CFM and static pressure rating for a Squirrel-Cage/Centrifugal fan?

[ScatterPlot]

First off I'd like to say if this belongs in Friendly then please move it.


Anyways~

I am building a hovercraft, and have used some online calculators to determine how much CFM, motor horsepower, etc. that I need to use, and it even tells me the type of fan and all that I need to get my desired results. However, these results are for obviously a normal fan (X shaped). I don't want to get one, since they'll be expensive to have one custom made, shipped, and all that. I already have a very large centrifugal fan that I would like to use, since it's very big and can handle static pressure. I was wondering if there's any way to find out how well the fan would work by means of some equation or online calculator or something. If any of you engineers wanna gimme some help with this I'd be grateful. It's not an absolute need-to-know type thing, since I could just stick it on and try it, but the construction is still in the design phase. Anyone know how I can find out what kind of power this thing can put out? I've looked all over it and there's no manufacturer or anything like that to call, only an inspection stamp thing. Even if I were to find a number they'd probably be out of business, since it was inspected in like 1950 something. So anyway, can anyone help me with this? Thanks
Bert McMahan

[Redkey]

questions

Since I know nothing of hovercraft design my questions may not apply.

I assume the fan works and that you are trying to figure out how much lift potential it has?

Would it be worth while to build an assembly so the fan could be mounted firing in the upwards direction... then build a "tray" that is lifted by the force of the fan. Stacking weights on the tray would give you an idea of the amount of lift generated by the fan. Probably more work that it's worth.

Can anything useful be gained by knowing the output velocity of the fan and its cross-sectional area?

Is there anyway you can back-calculate the fan parameters by knowing the diameter, blade size, blade pitch and rpm?

good luck

[ScatterPlot]

The problem is that all the calculations I can find are for normal propellors. Also to make a mount upside down like that would be kinda hard, and also the fan will behave differently with a skirt than just with a board on it. I was thinking if anyone knew of any formulas for this kind of thing if they were some kind of engineer or something. Any of your friends an engineer that might know this ?

[SlartyBartFast]

You'd have to test the fan.

Or, find the manufacturer. They'll give the static pressure and CFM rating on their tech sheets.

[ScatterPlot]

I wish I could find the manufacturer, but they're probably out of business since it was made in the 50's. I was hoping for a formula or something that someone out there in Internet land would have. Guess not, thanks for the help though.

[Jade]

Bert-
Since you cannot seem to get a fan curve for this particular fan, your next course would be either to generate a fan curve with a flow bench, or get a fan curve for a similar fan and apply the fan laws. The fan should be similar in the no. of blades, width, etc..
Remember there is a difference btwn the fan curve and the system curve. Your installation may add additional static pressure that will need to be considered. I don't know of any calculators that will relate a fan performance to a hovercraft, but I imagine you could start with f=pa. Good luck.

[ScatterPlot]

No no no, it's not a fan blade, it's a centrifugal fan. Looks like this:
http://cgi.ebay.com/ws/eBayISAPI.dll...category=42225
(kinda).
It's not like a fan in a tube, so those fan laws do not apply.

[Jade]

um yeah, I understand that. Fan laws apply to centrifugal blowers too! What is the diameter of the wheel? Is it a backward curve design? How many blades are on it? What it the wheel width? Backward curve is common, so if it is you could get a fan curve from another product of similar dimensions and extrapolate. Every fan is a constant volume air pump of sorts. Axial and centrifugal and every derivitive within are just better for different things (eg. centrifugal blowers work nicely against higher pressures, but tend not to move as much air as a similar sized axial...)

[ScatterPlot]

Ok sorry, I thought you meant a turbine type thing. However its kinda weird; it doesn't have propellors in it. Roughly the disc is like 15 inches in diameter. But like I said, there's no prop, its like you took a picket fence, rolled it into a circle, and tilted the blades some. The blades are a little under a half inch or so thick and are tilted at like a 40 degree angle or so, again without measuring. It's about 16 or so inches wide and I havent messed with it enough to determine if it's backwards or whatever. Thanks for your help and...


WELCOME TO AO!! I hope you enjoy the forums! But anyway I'll check into the fan some more, and see if I can find out the manufacturer written somewhere obscurely on it and figure something out. Ill check those laws again and see what I can come up with.

[Redkey]

does the fan work?

what kind of power requirements does it have?

What is the HP of the fan motor.

[ScatterPlot]

The fan works. I dohn't know it's power requirements, nad that's kinda the reason I needed an equation- to determine motor size and all.

[athomas]

Build a box on the output of the fan. At the outlet of the box put a trap door on a hinge. Start the fan and run it allowing the air to force the trap door open as it escapes. Now push against the door to hold it shut. Measure the amount of pressure(lbs) it takes to hold the door closed or near closed. Measure the current draw of the fan as well. This will give you an idea of how the fan will react under load and the load that it can handle.

[SlartyBartFast]

For static pressure, run the fan blowing into a closed box with a pressure guage on it.

Keep increasing fan speed until the pressure drops instead of rises. Check the current and rpm of the motor and compare it to the motors capacity/rating. Change motors (larger if fan doesn't top out on static pressure) if necessary.

[ScatterPlot]

It's pulley powered, so it'll be gas-powered. I might try some of that though. Thanks for you input.

[phyregod]

The Tao of Junkyard Wars

They built personal hover craft on junkyard wars last year.. One of them was using a fan like the one you are describing. Both of the hovercraft worked, So it must not be too exact a science. If you don't get enough air it will be obvious, if you get too much air it will just blow out the bottom. Try and see? Just thought you would like to know that two teams were able to build two hovercraft out of junk in ten hours. Should provide some encouragement.