Question of the Day:

"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
04/25/2016 at 16:05 • Filed to: None

How many CFM is the typical hair blow dryer?

DISCUSSION (30)

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > Rusty Vandura - www.tinyurl.com/keepoppo
04/25/2016 at 16:07

Not enough to ‘turbo’ your car with one...

BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest. > Rusty Vandura - www.tinyurl.com/keepoppo
04/25/2016 at 16:09

I’d take a guess and say 50.

Tareim - V8 powered > BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest.
04/25/2016 at 16:10

I have computer fans that can push more than that (or so the spec says) and don’t feel as powerful as a hair dryer to me

BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest. > Tareim - V8 powered
04/25/2016 at 16:15

I guess my guess was just a guess then.

Also there’s a difference between pressure and CFM. I’m not versed in what that is though or how you’d measure it.

Maybe use a mass air flow sensor from a car?

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo
04/25/2016 at 16:15

I design ducting as one of my several hats, and we try to keep duct sizing so that the airspeed is <1200ft/min because of sonic effects. I’d expect a hairdryer to not be over maybe twice that, so that would make a hairdryer with a 2" bore something in the range of 50CFM.

TheTurbochargedSquirrel > BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest.
04/25/2016 at 16:19

CFM stands for Cubic Feet per Minute, and measures what it sounds like, how much air can be moved in 1 minute.

BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest. > TheTurbochargedSquirrel
04/25/2016 at 16:20

I get that, but there’s also pressure. What’s that measured in for fans?

Tareim - V8 powered > BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest.
04/25/2016 at 16:22

must be a more simple way of testing?

samssun > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
04/25/2016 at 16:26

Came here to post this. Might need to specify “even if it’s a Honda” just to be safe.

miadaman? yes please > BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest.
04/25/2016 at 16:26

Depends if you are talking about velocity pressure or static pressure. One (velocity) is used to calculate CFM using the cross section area, the other represent the force it exerts on objects blocking it.

RamblinRover Luxury-Yacht > BmanUltima's car still hasn't been fixed yet, he'll get on it tomorrow, honest.
04/25/2016 at 16:26

Typically you measure drops in pressure over a system with airflow in inches of water. It’s a much finer measurement than psi. It’s really easy to set up something to measure that with, too.

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht
04/25/2016 at 16:43

This,

but more like this (tidier, though):

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So I want to find a ball park CFM number, then buy a real squirrel cage fan with about a 2-inch output and a rheostat control so I can dial the CFM to mix with the propane, a la:

!!! UNKNOWN CONTENT TYPE !!!

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo
04/25/2016 at 17:15

I can tell you how to make a machine that will measure the flow from the hairdryer out of pipe, but unless you expect a lot of backpressure the result from that will be fundamentally useless. Even if you had a super-precise pressure meter. Other than guesswork, it will be hard to find out, and the more useful information is what you’d call a pressure curve - what CFM it will push against a given system resistance. That’s the problem with the comparison to a 50CFM computer case fan - it has very poor ability to put up pressure, which the hairdryer doesn’t necessarily.

Storytime! I once built what’s called an elbow flow meter so my company could calibrate a machine pushing 30-100CFM, but because an elbow flow meter depends on mass of air, you have to be moving a good bit of air to be able to measure anything. An elbow meter works like this: you set up laminar flow after the air source you’re using (so, you’d connect it to a piece of PVC or similar), then you force the air to make a 90 degree turn with a pipe elbow. The required centripetal force on the air to make it “make the turn” creates a pressure differential from the inside of the elbow to the outside. It’s a bastard to get working properly, however, because you also need to have a smooth transition into the “elbow” region to control turbulence, and you have to have a pipe leading away from the elbow to maintain smooth airflow - so it doesn’t “hiccup”. The pressure sensor I was using was extremely sensitive - it would measure down to something under 1% of an inch of water, but the pressure differences in a 2" pipe were miniscule. So, after all that and a great deal of difficulty getting the right math figured out, it was only reasonably accurate in excess of 30CFM (50CFM, better), and almost useless below that. To get even that far required a very expensive tool and 15' of plastic pipe and more fiddling than I care to recount, including calibrating the Excel setup for that particular elbow. And what, at that point, do you have? How many CFM the system will put up pushing air through a 2" plastic pipe 15' long. Probably not applicable to anything you’re doing.

From what I remember about airspeed from that project’s conditioner, a hair dryer *might* be as high as 70-100 CFM. At most.

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht
04/25/2016 at 19:10

Are you an engineer? A technician? I’m curious.

