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PSI - Not Sure How Much I Need

10/18/2012 2:30 PM

Hello, Would really appreciate any advice or feedback since I am not an engineer. I am looking to build a device that emits a continuous stream of air, similar to a hairdryer but without heat. The air stream is also more concentrated and smaller than that of a hairdryer, perhaps the width or circumference of an index finger. I need the air pressure to be significantly stronger than that of a hairdryer. If, for example, a glass of water were sitting on a table, the air pressure from this device would be enough to knock it over. The device would have an electrical cord so it would have a continuous power source to operate the motor, like a hairdryer. Any thoughts about the amount of air pressure needed? Thanks in advance.

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#1

Re: PSI - Not Sure How Much I Need

10/18/2012 2:55 PM

Ok, what do you need, pressure or velocity of a hair dryer. Because I think you may be confusing the two.

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#4
In reply to #1

Re: PSI - Not Sure How Much I Need

10/18/2012 3:30 PM

I believe I need the pressure of a hairdryer. I know there are tools to measure PSI, but would like to know if anyone might guess what the psi of a hairdryer is (roughly).

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#2

Re: PSI - Not Sure How Much I Need

10/18/2012 3:14 PM

Wouln't an air compressor work?

If you're not familiar with air compressors do a "google search" or go to global spec or visit a Home Depot or Menards or??

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#5
In reply to #2

Re: PSI - Not Sure How Much I Need

10/18/2012 3:32 PM

No an compressed air will not work, but thanks for that suggestion. This is much closer to a hairdryer. I actually did go to Home Depot and their compressed air machines are pretty heavy and loud. But they definitely are powerful.

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#7
In reply to #5

Re: PSI - Not Sure How Much I Need

10/18/2012 3:38 PM

Something in an air mattress pump, perhaps?


Coleman Ozark Trail Inflation Pump
$7.88


Coleman Quick Pump
$16.97


Coleman Ozark Trail 120V Electric-Powered...
$13.88

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#8
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Re: PSI - Not Sure How Much I Need

10/18/2012 3:40 PM

As to flow, 5-10 cfm at 5 psi maybe, for a hair dryer.

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#3

Re: PSI - Not Sure How Much I Need

10/18/2012 3:22 PM

Sounds like an air compressor with a regulator.

You'd have to know the duty cycle, (is it continuous or intermittent) rate of flow, nozzle size, hose length, etc.

A small compresser may not be enough if this is a continuous process. Sounds like roughly equivalent to running an air tool. That takes 90PSI and 4-10 CFM. (Cubic feet/minute) to run. That's not an air brush compresser. It's much larger.

Go to Home Depot, Harbor Freight or some place like that and ask.

BE CAREFUL. COMPRESSED AIR CAN KILL YOU, OR TAKE OUT AN EYE.

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#6
In reply to #3

Re: PSI - Not Sure How Much I Need

10/18/2012 3:36 PM

Thanks. I'm curious to know what the PSI and CFM would be for a hairdryer when it's blowing air? Would you be able to guess or is there a tool I can use to measure it myself? Thanks for your help and the warning about compressed air - I know it's so dangerous!

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#9

Re: PSI - Not Sure How Much I Need

10/18/2012 3:57 PM

A hair dryer puts out several hundred CFM but at less than 1 inch of water column or about .04 PSI or less on average.

The levels you are talking about to tip over a glass of water on a table (arms length I am guessing) would need flow rates in the thousands of CFM and tens of PSI using a nozzle the size of your finger which works out to a power input of tens of horsepower or kilowatts.

I have 20 HP worth of air compressor capacity in my shop which gives me a rough output of around 70 CFM at 175 PSI and I can guarantee that even with that I could not tip over a glass of water with it at 3 feet. Now with a short burst out of the 200 gallon air tanks through a 1/2 inch nozzle I could easily do it but then again that's dumping a massive burst of energy over a few seconds.

Now if you are looking to tip over a empty paper cup at 3 feet then yes that takes a few magnitudes of order less energy.

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#10
In reply to #9

Re: PSI - Not Sure How Much I Need

10/18/2012 4:30 PM

Thanks that is very helpful. Yes tipping over a glass of water at arms length. I'm wondering if my device was about the same physical size of a hairdryer but did not need a motor that could generate heat, only air, what would be the maximum amount of PSI or CFM's that could be generated from a device that size? Using a power cord to supply continuous power? I think roughly 13 inches in length perhaps 6 inches in width?

