Filter advice please on home made fume extractor

[HobGoblyn]

Hi all,

I’m in the process of making my own fume extractor.

I’ve been following the link at the bottom of this post.

I have now got everything except the actual filter medium as I’m not sure the way he’s doing it is the best way?

Basically it’s a deep plastic box with an in line mixed-flow fan, the exit going through the side of the box.  And a smaller box that sits inside with the filter medium in.

It’s just the design of this smaller box that I want advice on, the fan etc is already here, waiting to be installed. 

I’ve hopefully set the link to 19:36 where he shows how he does the filter.

He  uses two 30mm thick cooker hood filters on a cardboard gasket and covers the entire box with white wadding type stuff.

I was thinking that instead of cooker hood filters with cardboard gasket etc, permanently installing something like a couple of 200mm (8”) flanges (or smaller, maybe 150mm/6”) , like in the below pic,  but ones that allow me to put in metal mesh at the bottom of the flange.



Then I could simply fill with loose activated carbon and cover with hepa filter sheet?

I’d imagine in the long run this would work out a lot cheaper as those 30mm thick filters in the YouTube vid  are around £10 each + delivery and activated carbon for aquariums can be bought fairly cheaply?

(I will also make sure a hot knife is used to cut everything so the brittle plastic doesn’t shatter/break).

Anyone think my way is worse than his or can anyone think of a better way altogether?  Again I’m doing it roughly how he’s done it and already have everything except the filter medium.

Many thanks

YouTube clip below

https://youtu.be/JdrFsuAcm48?si=E_gZAbo8zq3uGZqv&t=19m36s

[thm_w]

Is he using any HEPA filtration at all? Didn't see it.
I would be tempted to use an off the shelf hepa filter and not some sheet because they are pleated which will increase the surface area. But if you have a really large surface you can drape it across maybe it would be fine.

The cheapest source of activated carbon is usually the grow filters you can get on amazon ($30 for ~2kg). I would avoid the pelletized fish carbon (appears to be less surface area), but if you can get the granular stuff for a similar price I'm sure its fine.

[HobGoblyn]

Do you think this will be OK?

£16.50 for 2.5 kg

https://www.ebay.co.uk/itm/351226809710

Thanks

 

[tooki]

Is he using any HEPA filtration at all? Didn't see it.
I would be tempted to use an off the shelf hepa filter and not some sheet because they are pleated which will increase the surface area. But if you have a really large surface you can drape it across maybe it would be fine.

I second this suggestion: you want a commercial pleated filter with a quite dense, deep pleating. Rosin clogs up filters (and can be sticky, trapping other dust), so a flat filter would probably clog very quickly. Look for “deep pleat HEPA filters”.

[shabaz]

Some vacuum-cleaner filters are a decent size, I gave it a shot a while back too, I can't say I was entirely happy with it all (it was a bit experimental; it would probably need a rebuild, with more air-tightness around the fans). I didn't complete : ( But I was happy with some aspects of it, it needed better execution. For the ingress end, I used a 3D-printed oval-shaped funnel, connected to a short length of conductive plastic hose (from eBay).

[tooki]

That’s tiny compared to what you want for a filter that shouldn’t make tons of noise (by needing a vacuum cleaner’s static pressure). And it has very shallow pleats.

Look at this instruction sheet for an idea of the scale one wants for a large-surface (=low pressure, quiet) filter system: https://www.southernfilters.co.uk/theme-content/uploads/2017/07/unpack-hepa-filter-1.pdf

(No, you don’t need one quite that big. But closer to that than to PC fan size. )

[thm_w]

Do you think this will be OK?

£16.50 for 2.5 kg

Looks good

Some vacuum-cleaner filters are a decent size, I gave it a shot a while back too, I can't say I was entirely happy with it all (it was a bit experimental; it would probably need a rebuild, with more air-tightness around the fans). I didn't complete : ( But I was happy with some aspects of it, it needed better execution. For the ingress end, I used a 3D-printed oval-shaped funnel, connected to a short length of conductive plastic hose (from eBay).

Tooki beat me to it. Your construction looks very good but, the vacuum these filters are from is probably using a 500W+ centrifugal motor. Those 3 axial fans you have most likely aren't powerful enough in comparison. You could 10x the surface area of that HEPA filter and greatly decrease the pressure needed, which might make those fans usable (depends on their static pressure rating, if its from a PC they are probably no good).

Also that mesh carbon filter is near useless. Get some granulated media in a mesh bag if you can.

[shabaz]

I used 120mm fans (since I had a load of them) and had to use 4 of them I think, to work through that filter!). A larger filter and different (and larger fan) topology for that sounds very sensible.

Also you're right on the noise (I was hoping to put it under the desk with just the ingress hose exposed, but it's still unnecessary sound that could be solved by better design). Also needed a soft-start (I used extremely crude series resistors shorted out by MOSFET delayed by RC circuit) since the fans have so much demand on start-up. I definitely would scale it up and do things differently if I were to do it again.