Shop air cleaner

[william watts]

That is totally intuitive. The more constriction means less air flow means less amp draw. That is the exact opposite of what you are saying.
A DC uses the most current, and moves the most air, when you remove the ducts and filter; and it uses the least current and moves no air when you put something over the intake.

Dale,Wade, and Art are in agreement. Every one is right. You can prove this for yourself by putting amp meter on your shop vac. Measure the current it draws, then block the intake with your hand, you will see the current decrease. It's not a centrifugal fan, but it's the same principle. Some folks are fooled because the sound from the motor/fan really starts to scream. The rpm increases because there is less air to move, but the current goes down. Dale has fought this battle before.

[Ole Anderson]

The rpm's increase on a shop vac with your hand over the hose largely because the fan is now spinning in a partial vacuum which of course has much less resistance than air at 14.7 psia. Almost like taking the fan off the shaft.

[Dale Murray]

To properly balance a forced air HVAC system you will, among other things, check The Supply Static Pressure (+) and The Return Static Pressure (-) to get the Total External Static Pressure. When adding the two numbers you ignore the -/+, just add the numbers, the is the TESP of the system.

"Most manufacturers’ engineering manuals rate airflow in furnaces, fan coils, and air handlers based on Total External Static Pressure (TESP). A typical furnace is designed to operate at a maximum of .50-in. w.c." * w.c. is for Water Column.

If a furnace blower is operating outside those specifications one of two things will happen;
- Zero static pressure (unrestricted airflow) will cause the motor to work harder (moving greater mass than design specifications) thus increasing the amp load on the motor and burning it up.
- Excessive static pressure (near zero airflow) will cause the motor spin faster (the motor is working against nothing) while moving nearly zero air (not enough to cool the motor) and burn up.

In my case my motor told me what the AMP load should be for each speed setting.
I then, using my clamp meter, determined the amount of restriction (restriction of air flow results in Supply Static Pressure as a negative number) required to run at the selected speed with specified AMP load. I then tested at the other speeds and AMP loads to be sure my level of restriction allowed the motor to operate withing design parameters (as specified on the motor).

Whether the static pressure is on the supply side or exhaust side is irrelevant.

So, I guess you can run one of these motors with zero restriction and burn it up, however, you will get optimum performance and air flow when it is operating within design specifications.

I have supplied several links and given enough terminology for anybody to do their own homework and figure this out. If you still have doubts, get a furnace blower and clamp meter and do some testing.

[Dale Murray]

Dale,Wade, and Art are in agreement. Every one is right. You can prove this for yourself by putting amp meter on your shop vac. Measure the current it draws, then block the intake with your hand, you will see the current decrease. It's not a centrifugal fan, but it's the same principle. Some folks are fooled because the sound from the motor/fan really starts to scream. The rpm increases because there is less air to move, but the current goes down. Dale has fought this battle before.

The difference is your shop vac will not burn up if offered unrestricted air flow, most furnace blowers will.

I believe that is the cause of disagreement; the idea furnace blowers are designed to operate with some form of static pressure (supply side or exhaust side), without it the motor will burn up.

[Ray Newman]

"DO NOT RELY ON A AIR CLEANER TO ACT AS A DUST COLLECTOR. The purpose of and air cleaner is to keep airborne dust in suspension and reduce airborne dust as quickly as possible AFTER THE DUST PRODUCER HAS BEEN TURNED OFF. "

--Howard Acheson

BINGO! The 1st step to "clean" air is collecting the dust and other particulates at the source.

[Peter Aeschliman]

"DO NOT RELY ON A AIR CLEANER TO ACT AS A DUST COLLECTOR. The purpose of and air cleaner is to keep airborne dust in suspension and reduce airborne dust as quickly as possible AFTER THE DUST PRODUCER HAS BEEN TURNED OFF. "

--Howard Acheson

BINGO! The 1st step to "clean" air is collecting the dust and other particulates at the source.

Yep! But this is not to say that we shouldn't also use air cleaners as a backup to get anything that escapes primary dust collection.

[David L Morse]

I believe that is the cause of disagreement; the idea furnace blowers are designed to operate with some form of static pressure (supply side or exhaust side), without it the motor will burn up.

GOT IT! Before you added the restriction your motor was drawing more than it's rated full load current. You then reduced the CFM just enough to bring the motor current down to rated FLA. I couldn't figure out what you meant by "optimum airflow/amp draw ratio". Now I understand that meant maximum flow without frying the motor.

[Dale Murray]

The thing is, the motor was not throwing an impressive amount of air until I restricted it; the velocity increased. Maybe it was the same volume however at a greater velocity.