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#11
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Re: PSI - Not Sure How Much I Need

10/18/2012 4:35 PM

You do not have to look at psi completely, but the velocity of the air.

Have a look at this here on CR4

wind load calculation

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#12

Re: PSI - Not Sure How Much I Need

10/18/2012 4:45 PM

A regenerative turbine type of air blower might work. Around the house, a leaf blower might be about the closest thing you could try. It will need a combination of cfm and pressure (the more of one the less of the other, within a range). If I get the chance, I'll play around with some equipment after hours where I work.

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#13
In reply to #12

Re: PSI - Not Sure How Much I Need

10/18/2012 5:07 PM

I'm liking electric leaf blower, too.

It shouldn't be too difficult to fashion a reducer out of plastic pipe to fine tune it, if needed.

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#16
In reply to #12

Re: PSI - Not Sure How Much I Need

10/18/2012 8:42 PM

A couple more references: a good home vacuum cleaner moves about 100 CFM with no restriction, and has about 100 "H2O or 3.5 PSI of vacuum (or pressure) with blocked flow. A human can 'blow' 3 to 5 PSI on a closed tube (not recommended for health reasons).

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#14

Re: PSI - Not Sure How Much I Need

10/18/2012 5:08 PM

The amount of air pressure required to overturn a glass of water at an arms length cannot be generated by a device the size of a hairdryer in a continuous stream.

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#15

Re: PSI - Not Sure How Much I Need

10/18/2012 7:11 PM

Questions, there are always questions. Do you really want to blow a glass of water over or is this your best shot at modeling something else? If the glass is short and stubby, it is more likely to slide rather than tip. It is more likely to tip if it is tall and/or narrow at the base. I'm not sure why I ask this but my head says "ask", is the device stationary or mobile?

Just to get a feel for this, 8oz of water weighs 0.5lbs, assume the glass weighs the same, total of 1.0lbs. Assume 3" high and 1.5" across the base.

The restraining moment =0.75"x1.0lbs. The overturning moment is Px1.5", so equating, the wind force P, must be at least 0.5lbs to tip the glass. But the coefficient of friction is, say 0.3. O.3x1.0lbs=0.3lbs which is less than 0.5lbs, and the glass would slide before it tips.

I just pulled these dimensions out of the air to get a feel for the problem, if the details are different, the result may be different.

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#18
In reply to #15

Re: PSI - Not Sure How Much I Need

10/19/2012 12:08 AM

Hilarious...good argument and made for some good chuckles tonight...but you don't consider the type of wax used on the table...which might drastically alter your coefficient of sliding friction...

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#17

Re: PSI - Not Sure How Much I Need

10/18/2012 10:49 PM

Use a carbon fiber tank for a paintball gun, they hold between 3,000-4,000 psi, will easily knock over ur glass, and aside from a very loud burst of air, there is no mechanical noise to speak of. I will leave the logistics of this experiment to some trial and error on ur part.

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#19

Re: PSI - Not Sure How Much I Need

10/19/2012 12:10 AM

Drill a hole near the end of your hair dryer. Use a Pitot tube and a magnahelic and make the necessary readings.
Then do your homework.
http://www.tsi.com/uploadedFiles/Product_Information/Literature/Application_Notes/AF-106%20Traversing%20a%20Duct.pdf

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#20

Re: PSI - Not Sure How Much I Need

10/19/2012 3:31 AM

If one is overturning a glass of water, why not a jet of water instead of a jet of air?

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#21

Re: PSI - Not Sure How Much I Need

10/19/2012 4:10 AM

Do you want a continuous stream, or, a quick periodic pulse.

It might be worth looking at the design of an airzooka toy (google it and look on You Tube).

The design really focusses the pulse of air. If you stand three paper cups in a line with 1½ ft between each cup you can easily knock over any selected cup from a distance of over 10 feet.

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#22

Re: PSI - Not Sure How Much I Need

10/19/2012 4:42 AM

You get a good blast of air from these hand dryers, they exert a good pressure on your hands although that is obviously close to the outlet. From the bulk & noise they obviously have a much bigger motor than a hair dryer.

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#23

Re: PSI - Not Sure How Much I Need

10/19/2012 4:45 AM

...and these (usual disclaimer).

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#24

Re: PSI - Not Sure How Much I Need

10/19/2012 10:21 AM

Might want to try a vacuum motor and use it as a blower. Or An electric leaf blower.

