Teaching Capacitive Reactance

A capacitor is basically an electrical spring. It stores an electrical charge when the voltage increases, and it releases that electrical charge when the voltage starts to drop. Capacitors are used smooth out the output of DC power supplies. When a capacitor is connected in an AC circuit it alternately charges and discharges with the AC current. This has the effect of causing voltage to peak after the current peaks. Note that this is exactly the opposite of inductive reactance which causes current to peak after the voltage peaks. Adding a capacitor in series with an inductive load offsets the inductive reactance so that the voltage and current peak together. Capacitors are used with PSC motors to improve their efficiency by offsetting the inductive reactance of the motor windings. This is why PSC motors became the standard motor for compressors and fans. Several years ago I had a furnace with a shaded pole indoor blower motor. When it died, I decided to replace it with a similar size PSC motor. The replacement PSC blower motor was slightly larger in horsepower than the original shaded pole motor, but drew only half as many amps while operating. Of course today PSC motors are being replaced with brushless DC, or ECM motors for efficiency savings that are equally dramatic.

You can do a demonstration of the effects of inductive and capacitive reactance by wiring a 60 watt light, a 120 volt 10 watt unit bearing shaded pole motor, and a 10 MFD run capacitor in series to a 120 volt power source. Wire a switch in parallel to the shaded pole motor and another switch in parallel with the capacitor. The switches will allow you to take the motor or capacitor out of the circuit by simply bypassing them. Ideally, you want to use an oscilloscope to show the source voltage and the voltage across the light. The light will remain in phase with the circuit current since the light is a resistive load. The phase relationship between the source voltage and the voltage across the light will show the relationship between voltage and current. Operating the switches will show the effects of inductive reactance, capacitive reactance, and the combination of the two. Even without an oscilloscope you can show that the voltage across the three devices adds up to more than the applied voltage. This shows why run capacitors must be rated for a higher voltage than the voltage to the unit.

I did a powerpoint presentation for the National HVACR Educators & Trainers Workshop this past March that includes an interactive virtual lab to show inductive and capacitive reactance. You can download the presentation at the HVAC Excellence web site. It is listed in the free downloads as "Practical Labs." Students can click on the switches to control them and see the results. For more ideas on teaching alternating current fundamentals check out Unit 29 Electrical Power and Circuits in Fundamentals of HVAC/R.

Dry Out!

Its spring, the weather is warming up, flowers are blooming, and our thoughts turn to dehumidification. In humid climates like the southeast, removing humidity from the air in warm weather is just is just as important to comfort as reducing its temperature. Being warm blooded, our body normally produces more heat than it needs and then regulates our temperature using different cooling mechanisms. The primary cooling mechanism is evaporation of perspiration from our skin. Dry air makes us feel cooler because it accelerates the evaporation from our skin. Humid air makes us feel warmer because the evaporation process is slowed down. Dehumidification can be the difference between being comfortable and being uncomfortable at 78°F. Dehumidification can save energy by reducing the amount of sensible cooling required for comfort. Many people with oversized air conditioning systems essentially over cool their house to be comfortable because their systems do not run long enough in mild weather to reduce humidity, so they do not feel comfortable until they reach temperatures of 70°F in their house. It takes several minutes for most air conditioning coils to get cold enough to sweat. An over sized system will often satisfy the thermostat and shut off shortly after the coil reaches dew pont. A properly sized air conditioning system will run longer, allowing longer operation with an evaporator operating below dew point and removing more water from the air. Systems with ECM blowers and thermidistat controllers have a special dehumidification mode that reduces system airflow for dehumidification. This increases the latent system capacity and decreases its sensible capacity. Two stage cooling systems can help by allowing longer system operation at moderate loads. All of these are a big improvement over the typical oversized single capacity system with a PSC blower. However, an air conditioner is still not a dehumidifier.

