Chalk and Talk 

Several years ago a fellow HVAC instructor told me he had been strongly encouraged by his immediate supervisor to do more lecturing. To the academically trained supervisor, working with students in the lab didn’t look like teaching, it looked like training. In the supervisor’s mind, training was a lower level pursuit – akin to monkey see monkey do. The school had just transitioned from a Technical School to a Technical Collage and they wanted to look like a college. 

The instructor was asking for any help I could offer, especially along the lines of “chalk and talk”, his description of what he felt was being asked of him. I think that phrase perfectly embodies why so many students really don’t get a lot out of lectures – they are just observers, not participants. The instructor is writing stuff on the board and talking while the students are just passive observers.  In the days of chalk boards, the instructor was not even facing the class when they were writing. It sometimes seemed like the teacher was engrossed in their own thoughts while they stare at the board and talk to no one in particular. 

We have advanced a bit since then. At least with Powerpoint presentations we are usually facing the class, even if we are staring at our computer monitor most of the time. However, it is possible to lecture and still rise above the mind-numbing norm of a monologue delivered to a captive audience of passive observers.  

Begin by facing your audience and making eye contact with people long enough so they feel you are speaking to them personally. If you are using a Powerpoint presentation, be familiar enough with the presentation and the material that you don’t have to read the notes while looking at the screen. It is OK to look at the screen occasionally, but if your discussion is just straight out reading the notes it will be boring no matter how good the presentation is.

Use voice inflection and hand motion to convey personal interest in your subject matter. How do you expect your class to maintain interest if YOU think the talk is boring.

Stop and take a breath. One dead give away of a nervous speaker is someone who has rehearsed their speech so much that all the words come out in a rapid-fire regurgitation that indicates they are just repeating memorized phrases, not really thinking about they are saying.

Whenever possible, involve your audience. People learn more if they do more. Ways to involve the class could include group questions, prompts for input, or even direct questions to individual students if you know them well enough. Be careful with the last suggestion, the goal is to include the students, not intimidate or embarrass them.

Use visual aids. This is pretty easy to do today. Just be careful not to overdo the videos and pictures you share. You should not be handing your class over to professor YouTube, just showing snippets that enhance and reinforce your talk.

When using online material, make sure and watch it yourself first. Even though you are not the person in the video, if you show it to your class you are in effect endorsing it. There are some things online that you don’t want to be associated with. 

Finally, this is an HVACR class. It is perfectly OK to have gauges, meters, gas valves, compressors, or any other tools or parts you want as visual aids. If you are talking about meters, every student should have one in their hands. You can even do some small exercises during the lesson. Remember, people learn by doing, so have the students do something. No, this is not your typical college class – it is a hell of a lot more interesting.

 Diesel Effect Compressor Explosions

There have been a few rare but potentially fatal accidents involving exploding compressors due to an effect known as diesel effect. If the gas mixture being compressed contains enough air, the heat of compression can ignite the refrigeration oil in the cylinder, much the same way diesel oil is ignited in a diesel engine. The heat of compression plus the heat of combustion from the refrigeration oil then ignites the refrigerant in the cylinder, creating a dramatic increase in pressure which blows the compressor apart. Note that this is not just a phenomenon limited to flammable refrigerants, but can happen with A1 rated refrigerants such as 134a or 410A. How? 

Refrigerants are rated for flammability according to ASTM E681 at a temperature of 60°C. Many refrigerants that will not burn under ASTM E681 conditions will burn at higher pressures and temperatures, including R-22, R-134a and R-410A. It is worth noting that in tests where they were trying to create diesel explosions, the University of Tokyo found no significant difference between the behavior of A1 refrigerants (R-22, R-410A) compared to A2L refrigerants (R1234yf, R32). They also found that compressing refrigerant and air mixtures without refrigeration oil did not create a diesel explosion. The refrigeration oil had to be present. 

So how can we avoid compressor diesel effect explosions? Simply put: keep the air out. With no oxygen you cannot have an explosion. Air is never good for any refrigeration system anyway. It should not be news to anyone who has studied refrigeration at all that air does not belong in a refrigeration system. However, you may not realize that leaving air in the system not only hurts system performance and reduces the equipment life, it can create a real hazard to service technicians. Here are a few precautions you can take to avoid the specter of a diesel effect explosion in your refrigeration system. 

• Check new installations for leaks using nitrogen and repair any leaks in the system.

• Never use compressed air or oxygen for leak testing refrigeration systems. 

• Thoroughly evacuate the lines and coil of new split system installations AFTER verifying they don’t leak.

• Never pump a system down into a vacuum. Reducing the low side pressure to a vacuum can suck in air through leaks or incorrectly positioned service valves.

• Never jump out safety controls such a low-pressure switches. Forcing a system to run when it is low on refrigerant creates the possibility of sucking in air through leaks on the low side.

• Don’t simply add charge to systems that are low on refrigerant. This is especially true for systems that are significantly low. You should find and repair the leaks.

You can read more about the research into compressor diesel effect explosions here. https://hpc2017.org/wp-content/uploads/2017/06/o324.pdf

Human Search Engine

 Instructors worry about not knowing all the answers. I know that I do. Let me put you at ease. You don’t have to know all the answers to be an effective instructor. In fact, I feel that anyone who knows all the answers hasn’t asked enough questions. Our job as instructors is not to be a human search engine, but to teach students how to search for answers on their own. In short, to help students become more proficient at the learning process, specifically applied to our field of HVACR. To be clear, I am not suggesting that instructors do not need to be competent in HVACR. Nor am I saying you shouldn’t want to know as many answers as possible. Just that knowing every answer a student might ask is not necessary. Discomfort with areas where you don’t know the answers can sometimes cause instructors to cling to the specific areas of information they already know and refuse to broaden their scope. This is particularly true when it comes to new technology and industry developments. I believe this is an unconscious effort to “know everything” by limiting the scope of knowledge you expose yourself and your students to. Venturing into areas of new development can be uncomfortable because you don’t have as many answers at the ready. However, it is perfectly OK to tell a student that you don’t know the answer to a question they ask. Help them by directing them to resources where they might find answers. They are going to need research skills when they enter the field. Teaching students to learn on their own is probably the most important thing you can possibly teach them. On those many occasions when you DO know the answer, it can be more helpful to guide them through a search process than to simply hand them the answer. People tend to remember things they discover on their own more than things that people tell them. It takes discipline to do this. Providing the answer immediately basically concludes the interaction with the student. Asking leading questions or discussing relevant informative resources makes the interaction more of a dialogue and requires more student participation. It definitely takes a little longer, but provides a better long-term result. Remember, students learn more by what THEY DO, than by what you do.