Fan Design - A problem 2

[dcharles]

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.

 

[swameet]

dear
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....

 

[dcharles]

The fan will that of an extraction fan like the ones found in bathrooms and kitchens. So I wouldn't have a clue with regards humidity levels as they could be varying. Also, it is the same with altitude, depends on the location of the house. With regards to this, could you give the scenario that can be more realistic based on the initial calcs in the first post. Could you select a suitable material for the fan blade.

Please can you help, its getting rather desparado.


Thanking you

 

[insideman]

Howdy dcharles.

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

 

[dcharles]

Thanks for the prompt reply.

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

 

[insideman]

Ns is customarily expressed in a form that is not dimensionally consistant. If you want a consistant form, use "gh" instead of "h".

Pat

 

[insideman]

I suggest contacting me by Email to speed the
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

 

[dcharles]

Yes But With THe Fugures Provided How Do You Get 5.3ft for the value of h?????? And therefore the value of 4850 for Ns ???? It doesn't work out.

Please help


Thanking You Loads