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#25
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Re: PSI - Not Sure How Much I Need

10/19/2012 12:54 PM

To build on Ozzy's suggestion, a good vacuum cleaner can generate a maximum pressure differential of about 5 psi and its CFM is about 100 but not at the same time. This is with a motor of about 3/4 to 1 hp (don't pay any attention to that stupid peak H.P. they like to tout on vacuums ) Look at the ampere rating on the motor plate. You want to see about 15 amps plus to get about 1 h.p. of work out given the inefficiencies of the motor and blower assembly.

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#26

Re: PSI - Not Sure How Much I Need

10/19/2012 1:22 PM

I've been wondering about focusing a jet of air so that it will still be concentrated at arm's length, perhaps one of these would do it, it works with water.

The need at the glass is velocity, That is attained by pressure short of the nozzle. Wind pressure, P =0.00256V2, V in MPH.

The area of the glass that I used earlier was 0.25'x0.125'. 1/32of a sqft. If P/32=0.5lbs, P=16psf. 16/0.00256=V2, V= 79mph or 116ft/sec. 116ft/sec, at arms length, looks huge.

If the glass is round, the shape factor changes and more pressure would be needed.

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#27

Re: PSI - Not Sure How Much I Need

10/19/2012 8:51 PM

Pay a visit to a tyre company that fits truck tyres. There you will see a big compressor that won't be able to do what you want to do BUT they will have a hand held tank that they pressurise and fit with a nozzle that goes between the tyre and rim. As most modern tyres are tubeless it is impossible to simply blow up a new truck tyre as it is with a car tyre. ( it just doesn't touch the rim, leaving an opening for the air to escape ) The tank is pressurised and placed next to the tyre and rim ( in the gap ) and a ball valve is rapidly opened allowing the pressurised air to escape very quickly, inflating the tyre enough to push it out onto the rim and make a seal so normal inflation can continue. If you take a metal "glass" with you they would be happy to demonstrate it's ability to knock it over at the distance required. Once you have seen this demonstration i suspect you may drop your plans as the noise generated by the rushing air is high. On the plus side a small compressor will be able to fill the tank with compressed air, just taking longer. Large diesel powered compressors as used in mining will be able to supply enough air to give you the velocity and pressure you seek but certainly not my large, loud and huge 20HP unit. This last statement refers to supplying air constantly whereas the tank ( which is do-able ) will supply a short burst.

Just another thought, one i'm not knowledgeable enough to answer by the way. Will an orifice/tube the size of a finger EVER be able to supply a stream of air with the pressure and velocity required? Intuitively i would say that if connected directly to a gas bottle at 10,000 psi it would, buuut. Certainly a larger orifice/tube would once connected to a mining compressor.

Any thoughts?

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#29
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Re: PSI - Not Sure How Much I Need

10/20/2012 5:42 PM

Truckers inflate tires by injecting starter fluid and igniting with a flame to seat the tire before inflating; same principal.

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#30
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Re: PSI - Not Sure How Much I Need

10/20/2012 7:11 PM

That's clever. Not necessarily good, but clever. I guess an explosive would work as well: New thread? How do you 'blow up' a truck tyre?

Jim

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#28

Re: PSI - Not Sure How Much I Need

10/20/2012 3:30 PM

Where did you go, Melissa?

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#31

Re: PSI - Not Sure How Much I Need

10/23/2012 10:22 AM

Put a cat on the table, that'll knock over the glass of water
Del

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#32

Re: PSI - Not Sure How Much I Need

04/21/2013 10:32 PM

Hello - Reading through this thread again, I appreciate all the responses. I'm not sure if anyone is still active on here, I know I disappeared for a time, but would like to revisit this project/problem to solve. I think the question of whether a nozel the size of an index finger could handle enough pressure to knock over a glass of water is a good one. I wonder what the maximum amount of pressure passing through a small nozel (size of two index fingers) would be? I know some dentists have a tool that delivers air with quite a bit of pressure through a very small hole. It feels almost like a pin prick. Has anyone seen such a device? I don't need to knock over a glass of water per se but was just trying to suggest an example of strong pressure. I think this is a really good forum and appreciate hearing from all of you!

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#33
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Re: PSI - Not Sure How Much I Need

04/22/2013 5:20 AM

Do you want a continuous stream, or, will pulses of air do the job?

There is a huge difference in the total average power required, and, using pulses opens up a lot more possible design techniques.

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