refrigerant to increase pressure as it tries to make its way past a restriction. I use the analogy of all of the students leaving for the day. As they approach the only exit they tend to bunch up and slow down. After they squeeze through the door they can spread out and move easier. This is what happens to the molecules of refrigerant as they are forced through the restriction. First they pile up and condense at the entrance of the restriction. Once they are through the restriction the molecules can spread out and they expand into a gas. At that point they will release latent heat. I tell them that the refrigerants we use today basically boil/evaporate at about 32 degrees. From there I lead them to the idea that if we have to do this over and over again we need to now re-condense the refrigerant by re-pressurizing it. So back to the compressor it goes!
I start my Power Point which covers the theory, operation and components of the A/C system. The students are encouraged to take notes on the ppt. notes pages that were posted on the school's web site. Each slide is discussed at length. Common normal pressure readings and the manifold gages are also discussed.
- Classroom Power Point [ Download ] This is a modified version of the Power Point for chapter 7 supplied with the text "Automotive Heating and Air Conditioning" (5e) by Tom Birch
- student notes pages [ Download ] This a condensed version of the classroom Power Point with three slides per page and a section for notes.
- Introduce and demonstrate A/C manifold set - Demo / Modeling
Note: for this activity I have a video camera set up with large monitors for students to observe.
I demonstrate the correct process for determining if the system is checkable, while wearing my safety glasses and explaining safety items to be aware of when working with AC systems. I will pose the following questions:
1. Does the vehicle run?
2. Do the A/C controls appear to work?
3. Does the compressor clutch engage?
I then demonstrate the correct and safe process to connect the A/C manifold set.
1. Identify the components of the system.
2. Identify the high and low sides.
3. Confirm the manifold valves are closed.
4. Connect the appropriate hoses to the ports.
5. Take a static pressure reading.
6. Start the vehicle and turn on the A/C system. Listen for the click of the compressor clutch engaging, and for a noticeable engine RPM change.
7. Take the running pressure readings, record and compare to specifications.
8. Locate the orifice tube and feel both sides and notice the temperature change.
9. Locate the pressure switch and jumper to force the compressor to continue to run. Then record findings.
10. Turn off A/C and engine. Safely and properly disconnect the manifold set.
- Using the A/C manifold gauge set - Lab / Shop
I will now ask the students to wear their safety glasses and ensure that they follow safe shop practices while I assign the students a vehicle and have them check out the appropriate manifold set (r134).
They now will assess their vehicle, hook up, read and asses their A/C systems and record on worksheets from their lab book. They are encouraged to “feel around” the system and notice where there are temperature changes. While they are doing this I wander from group to group questioning and assessing their findings.
- Review findings - Group Work
I now bring the students back into the classroom and bring up selected slides (the slides I bring up depends on my conclusions derived from questioning the students) and we discuss what we found. During this activity, I try to get the students to answer their own questions. For example, one student may ask, “Why is it cold by the firewall and warm at the compressor?” I’ll ask the whole class if anyone can answer the question. If no one speaks, I review a slide that shows the relationship of heat to pressure, a slide that demonstrates latent heat, and then a slide that has a diagram of the whole system. If a student volunteers to answer, I’ll listen to their explanation and try to find slides to support them and help them fill in the gaps.
- Closure - Closure
Once in the classroom I’ll let the discussion dictate the questions I ask but they are typically similar to the questions I ask of the groups in the shop.
Work sheets are reviewed for accuracy.
Eventually they will be assessed by a written test.
Finally, when I see the students at the next class meeting I ask them: “Can you see it?”
- Assessment Types:
- Demonstrations, Interviews, Teacher-Made Test, Observations,
While the students are working with the manifold sets I’m visiting each group of students (2 to 3) questioning them, sometimes individually & sometimes as a group. I vary the questions such as:
- What is this component?
- Where is this component?
- How does the component work?
- What is this part going to feel like?