Force required for small blower motor

[JoeDeschesnes]

Hi! I'm trying to figure out what kind of specs a small electric motor would need to power some small "positive displacement" blower.

According to my design, the total amount of space in the chambers would be approx 50 cubic inches (0,0861 cu feet). Then to reach a required 400 cfm it would need to go at 5000 RPM (giving around 430 cfm). Since the blower tranfer the air to a pierced cushion that needs to be maintained between 1 and 2 psi, the blower itself must still give the minimum 400 cfm at those pressure if I'm correct..
Considering this is just an approximation, and therefore doesn't need to consider the temperature or the composition of the air, etc.
What power (in watt) would need the motor to provide those 5000 rpm (430 cfm) even at 1 or 2 psi against it?

 

[Merlin3189]

I wonder where you get the requirement for 400 cfm?

Otherwise, you can get an approximate measure of the power, from the flow x pressure
$\frac { volume } {time} \times \frac { force} { area } = \frac {distance \times force } {time} = \frac {work} {time} = power$
All you need is to get your data into consistent units or add conversion factors.

I make it about 2600 W which I wouldn't call a small electric motor.
1 psi would obviously halve that to 1300 W. At your low pressure are you really leaking 400 cfm?

 

[JoeDeschesnes]

Thank you very much for the reply! Actually, I'm not even sure if the minimum cfm for my device to work would be 400. In fact, I just based this requirement on an observation from some experiment seen on youtube. Since you've been kind enough to take the time to answer me, I'll reveal a bit more about my project.

I saw people building small hovercraft out of a 2 feet diameter board and a 400 cfm leaf blower. I did some little research and found out that to provide lift, the air pressure under the surface must be higher than the weight divided by surface. So if I weight 180 pounds and my hovering surface is 120 in^2, then the pressure under it must be 1,5 PSI minimum.

Since I'm willing to lift a board of max 120 in^2 with weight around 120 to 180 pounds I'd need a blower of around 1,5 psi. But since most blowers doesn't seem to provide a lot of pressure and most compressor doesn't provide a lot of air, I tought of using a "supercharger" kind of blower since it seem to give enough cfm and pressure. Still, while looking at their specs of thousands cfm, multiple psi, and high rpm, I thought that by reducing the power of those types of blowers I may actually reach just enough psi, cfm and be able to power it with a small electric motor.

Do you think it eould be feasible somehow?

 

[CWatters]

Google can find lots of home made hovercraft. Picking one at random...

This one used a 4HP (750*4=3000W) engine.

http://www.instructables.com/id/Hovercraft/

 

[JoeDeschesnes]

Google can find lots of home made hovercraft. Picking one at random...

This one used a 4HP (750*4=3000W) engine.

http://www.instructables.com/id/Hovercraft/

Thank you but I am actually looking for something smaller

 

[anorlunda]

It takes relatively little power to life a hovercraft a fraction of an inch above a perfectly flat surface. Lifting higher, or lifting over rough ground needs much more power. Some hovercraft use a fabric skirt that drags over the ground to reduce the power required.

All that makes it difficult to give you a simple answer.

I think you should take @CWatters advice. Look at what others have done on that link he provided. Look on Youtube also.

 

[JoeDeschesnes]

I know it doesn't take much power to lift an hovercraft barely above flat ground. Thats exactly what I want to do. My problem is with the dimension of the hovercraft. The smaller it is, the more power it requires for the same weight... "SIMPLE! Just make it bigger!" That's exactly the opposite of what I'm trying to do. Thus my initial question: what kind of motor would I need to power a small fan that provide enough power to counter 1.5 psi considering that the closest experiment I've seen for what I'm trying to do required 400 cfm.

 

[Tom.G]

From the above calculations from both @CWatters and @JoeDeschesnes, you need around 4 horsepower. A small ShopVac vacuum cleaner is about 1HP peak, so you would need at least four of them; if they would even work with your 1.5psi backpressure.

Fans are usually specified in inches-of-water back pressure, with a maximum around 0.5in to 0.75in being common for a common houshold fan. You are asking for one to work around 40in of backpressure, so it's not going to be simple bladed fan.

 

[JoeDeschesnes]

From the above calculations from both @CWatters and @JoeDeschesnes, you need around 4 horsepower. A small ShopVac vacuum cleaner is about 1HP peak, so you would need at least four of them; if they would even work with your 1.5psi backpressure.

Fans are usually specified in inches-of-water back pressure, with a maximum around 0.5in to 0.75in being common for a common houshold fan. You are asking for one to work around 40in of backpressure, so it's not going to be simple bladed fan.

Yes that's why I'm considering a supercharger type of positive displacement blower.

 

[jedishrfu]

As the OPs question has been answered in multiple ways, it’s time to thank everyone here and close the thread.