If you are really serious about dehumidification you need a dehumidifier. In a nutshell, a dehumidifier is an air conditioner with a single blower that moves air first over the evaporator, and then over the condenser. The air first passes over the evaporator where it is cooled to dew point to remove water, and then the same air passes over the condenser where it is reheated to a temperature slightly above its original temperature. If you have a basement in the southeast you NEED a dehumidifier. My basement stays below 80°F all the time without air conditioning, but it feels warm without my dehumidifier operating. In the past most dehumidifiers have been small console types that are noisy, inconvenient, and typically undersized. Several companies now offer whole house dehumidifiers that can be integrated into a complete comfort system for your house. They have enough capacity for a house, are far quieter, and do not require you to empty a bucket twice a day. Therma-Stor has a good short animation on why basements have high a relative humidity and how a dehumidifier addresses this problem. You will find other articles on their website which help address specific dehumidifier applications. They also have a psychrometric chart in a round format that looks like a ductulator that are great for teaching psychrometrics. To read more about the effect of humidity on comfort, check out Unit 61 Fundamentals of Psychrometrics in Fundamentals of HVAC/R. To download an interactive pschrometric chart for free, go to HandsDown Software.

Free RSES Jounal for Students

I truly believe that one of the best career decisions anyone can make is to choose something they love. After all, most of us are going to spend the majority of our conscious lifetimes at work. So if you hate your work, your life will be miserable. By contrast, if you enjoy your work, your life will be more fulfilling and enjoyable. However, young people often have not experienced enough of the world to really know what they will like. Unless they grow up in a family that works in the HVAC/R field, most teenagers really know very little about it. We should be cultivating a love of the HVAC/R field. One way is to expose your students to the wide array of careers in the HVAC/R field through professional journals. I bring in my RSES Journals to school and strategically place them around the class and lab for the students to pick up and read. OK, so maybe I leave them laying around – the effect is that students do read them. They will ask me about articles they read and are interested in. Many students use them as resources in other classes. Students have used RSES Journals for research articles and even for speech class. One student discovered that he could stand up and give an extemporaneous speech on air conditioning anytime based on RSES articles he read. After a few speeches on the refrigeration cycle, scroll compressors, and heat pumps the teacher told him the next speech could not be on air conditioning. She later told me that she never met anyone before who loved his field of study so much. That should be our goal – to instill respect for the craft and pride in learning and practicing it. RSES would like to help us out. They are offering FREE one year subscriptions to HVAC/R students. To get their free subscription they need to go to the RSES website http://www.rses.org/studentjournal.aspx The good folks at RSES did stress that this offer is for students only.

I think getting the RSES Journal for free is a great start at getting a whole new generation of HVAC/R technicians interested in a great organization and a rewarding career. Right after getting all your students signed up for a free RSES Journal subscription, consider thanking RSES by joining JOIN RSES. If you are already a member, thanks!

Flooded Equipment

Large portions of the country are experiencing flooding from unusually large amounts of rain falling in a short period of time. Areas that have experienced flooding typically have a big cleanup job and lots of rebuilding ahead. Make sure your students know to be careful when working on equipment that may have been flooded. Even after drying out, the corrosion and debris left behind can make the controls and motors inoperable. Worse, they can be dangerous. Safety controls that have been under water cannot be relied on to work. Silt and debris can create potential shorts. Another concern is for the health of the building occupants. Floodwaters contain all types of things that you really don’t want in your air conditioning system: chemicals, gasoline, dead animals, and sewage to name a few. Coils , equipment cabinets, and insulation all will retain some of these undesirable things even after the water has receded. Technicians may be asked to repair flood damaged equipment, but in most cases the proper repair is more costly than replacing the equipment. It is hard to tell someone who has just lost most of their possessions that you cannot fix their flood damaged equipment, but that is exactly what you should do. Then there is the matter of ductwork: obviously a good place to catch things. Again, cleaning may not really be practical. A danger to technicians is the muck they will often be working in. They will be walking, crouching, crawling, sitting, and laying on this muck when they go in the house to look at the equipment. Again, this is not just mud. This is a combination of nasty stuff that is definitely hazardous to your health. For more information on flood damaged equipment check out the AHRI page on flooded equipment.