So if I had a small centrifugal fan and a rheostat such that I could generate 0-100 CFM, I’m sure I’d be golden, and I might only need something like 10 or 20, because there would be the pressure of the propane entering the little combustion chamber, heat convection, et cetera. So where would I find a fan like that? With a 2-inch output? Yes, like Mr. Kingofrandom, I could buy a blow drier at the thrift store for $2, but as Ttyymmnn points out, O.C. doesn’t like to do things halfway. I have a reputation to establish.

Any ideas?

Santiago of Escuderia Boricua > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 07:19

Interesting... I’ve been thinking about picking up some wax filament to print casting blanks http://www.machinablewax.com/product.php?pr…

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 09:26

I’m trained as a mechanical engineer, and while most of what I do is parts design, I also do sales drawings and some test engineer type things. Sadly, I don’t have much in the way of recommendations for fans because most of what we do is with very expensive commercial hardware - a 200-400CFM impeller and motor unit could cost $700.

As to your best choice of fan, it seems you could actually try using a computer case fan, because it seems that some people have gone to a great deal of trouble to develop pressure curves for them:

Kinja'd!!!

...except that, as you see, you’d have to not have a whole lot of backpressure. The best case fan on that chart, a Corsair 120mm, chokes to below 40CFM when the backpressure exceeds 2mmH20, and falls below 10CFM at 3mmH20. That’s 30 Pascal, or about 4 thousandths of a psi. Almost literally nothing. Anything forced air in any way is going to be more than that, I would think.

Add in that you often need pulse width modulation to get different speeds with these properly and need to run them on 12V, and you can see why the hairdryer solution looked so attractive. The hairdryer blower is designed to run on A/C already, meant to run with a lot of backpressure, etc. etc. I regret to say that other than using a small automotive blower, I can’t think of anything off the top of my head that would work better, unless maybe a model airplane ducted fan, and that could be very pricy.

RamblinRover Luxury-Yacht > Tareim - V8 powered
04/26/2016 at 09:32

The reason a computer fan doesn’t feel as powerful as a dryer fan is that it can only put up high CFM numbers with no resistance. Add even a tiny amount of resistance and it drops away very fast. The dryer may only ever turn at a certain speed (to push a given amount of air), but if it can keep up that speed building compression, the pressure curve will be very, very different than the case fan. It’s a lotus to road tractor comparison.

Rusty Vandura - www.tinyurl.com/keepoppo > Santiago of Escuderia Boricua
04/26/2016 at 11:28

Cool. I’ve been thinking I should have my head examined, but I think this looks like a really fun project. I don’t know what I’ll cast yet...

Santiago of Escuderia Boricua > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 11:32

Stuff for cars, of course

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht
04/26/2016 at 11:32

Thanks for taking the time to share that. The automotive blower is an interesting idea, but I suspect it’s too much. I’m beginning to think that I’ll stick with the hair blower... My wife would have a Duh! moment about all of this if she knew about it (the time-and-effort-and-expense part of not buying a blow dryer at the thrift store...). The truth is, after 26 years, I don’t surprise her too often any more.

Rusty Vandura - www.tinyurl.com/keepoppo > Santiago of Escuderia Boricua
04/26/2016 at 11:33

Radical hood ornaments, perhaps... First, I need to get the furnace going and melt the aluminum into ingots...

Santiago of Escuderia Boricua > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 11:37

That would be sweet.

A friend of mine who was looking into this stuff suggested grabbing broken aluminum heads from a junkyard, since it’ll already be a casting alloy. Then campfire + hammers to break it into small enough pieces to put in your forge.

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 11:40

I have a casting project in the works that I’ll probably do a few months down the line, but that will likely be with heat supplied via acetylene torch and a plaster of paris mold pre-heated in an oven. Casting zinc, so the temperatures involved are neither ludicrously high nor terribly exact.

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht
04/26/2016 at 11:55

What are you going to make?

I want to get an aluminum cylinder head and melt it down into ingots. Just for fun.

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 12:03

I’m contemplating an Art Deco dragon hood ornament for the ‘59 Lincoln, which came without one of any kind.

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht
04/26/2016 at 13:19

Sounds good.

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht
04/26/2016 at 13:28

Are you going to make the model from scratch?

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 13:38

I’ve got a 3d shape made up from several 3d blanks, which I then need to transfer to slabs of loss material, mill/carve out, and assemble, then place in the plaster of paris. No original source to work from - Agrajag and I did a back and forth on Oppo about design inspirations and elements to combine until we’d come up with something.

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht
04/26/2016 at 13:39

Agrajag?

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo
04/26/2016 at 13:41

https://kinja.com/agrajag is an excellent pencil artist and was interested in the project, so I worked with him on conceptual stuff.