Model Forum / General / Models / February 2010

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 http://uk.rs-online.com/web/search/searchBrowseAction.html?method=
getProduct&R=5441018

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

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0
221077

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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