Further on sorting alder seeds by falling speed in air.

I have had mixed success with my winnowing machine and I want to do better next year.

The machine, as described below, throws up and out about half of the alder seeds, and lets fall through for collection the other half, including a small amount of cone fragments. What I really want is to be able to throw out MOST of the seeds and keep back maybe 1 in 1000 or maybe even 1 in 10 000. Final sorting will always be by eye, some of the "accepted" material will be rubbish such as cone fragments, insects, stones, etc. To make it possible to sort through really large numbers, I want to get rid of the vast bulk of uninteresting seeds using a mechanical method.

(I needed to plant last year's seeds now, so I sorted through them by eye, using a large magnifying glass of the kind used for embroidery and sucking the ones I wanted up using a pooter)

The equipment as it is now uses a Sunon fan,

Manufacturers Part No. KDE1208PKV1.13.MS.A.GN R S Components Stock No RS Stock No. 544-1018 URL

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on top of a "Persepx" cone, 8 cm diameter at the top, tapering down 14 cm to 2.5 diameter and a straight "Perspex" tube 2.5 cm diameter length 13 cms.

The seeds are fed in through a small hole 2/3 the way down the cone. Connecting a pipe to this hole, the suction was a startlingly low 2 mm water gauge. I noticed that the seeds were swirling round beneath the fan, evidently the fans blades were stalled. I put a 4 cross pieces below the fan, which stopped the whiling, but didn't improve the performance.

Evidently I am not getting enough airspeed through the final tube at the bottom. What could be done to increase it?

The neatest solution would be a contra-rotating pair, of fans, I haven't searched through the catalogue, but it is unlikely that I will find a fan turning the other way, so the only other way of going that way would be to make circular turning vanes like this

) ) ) ) )

around the axis. The other way of going would be to use a centrifugal fan, they don't stall in the same way and possibly suitable ones are available from R S Components. They would have another advantage, when in field use they could be set to blow the rejects away from my face, instead of at random upwards. The upward airspeed has to exceed 3 metres/second by some margin, exactly what margin is one of the uncertainties.

The apparatus as it now is, is about 90 cms high, (the top to be held at about eye height) and 10 x 10 cm across, and weighing 800 gms, and powered by a lead-acid battery (1.2AH) weighing 500 gms in my apron pocket. That's the kind of size and weight I am willing to carry around.

Let's say I go for a testing tube with cross section of 0.01m^2, I did this purely for simplicity of arithmetic, but by chance it gives a tube diameter of 3.0 cm, close to what I have now.

Lets say I go for an airspeed of 5 m/sec. In one second that will amount to 5 x 0.1 = 0.05 m^3/sec.

To get it into hour terms, multiply by 3600, so

0.05 x 3600 = 180 m^3/ hour

This one seems to meet the bill

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RS Stock No. 221-077 Manufacturers Part No. RG160-28/12N

BUT at that current drain, I could only use it for an hour, but that it probably acceptable because it will only be switched on for short periods, and I can have another battery in the car for the afternoon. WORSE, it is much wider than what I have now, trying to get air through a much narrower inlet. Going from axial flow to centrifugal has not been a magic bullet, nor should I have expected it to be.

I am not very good at arithmetic. Comments and suggestions would be most welcome. Just to write this has helped clear some of my confusion.

Michael Bell

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Reply to
Michael Bell
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You have ignored the advice given. The cone is far too steep. It has an angle of about 3 times the ideal The pressure loss in such a cone will be about 6 times that of a shallow one The advice was to make it 46cm long The inlet tube is unnecessarily long. More pressure losses The debris sitting below the fan won't help either. Your inlet duct and cone amount to 27 cm so what is the rest of the 90 cm length quoted? Hopefully the material is collected in a container clear of the inlet tube or it will get blocked and air can then only enter via the small hole in the cone.

The fan you have is specified as moving 220m^3 /hr It will only shift that if it has unobstructed inlet and outlet. Ideally that would give 120 m/sec through the 2.5cm duct and 12m/sec through the fan.

1/3 the way up the cone the velocity would be 40 m/sec with a pressure drop of 99 mm water gauge. All of which shows that your device is far from optimum and that losses mount up very quickly. The fan is not designed for a large pressure rise across it as would be needed to overcome the losses. Shifting air takes more power than you might think!

It is possible to calculate the losses in a given duct system ( long time since I did that!) and there are online calculators to help. However you also need to know the pressure rise characteristics of the fan so that a match can be found. RS don't supply that information. I found some small PAPST centrifugal blowers but even they had little pressure rise.

Henry

Reply to
Dragon

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