Promote Active Listening With Questions

I believe that what the students do is just as important as what the teacher does. One of the big problems with traditional lecture is that most of the students are doing very little, the teacher is doing all the work. The teacher talks and the students listen. In the days before whiteboards and powerpoint this was often referred to as “chalk and talk.” I must confess that I can slip into this mode if I am not careful. Delivering a well organized lecture and writing notes as you go is not bad, but keep in mind that just because YOU SAID IT does not mean that the STUDENTS LEARNED IT! It is important to use delivery techniques that keep the students engaged. One simple technique is to ask questions. Three types of questions I use are volunteer responses, shout outs, and directed questions. Note: these are my own labels. Other folks probably have their own descriptive labels for these common methods.

I often open a topic or discussion with volunteer responses. This gives me an idea of what my students know and opens a discussion on the topic in a relatively low stress way because students are not put on the spot. Students that have done their homework or already have knowledge in the topic are the most likely to respond. Volunteer responses are probably the most often used in many class rooms. Two potential hazards of volunteer responses are that the same students tend to do all the answering, and the answers you receive can be wrong. Wrong answers are OK, just try and steer the class in the correct direction without embarrassing the student that offered the incorrect answer. If people are shot down when responding, the responses will stop.

Shout outs (my term) are questions directed at the audience in general where the answer you expect is fairly obvious and several people are likely to shout out the answer. These work to reinforce material, review main points, and provide you with feedback before you go on to another topic. I just have the class finish my sentence. For example, after discussing the states of matter and the properties of each state you might say “So, the three states of matter are … and the class will respond “solid, liquid, and gas.” Then follow with some more specific questions like “The state which has both a definite shape and volume is...”

Both volunteer responses and shout outs suffer from a common flaw: they allow a small core of dominant students to do all the answering while less confident students hide. Directed questions ask a specific student a detailed and specific question. This can be done using homework questions. The idea is to make sure everyone has to answer at least one question. I had a professor in college that was great at this. He would ask a few questions over homework or the previous day’s lecture to start each class. He would ask the question before calling the name of the person who was required to answer. This kept all of us listening to the questions and the answers because you did not know when you would be required to answer. I can tell you this technique is effective at getting lazy students to study the material before class – it certainly worked on me! However, it does also create a fair amount of stress. I use directed questions about once a week when we go over homework. Everyone knows they will be embarrassed if they show up for class without having done their homework. I reduce the stress level by going around the room in order, so students have a good idea when their time will come.

The simple act of asking questions and requiring students to do more than sit quietly improves retention of the material. After all, the goal is not for you to say everything the student needs to know, it is for the students to learn it.

Manual J 8th Edition

Many industry professionals that were familiar with Manual J7 find Manual J8 hard to navigate. The increased amount of information, detail, and forms discourages even seasoned pros who are used to the 7^th edition. Similarly, I have found that students are often intimidated and confused by the level of detail and the wide variety of forms they must learn in Manual J8. I try to make it more manageable by only discussing the averaging method and keeping the first calculations simple. Stay away from peak loads and zoning until the students have got a good handle on a straight forward load study. Also, I discuss the difference between the way heat loss and heat gain are handled. Heat loss HTMs (heat transfer multipliers) are all calculated using the temperature difference between the outside and inside design temperatures. Heat gain HTMs must also take heat storage and climate variation into account.

The most obvious difference between the 7^th and 8^th editions is how HTMs (heat transfer multipliers) are determined. The seventh edition provides tables that have basically taken the U value and multiplied it time a range of temperature differentials to produce a table of HTM factors. This saves the user from calculating the HTM themselves. The 8^th edition just gives the U values and leaves it to the user to multiply the temperature difference times the U value to get the HTM. For heat loss this is a very straight forward process. For heat gain it becomes more complicated because the temperature difference is usually not simply the difference between the outside and inside temperatures. Thermal storage of different materials and climate variations both affect the cooling temperature difference. Tables in the 8^th edition show both the U value of the material and the effective cooling temperature difference, abbreviated CTD. These can become confusing because they are arranged differently for different constructions and different building components. I find that if people understand some of the underlying reasoning they can do a better job managing the details. Explain to your students why the cooling temperature difference is different for each construction and material. Attic space and ceilings are a good example. Everyone has experienced how hot an attic can get in the summer. Clearly, the temperature difference across the ceiling is greater than the difference between the inside and outside temperatures. This is due to thermal storage in the attic. But this effect is not the same all across the country. Areas that do not have a wide variation in temperature store more heat, while areas with a wide temperature swing through the day store less and have an opportunity to release some of the stored heat. The low, medium, and high daily ranges found in the weather data are used to help determine the cooling temperature difference for most materials. Low daily range climates have higher CTDs because there is not much difference between the high temperature and the low temperature of the day: they get hot and stay hot. High daily range climates have lower CTDs because there is a big difference between the high temperature and the low temperature of the day: they get hot during the day and cool off at night. The combination of material heat storage and daily temperature variations makes cooling temperature differences complex. Once students understand the why, it is time to dig in and learn the how. There is really no substitute for working through several examples on each table. You can do worksheets for students to calculate heat transfer multipliers until they are comfortable. Then let them try an entire house, starting with a basic house.