Think of a squirrel fan as a circle with a bunch of shovels scooping air and the throwing it; that is effectively what it does.
Now lets imagine a hundred men with wheel barrels;
- if you load each barrel with 300lbs they will all be very labored moving dirt and not move very fast and get worn out quickly.
- if you load each barrel with 100lbs of dirt they can nearly jog to the destination and quickly return for more, this will likely improve productivity.

Make sense?

Do not think of these devices like a vacuums or house fans, they are a different beast altogether.

It had my mind bent until I did the actual testing and found it to be true. Unfortunately I dont have my notes from testing where I tracked the amp load.

[Wade Lippman]

There are three claims.
1) Squirrel cage fans will burn out if they don't have enough resistance.
I don't find anything definitive on this, but everyone seems to think it is true; so it probably is.
2) 3 heavy filters don't provide adequate resistance to avoid burning out.
This seems highly unlikely, but I surely haven't tested it, so it might be true.
3) "Furnace blowers work best when there is considerable constriction on the inlet and outlet pathways. This allows the motor to turn higher RPM while drawing fewer amps thus moving more air."
This is just crazy. It might burn out doing it, but an unrestricted fan drawing more current will certainly move more air than a heavily restricted fan. A restricted fan uses less current because it isn't doing anything.

[Dale Murray]

What fascinates me is I have delivered links to professionals stating the very things you doubt.

http://hvacrfundamentals.blogspot.co...fan-motor.html

Since you apparently have not followed or read the links provided, here is the text:

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.




At this time I feel I have delivered enough information and sources. If you continue to have doubts then I suggest you get yourself a blower and do some testing.

[Wade Lippman]

The amount of air the fan is moving decreases as the resistance to airflow increases.
The amount of air the fan is moving decreases as the resistance to airflow increases.
The amount of air the fan is moving decreases as the resistance to airflow increases.
The amount of air the fan is moving decreases as the resistance to airflow increases.
The amount of air the fan is moving decreases as the resistance to airflow increases.
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.

What part of that can't you understand?!
You have delivered plenty of information; you just don't seem to understand what it is saying.

[Dale Murray]

Put 80" tires on your car and tell me how much faster it is because one revolution of the tire covers a greater distance, never mind your engine and drive train were not designed to deliver that much torque. Most likely you will burn up the engine and transmission and cover very little distance.

You need the torque to overcome the resistance of the large diameter tires.

Put normal sized tires on your car and tell me how much slower your car is because those little tires do not cover as much ground per revolution. Oh, wait. You actually go farther and faster more efficiently because it requires less torque to spin the tires.

Your engine and transmission will spin more revolutions per mile covered but using less torque thus operating more more efficiently.

In the case of a centrifugal fan (from a HVAC system) the lack of static pressure is like putting huge tires on your car.
The fans diameter does not change but the amount of air it is trying to move per revolution is at its maximum. As a result more torque is required to turn the fan and move air. More torque = higher amp draw = more heat = burning up the motor.

By offering a centrifugal fan some form of resistance (in the form of vacuum on the supply side) the fan has less air to push per revolution but is able to spin faster allowing it to push more air.

If you completely block the air flow it will burn up.

[Wade Lippman]

I suspected you were goofing on me. Now I am sure. Okay, you hooked me, but didn't succeed in landing me.

Using documentation that contradicted your point, while claiming it was proof of you point was inspired though.

[Curt Harms]

Perhaps something constructive here - a source for smaller squirrel cage fans. The recycled ones from furnaces are great and typically cheap but sometimes they're a little large, especially if you want to hang it from the ceiling. I have a hanging air cleaner and was looking for replacement blowers just in case. Apparently greenhouses use them - here's one example:

http://www.cheaphydroponics.com/product-p/active465.htm
ACTIVE465-2T.jpg

I have a similar blower in my air cleaner with a coarse precleaner and bag filter. It seems to work pretty well. If I were doing it again I'd use a furnace filter(s) in place of the bag filter - easier and cheaper to replace.

[Wade Lippman]

Perhaps something constructive here - a source for smaller squirrel cage fans. The recycled ones from furnaces are great and typically cheap but sometimes they're a little large, especially if you want to hang it from the ceiling. I have a hanging air cleaner and was looking for replacement blowers just in case. Apparently greenhouses use them - here's one example:

http://www.cheaphydroponics.com/product-p/active465.htm
ACTIVE465-2T.jpg

I have a similar blower in my air cleaner with a coarse precleaner and bag filter. It seems to work pretty well. If I were doing it again I'd use a furnace filter(s) in place of the bag filter - easier and cheaper to replace.

Grizzly air filters with remotes are only $150. Hard to justify $80 for the fan.