## Fan Design - A problem

## Fan Design - A problem

(OP)

As part of a small project for uni, I am looking at extraction fan design around the home. You know the ones I mean, the little ones in bathrooms, kitchens etc. I need help with some basic calcs. The way I have started is by calculating the initial velocity of fan to provide an extraction rate of 0.06m3/sec which gives me approx. 6.42m/s.

From that I have calced the Velocity Pressure of around 0.106 x 10-3 Units????

What else do I need ? help ?

Is the density ratio = 1 because the standard for air will be the same for operating in a bathroom ?

Please help with some more calcs.

From that I have calced the Velocity Pressure of around 0.106 x 10-3 Units????

What else do I need ? help ?

Is the density ratio = 1 because the standard for air will be the same for operating in a bathroom ?

Please help with some more calcs.

## RE: Fan Design - A problem

always calc. the values of the humidity,altitude,sea level,max and the min temperature, mean temp., material for the blades of the fans and the motor details.

please kindly mention the above details then we will solve ur problem.

regards

s....

## RE: Fan Design - A problem

Please can you help, its getting rather desparado.

Thanking you

## RE: Fan Design - A problem

You are worried about variation in density due to moisture and temperature changes. Don't worry about these things. Both increased humidity and increased temperature

bring decreased density and decreased load on the fan.

In English units, fan engineers use 0.075 lb

per cubic foot for a "standard" density.

I worked part of your problem after converting to more comfortable non-metric units. q=126cfm u=21 ft/sec

We need to be concerned with "specific speed"

of the blower. In English units, this is

Ns=(rpm)(cfm)^0.5/(h)^0,75, where h is the head developed. You have identified the exit velocity as 6.42M/sec or 21 ft/sec. The inlet velocity is assumedto be zero. We are neglecting any upstream or downstream losses.

h=(density/2)u^2= 5.3 ft.

Assuming a shaded pole motor with rpm=1500,

Ns becomes 4850. From experinece, this is a comfortable Ns. If it had been as low as 2100 or as high as 11000 I would begin to squirm.

The next step would be to find a good hub ratio. This is hub diameter/outside diameter.

Experience shows that values of Dh/Do=2000/Ns

(+ - 10%) will work nicely and avoid problems

with awkward solidities or overlapping vanes.

So we let Dh/Do = 2000/4850 = 412.

Now, the flow area through the fan is

Af = (pi/4) (Do^2 - Dh^2)= (pi/4)(Do^2)(1-[do/Dh}^2)=(pi/4)0.8305 Do^2

But it is also equal to the flow rate divided

by the velocity through the fan.

Af = q/u = 2.11 cu ft/sec/21 ft/sec

Putting these two together and solving for Do

gives Do = 4.70 inches.

We can go on to determine the correct inlet and outlet angles for the blades at several

radii, the blade width, the number of blades,

and similar details for a set of outlet guide vanes. Also, we can calculate the power required to drive this machine.

Materials: This should not be much of a problem. For large quantities (15,000 up)

I am inclined toward molded plastic. For an off-the-top-of-the-head choice, I would go with ABS. I would be concerned about adequate

long lasting attachment to the motor shaft, but feel that these could be solved by adding longitudinal scratches to the shaft and a spring clip to the plastic hub to reinforce against creep.

Less costly materails like polypropylene might be feasible. But the savings would be small in comparison to the engineering effort

involved to insure that PP would serve adequately.

My calculator refused to turn on so I did this with my old old slide rule!

Hope this helps.

Pat http://www.aquasite.com/skonkworks

## RE: Fan Design - A problem

I am taking ^2 To Be Squared.

I cannot get your equatoins for h and Ns To Work With The Figures You Have Provided.

Density = 0.075 lb per cubic ft

u = 21 ft/sec

Therefore for h to work out in ft you need another value to elminate the time sec. This would make it dimensional correct ?

Please can you calrify bot the above queries.

Thanks again.

I will acknowledge you in my report

## RE: Fan Design - A problem

Pat

## RE: Fan Design - A problem

discussion.The design is far from complete, but can be completed with not too much work.

The hard part for a molded fan is getting the

contour defined adequately for pattern and

mold makers to work with.

Pat insideman@worldnet.att.net

## RE: Fan Design - A problem

Please help

Thanking You Loads