ACCA has an excel file on their website that helps you perform a Manual J8 calculation. It does not replace Mnual J, but it does make using it much easier. The file is called a Manual J Speed-Sheet and is available at http://www.acca.org/speedsheet/ Another resource is my book.

Fundamentals of HVAC/R walks students through a simple house, filling in the Manual J 8^th edition form one step at a time in Unit 64 Residential Load Calculations. Remember, teaching your students how to do a proper heat load study is the first step in correct system sizing and application. Correctly sized and applied systems operate more efficiently saving energy and money. They are green two ways.

Why Teach Manual J?

ACCA’s manual J has been the gold standard for residential heat load calculations for many years. The latest edition, Manual J8, is frightening to students in both its size and cost. Many people ask why we teach Manual J calculations when everyone does load studies using computer software, especially after they get a look at the size of Manual J8. Others think that the sheer volume and complexity of Manual J8 makes a good argument for 600 square feet a ton ball park guesses. However, using the same logic you might ask why anyone bothers to learn their multiplication tables when they are always going to be using calculators and spreadsheets to do their calculations. I believe that understanding the underlying process is important to intelligently using the available tools. During a sale where items were being priced at 80% of their original price a clerk insisted that 80% of $120 was $150. They simply could not grasp the obvious; 80% of something could not possibly be more than the original amount. Their proof that they were correct was the fact that the answer was arrived at on a calculator, therefore it could not possibly be in error. Of course they had divided by 0.8 instead of multiplying, arriving at $150 instead of $96. The clerk’s lack of understanding of percentages and total reliance on technology had produced a ridiculous result that they were unable to recognize. Similarly, performing load studies using computer software by just dropping in numbers into a program provides many opportunities for error. If you don’t understand the process you may not recognize a ridiculous answer. Since equipment selection and duct sizing both rely heavily on the load study, the cost of a mistake is multiplied. One way to make Manual J8 more approachable is to use Manual J8 Abridged. It is more the size of previous Manual J editions and costs about half of the full J8 edition. All versions prior to Manual J8 used the averaging method. The eighth edition added peak load calculations that are similar to commercial calculations. The eighth edition also added factors for a plethora of unusual construction types and nearly 40 pages on duct loss and gain. The abridged version achieves is size reduction by only doing averaged calculations, removing factors for some of the more esoteric constructions, and drastically reducing the duct loss section. These things are not missed when teaching people to do their first load calculations.

One way to reduce the cost of Manual J to the students is to have your school join ACCA for $250 a year. If your school joins ACCA the students can buy Manual J8Abridged for $50 instead of $72. Another way is to have the school buy several copies and keep them in the library for student use.

Fundamentals of HVAC/R devotes an entire unit for load calculations using Manual J8. It takes the student step by step through a simple house, showing how the worksheet is filled out as you go. It was written to work with the full eighth edition, but also works well with the abridged version.

Sure, some folks still use 600 square feet per ton. The same folks do the duct system using the two sixes to an eight rule so they don’t have to do any duct calculations. When they go to charge the unit, they just add gas until the suction pressure is 70 psig. Many of the systems installed this way kinda sorta work. Trouble is – kinda sorta just ain’t good enough any more.