Understanding Centrifugal Fan Motor Performance

Since air is what we work with it makes sense to insure that our students understand airflow and fan performance. Fan motor performance is one of the most often misunderstood aspects air conditioning systems. The amp draw on a centrifugal fan goes down as resistance to airflow is increased. For most people this seems counterintuitive. It is easy to picture the fan motor pushing harder to overcome the resistance and increasing in amp draw. However, this is exactly backwards. Centrifugal fans move air by throwing the air outwards through centrifugal force. The amount of air the fan is moving decreases as the resistance to airflow increases. If the fan blades are moving less air, they can actually spin easier because there is less air to sling. This causes the motor RPM to increase and the motor amp draw to decrease.

The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease. Allow them a few minutes of play time to convince themselves. This experiment does more to explain centrifugal blower motor performance than a week’s worth of lectures.

Now that you have them hooked, refer them to Unit 56 Fans and Airflow inFundamentals of HVAC/R where they can read about the characteristics of different type of fans used in the industry and the basic principles of airflow. There the students can see examples of the different types of fans and read about their performance characteristics. Unit 56 Fans and Airflow wraps up with a discussion of the fan laws and using fan perfromance tables and curves. As always, examples show in detail how to apply each of these concepts.

Note that what I have been discussing assumes a "regular" PSC blower motor. ECM blower motors behave differently because they are programmed to adjust their output according to the resistance they are working against, but that is an entirely new discussion which I will save for another article.

Safety note: If you are not sure all of your students understand that it will hurt to put their hands into a moving fan blade, you should put a wire gaurd over the intake and exhaust to keep hands and fingers out. For a more polished trainer build sliding sheet metal baffles for both the intake and exhaust and mount the blower to a stand.

To see all my posts be sure to vist hvacrfundamentals.blogspot.com

Teaching Residential Design

One of the challenges that all HVAC/R educators face is convincing future contractors to ignore all the quick and easy rules of thumb for system sizing and duct layout and use design methods based on research. The problem with the ACCA standard design methods for sizing equipment and designing systems is that a considerable amount of time must be invested to understand how to use them. The first few systems someone does with Manual J and Manual D seem to take forever. Even using a computer design program requires the user to learn how to use the program. These methods appear overly difficult when compared to simple square feet per ton rules, or air changes rules. Still, the question really comes down to “do you just want an easy answer, or do you want a correct answer?” If you think from the customer’s perspective, they really want and need a correct answer. I believe that it is also in the contractor’s long term best interest to get correct answers as well. Avoiding a gaggle of unhappy customers that got the quick, easy, and wrong answer can be more time consuming than taking the time to get it right the first time.

I don’t think every Air Conditioning Technician needs to be able to whip out a Manual J load study, but I do think all technicians should be familiar with the overall concepts and general procedures used. Sometimes, the first step to learning is understanding how much you don’t know. Introducing technicians to the proper way to do a load study can be the light in the darkness that illuminates the path to greater understanding.

The truth is that the procedures are really not that difficult once you understand them. They will still take more time then dividing the square footage by a magic number, but an accurate load study on a typical residential home generally takes less than 30 minutes for someone who understands the process. It is true that you will have to invest hours of your time to get to that point, but that is really no different than any other skill that is required to perform HVAC/R work. The fact that time and effort are required to develop the skills and knowledge required to perform the job is part of what makes HVAR/R jobs valuable. The job is not as simple as memorizing a handful of rules of thumb.

Section 7 in Fundamentals of HVAC/R gives students the information they need to understand residential air conditioning application and system design with several unique features not found in other major air conditioning texts. The System Design, Sizing, and Layout section begins with Unit 54 which describes how buildings are constructed. It is important for students to understand how the air conditioning system fits into the building. Decisions about unit location, duct location, general duct design, and register placement are all very much affected by the building construction. Unit 55 shows students how to read and interpret drawings. Often, the first “look” an air conditioning technician has at a building will be in the form of a drawing. Teaching students how to interpret blueprints allows them to understand the building construction, even before the building is built. Unit 64 takes the students step by step through a manual J8 load calculation. Fundamentals of HVAC/R is the only major text to discuss the use of Manual J8, the latest version of ACCA Manual J, the ANSI standard for residential load studies. Unit 57 shows students the types the different types of duct systems available and how they fit different types of building construction. Unit 58 shows how to install duct systems and how to use friction charts and the ACCA Duct Calculator to design duct systems.

Education Requires Participation

The most consistently successful teaching methods are those that require students to be an active participant in learning. Very few people can sit passively through a lecture and remember most of what was discussed at the end. Actively listening improves their retention. This could include asking questions, answering questions, or taking notes. I remember one class in college in which the professor would ask a handful of students to answer questions at the beginning of class. These questions covered homework and the previous day’s lecture. He made notations in his grade book to keep track of how often you answered correctly and incorrectly. You could not relax even if someone else was on the spot because if they answered incorrectly, he would ask someone else. He quizzed every student at least once a week. This enforced active listening, but at the expense of great student anxiety. However, we paid very close attention to what he had to say. I prefer a dialogue. Ask leading questions to the group and get the students comfortable with answering. They will soon start asking questions, and you can directly address the areas where they need the most help. Reading retention can be improved by asking the students to answer questions. This requires them to look at the text carefully enough to pick out specific pieces of information. Better yet, ask questions that are not directly addressed in the text which require the students to use the information they have read to come to a conclusion. Well done interactive computer programs draw students in. The satisfaction of immediate feedback and reinforcement is extremely powerful and effective. Even without the high end graphics and sound effects, interactive programming hooks students because they receive immediate feedback and reinforcement. But I believe shop work is the gold standard. Nothing teaches students how to build circuits like building circuits. If a picture is worth a thousand words, a shop project is worth a thousand pictures. Labs do not have to be complex to be effective. A simple circuit consisting of a power cord, light switch, and light will show the student how a basic circuit works and how switches work in relation to loads. The circuit can be expanded by adding switches in series or loads in parallel. Now you are teaching series and parallel as well as circuits. If the students build simple circuits with heaters they can measure the resistance, voltage, and current to illustrate the principles of ohms law in series and parallel circuits. Keep in mind that many of your students have never wired anything. To them, wiring a few lights is exciting. I have seen students who could not wire a switch and light at the beginning of the week wire a heat-cool thermostat to a transformer, three relays, and three lights by the end of the week. Because I believe that students must be active participants in order to learn, I really prefer that each student do their own project. Often, if five students are assigned to wire a circuit, only one does the actual work and the other four write down the answers. In other words, one student does all the learning. If students must work in teams, make an effort to insure every student gets their hands on the project. The least confident students will stand back. However, they are the very ones that need the most hands on work.

When planning lessons, keep in mind that what the students do will have more impact on their learning than what you do. No matter how wonderful your powerpoint presentation is, if the students just watch it passively they will not get a lot out of it. For ideas on active lessons, check out the MyHVACLab online companion to Fundamentals of HVAC/R. Besides an e-book copy of Fundamentals of HVAC/R , animations, and a powerpoint presentation for each unit, MyHVACLab includes interactive activities and questions to insure that students are actively involved in the learning. A companion lab manual is also available that complements the Fundamentals of HVAC/R text. Among the unique labs included are labs illustrating the physics principles that the refrigeration cycle is based on. Heating and cooling a refrigerant cylinder while measuring its pressure and temperature helps students understand the pressure-temperature relationship and P-T charts better than a week of lectures will.