Airflow v Airspeed

[vacfan1982]

Can someone explain to me in laymans terms the difference between airflow and airspeed and which is more important to get clean carpets?

Here's an analogy.

A wide river could have lots of flow but the flow is slow and you could safely walk across it.
A narrow river could have less flow but it is flowing fast, if you tried to walk across this river you would be swept away.

So is it airspeed (velocity) That is more important than airflow?

This is complicated science for me haha

 

[vacfan1982]

Is Airspeed through the pile more important than airflow?

You could have large airflow but low air speed as in my river an analogy.

 

[kirbysthebest]

From your example I would surmise

but I could be wrong. . .
What you are describing is in part due to the Bernoulli principal.
https://en.wikipedia.org/wiki/Bernoulli's_principle
with a wide open river the same amount of water is moving, but with less force as it is non restricted. When you narrow or restrict this flow the same amount of flow is attempting to move at the same rate through the restricted exit hence increasing the speed and force of the liquid. (Yes air is actually a liquid).

An example I have used often regarding CFM, flow, and force:
A clothes dryer has a blower that moves air through your clothes as they are tumbling at about 180 CFM (Cubic Feet Per Minute). Because this flow is for the most part unrestricted it just moves through the dryer and carries the moisture laden air away from the clothes.

A vacuum cleaner, on the other hand has a restriction where the air speed is increased so in effect 180 CFM of air moving through a 1 1/2 inch tube will have more force than the same amount of air moving through a six inch tube as in the dryer exhaust.

This was a hurried, and probably not too well explained; perhaps others can add to the discussion.

Another great example of Bernoulli effect, Chicago, the windy city.
Wind can be blowing 30 MPH, but when it is being forced through narrow spaces between tall buildings the force is increased to more like 60 MPH, as the same amount of air is trying to move through a smaller space.


http://www.validyne.com/blog/application-note-basics-of-air-velocity-pressure-and-flow/

 

[vacfan1982]

Thank you for that Harley. Yes are correct I was reading about the Bernoulli principal earlier. How sad am I haha

But I do find all this interesting being a geek

 

[vacfan1982]

I just read the link you sent. My head is hurting now haha

 

[kirbysthebest]

Headache??

Take two Vicoden and call me tomorrow.

 

[vacfan1982]

Thank you Doc

 

[vaclab]

I see the two like this...

Airspeed is only the speed of a certain sample of air. For example, air travelling at 60 MPH or 10 M/S. Airflow takes into account a volume moving at a certain speed. For example, cubic feet per minute or liters per minute.

 

[vacfan1982]

So is Johnny correct in saying that airspeed is what you need to clean carpets well? The faster the airspeed through the pile the more dirt will be picked up?

 

[vaclab]

Yes, Airspeed Over an Area is The Most Important Thing

in addition to agitation. Remember, you can't have airspeed by itself. The speed of the air will have some sort area (i.e. nozzle opening) to flow through.

For comparison, a Dyson DC65 has 30 MPH air with the nozzle removed (ball bottom) and a Kirby Sentria II has 77 MPH air with the nozzle removed (on low speed). The Kirby easily deep cleans through a pile carpet while a typical bag-less machine can't.

Bill

 

[vacfan1982]

Thank you for clearing that up Bill

 

[n0oxy]

you need both to clean well

For cleaning carpets, you also need good agitation but let's set that aside and focus on the two main specs of a vacuum, airflow or cubic feet per minute, and lift, this would probably be the same as air speed, it's also known as the amount of suction. Both are needed to clean well, as one increases, the other decreases. Too much of one and too little of the other will not clean well. For example, when you use a table or floor fan, the area is very large, you have lots of airflow but not very much lift. Sure, it would blow large things like papers around but if you tried to use that set up to remove dust from the floor, you probably would not get very far. On the other hand, if you have lots of suction but not very much airflow you will also not clean well. If you attach a crevice tool to the end of a vacuum hose, the opening is very small, you will have lots of force to the air but not nearly as much air is being moved. As with many things in life, a balance is good. In order to clean well, you need to be moving enough air to encourage the dust and dirt to move from its place, and that air needs enough force to overcome the weight of what is being vacuumed up.
Mike

 

[vaclab]

Mike, I must disagree with you on this one

My entire youtube channel consistently shows that fine dust pickup requires high airflow coupled with a reasonable amount of agitation. High suction is a lie perpetuated by the bagless manufacturers in order to make their machines seem as good as dirty air machines.

Case(s) in point: Hoover Convertible with 25" of lift but 107 nozzle CFM. Cleans fine dust and grit very well. Next, a typical Kirby with 34" of lift and 120-137 nozzle CFM. Cleans so well, it easily cleans the carpet under the carpet, when the carpet is flow through of course.

With my extensive testing, I will not "drink the suction koolaid". Suction is good for lifting large objects, like bowling balls, bolts, pool balls and probably two by fours, not dust and grit though.

 

[vacfan1982]

I'm confused again now

 

[sptyks]

Suction vs airflow

I would have to agree with Bill.


 


Airflow is much more important than suction for deep cleaning carpets.


 


Suction is only good for surface pickup which is why you won't find any bagless vacuums that can deep clean carpet. Bill has videos that prove this.


 


 

 

[vacfan1982]

Airflow alone will pick up nothing. A desk fan produces lots of airflow but without suction behind it, it will pick up nothing.

In my opinion airflow and suction work together. And suction is more important when using a hose with tools.

 

[vaclab]

Airflow with relatively low suction

Can pick up houses. Think of a tornado. They are rated by wind speed, not suction. Also introducing wind driven tools changes the equation quite a bit. You are now making the air do work by forcing it to spin something.

Non mechanical tools work better with high airflow, not high suction. I have many videos showing this repeatedly. Maybe I should make another one...

 

[vacfan1982]

Surely you need suction to keep the airflow strong at the end of a hose?

 

[vaclab]

Ah, now I'm seeing where you're coming from I think

The strength you feel is a pressure (suction). If you need to force air to do work (spin something) or traverse long distances (say 50 or 100 feet), then higher suction will maintain more airflow. BUT, if your airflow is restricted too much, like cyclones, air driven tools, etc., then there is not much airflow remaining to pickup dust anyway. Turbine tools reduce airflow horribly just like cyclones unfortunately.

A Kirby Sentria 2 has 120 CFM at the end of the hose, but only 40" of lift. Standard non-Kirby air driven tools will work, but you won't get the best performance due to low suction.

See my turbine tools versus suction video for more info.

Bill