Practicing for Success

"We are what we repeatedly do. Excellence, then, is not an act but a habit." – Aristotle

I would like to propose a slight revision to Aristotle’s line of thought: Success is not a condition, but a habit. Have you ever known someone that is lucky and seems to go from one success to another? It is sometimes difficult not to be envious of their success and wish we were so lucky. If the primary action you take towards becoming successful is to wish for success, chances are you will not find it. People are not successful because they are lucky; they are successful because they prepare for their success through repetitive practice. So the question is: “Are you practicing for success or failure?” You might ask “Who would practice for failure?” However, I have seen many students diligently practicing for their inevitable failure. They practice for failure by not preparing for class, arriving late, and making endlessly creative excuses for their failure to complete their work. Eventually they become expert at failing – eliminating all chances of success. I will not tell you that if you work hard and put your best effort into everything you do that you will always be successful. But I can guarantee that if you do not work at being successful, you will most certainly fail.

A big part of achieving success is preparing for opportunity. Opportunity comes in different forms at different points in everyone’s life. But to take advantage of opportunity when it comes around, you have to be prepared. That is one of the main reasons for going to school – to prepare for your future opportunities. You should practice being successful to prepare for opportunities that are sure to come. Show up to class early, prepare for class by studying the assigned material, ask questions when you don’t understand something, and don’t settle for good enough to get by, always strive to improve. I can not tell you exactly when or where opportunity will present itself in your life, but I do know that you must prepare yourself to take advantage of it when it arrives.

So if you have been practicing for failure, I suggest you reconsider your strategy. Put yourself out there, throw yourself into your studies, and risk being successful.  

Centrifugal Blower Motors

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 with a standard AC inductive motor 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.

Although the noise and increased air velocity make it seem like the fan is actually moving more air, the truth is that it is moving less. The air that it IS moving is traveling at a very high velocity, or speed. This is what makes the increased noise. But since you are effectively making the hole that the air travels through smaller, less air is able to get through, even at higher velocity. This is more difficult to show. To get accurate readings, you really need the air to be moving through some ductwork.

Another point to discuss is the difference between a blower with a traditional PSC motor and one with an EVM blower. The behavior we have been discussing is typical of an AC induction motor, like the PSC motors that come standard on most blowers. However, an ECM blower motor senses the change in work and increases its speed enough to actually overcome the resistance, so that the fan moves the same amount of air even against increased resistance. Since this requires more electrical energy, the amp draw for an ECM blower will increase when the fan is restricted. The ECM technology solves one problem: losing airflow due to increased resistance. But is creates a new one: increased electricl use to overcome the resistance. 

To read more, check out Unit 41 Fundamentals of Psychrometrics and Airflow and Unit 75 Fans and Air Handling Units in Fundamentals of HVACR 2nd edition. You can find the blower labs in the new Lab Manual for Fundamentals of HVACR, 2nd edition. They are labs 75.3 AC Induction Motor Blower Properties and lab 75.4 ECM Blower Properties.

Why 120 + 120 = 208

Have you ever wondered why in a standard three phase system that the leg to leg voltage is a little lower than the two leg voltages added up? Typically, if your two legs are 120 volts to ground, the voltage between them will only be around 208 volts. A system like this uses a transformer arrangement called a wye because it looks something like a wye on paper (page 544 of Fundamentals of HVACR). The three windings are tied together in the middle, and that is where the transformer is grounded. Each leg to ground is around 120 volts, but when you check the voltage between legs, you only get 208 volts. Remember that voltage is a measurement of potential difference – it is measuring the difference in electrical potential from one point to another. That is why if you put both leads on the same point you read 0 volts. That does not mean there is no electrical potential at that point, just that the difference between your two probes is 0. When measuring the voltage between two phases of a 3 phase system, the meter is reading the difference between the two phases. If you were to draw both phases on a graph and plot another curve that represented the difference in the two phases, you would be plotting what the voltmeter reads. This can be represented mathematically by vectors.

A vector has both a direction and a strength. An easy way to understand vectors is to imagine a wagon with a rope pulling it. With one rope, the wagon will go at the same speed and direction as the person pulling the rope. Tie a second rope to the wagon and have two people pulling in different directions, and now the wagon will travel a path somewhere between the two people. Now have one person pull faster than the other, and the wagon will travel a little more in that direction. Adding different AC voltages and currents is done with vectors because vectors can account for differences in strength and direction. If the two legs were 180° apart from each other pulling in completely opposite directions, the voltage would be the sum of the two legs, as in single phase systems. Because the two phases of a wye type 3 phase system are not pulling in completely opposite directions, the difference is not simply the sum of the two voltages. You can see this by looking at a drawing of a Y type transformer, like the one on page 544 of Fundamentals of HVACR.

Zero is Not Nothing!

The other day a technician asked me why there appeared to be refrigerant coming out of a compressor that he was de-brazing. He had recovered the system refrigerant, so where was this refrigerant coming from? First I asked now deep a vacuum he pulled with his recovery machine. He had pulled down to 0 psig. I pointed out that 0 is not nothing. Even though I am from Georgia, “0 is not nothing” is in fact not a double negative. Remember that there is still 14.7 psia worth of pressure in the system at 0 psig, so 0 is really not nothing. Something is still in there. You would have to pull a deep vacuum to get close to nothing. Additionally, the compressor oil holds lots of refrigerant. So if you want to reduce the refrigerant coming out of the system, you need to go lower. For a system that has an operating compressor, operate the system until the compressor gets warm before starting recovery. No recovery machine can pull refrigerant out of the oil as fast as you can by operating the compressor. However, if you are changing the compressor, that probably won’t work. Instead, get out your hair drier and heat up the bottom of the compressor with hot air. While you are at it, heat the accumulator and filter drier as well. If you don’t plan on dining out while the recovery machine operates, use core removal tools to take the Shrader valve cores out to reduce restriction. Trying to recover through a Shrader valve with the core in is like drinking a milkshake through a coffee stirrer –not impossible, but really slow. But back to the 0 is not nothing story. 

I have done an experiment several times now that shows how much refrigerant can be left in the compressor oil. After recovering a 2 ton R22 packaged unit down to approximately 5” hg vacuum, we let it sit to see that it would not rise above 0 psig. Convinced that we had achieved 0, we operated the unit. After a few minutes of operation, the system was operating at 150 psig on the high side and 25 psig on the low side. There was a lot of refrigerant in the compressor oil. Now I must warn you that compressor manufacturers would frown on my experiment – so don’t do this at home – use someone else’s compressor! Operating a compressor on “nothing” is a sure way to kill it. One last safety tip: instead of de-brazing the compressor, try cutting it out. No fumes, no flames, and far safer if you mistakenly left a lot of “not nothing” in the system.