I’ve never used a fabric filter, but I will step in with what I think I have learned. Max is a proponent of fabric filters, but he says that you cool the gas down to below the dew point. Then you heat it up enough to stay above the dew point and run it through the fabric. This will eliminate moisture in the gas which if left in the gas will cause the soot to cake on the fabric and clog or partially clog.the fabric.
About fabric; I go to a boat repair place and buy fiberglass fabric. It comes in different deniers. On one end it can be made out of almost a clothes line size rope. At the other end it is almost like polyester shirt material. TomC
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Max also says this method will work even with a paper element filter, provided you actually get the moisture out and reheat the gas to dry it out more.
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If you look at some of the world war 2 gasifer plans you will see this method being used. I always wanted to try it. One more thing on my DOW bucket list.
Bob
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I’m just wondering how to do it and make it look clean as the car guys say.
One cooling rail runner after the cyclone or drop box, bring the temps down to drop moisture, then have the gas go around the drop box to reheat and filter then cool again with a second runner.
Plumber’s Nightmare.
One of the places I thought it could be done easy is around the gasifer itself. On my WK Gasifier 2 or 3 Mississippi’s is all I can hold my hand on and this is when I am stopped. Driving down the road probably 1 Mississippi hot. So my thinking was to weld a jacket around the barrel. On my new build the ribbing on the barrel is 3 times as large as a regular barrel. So this will work. Coming off the pre heated gas you need to filter it now then send it to the engine. I have thought about stacking some regular filters up and put a pleated welding blanket round the filters. All of it goes into a barrel sealing it top and bottom. The gases then would go through the filter barrel side through the welding blanket/filter ring and out the bottom of the stacked filters. Just 4 long threadall to clamp it together. I have a drawing of it some where. In my pile of drawings. 3" pipes in and out. Just one more barrel in the back bed area. But boy would you have some very clean gases to burn in the engine.
I called it my not so hot filter.
Bob
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The gas into the filter must be at least about 300f, so that water does not precipitate, that is why it is made of heat-resistant fiberglass.
The filters I have looked at can handle about 1100f, it is a welding blanket.
The temperature drops approx. 300F when the gas passes through the filter.
What Max was talking about was fabric filters in combustible materials, then they cooled the gas so as not to burn the filter.
The filter they had for tractors and trucks during the experiments in the 60’s looks something like this.
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Hello Jan,
Vesa Mikkonen uses Nomex filter cloth on his fabric filter. He uses many small round filter bags, as he says the envelope type shown in post no. 2 are hard to clean. The round filter bags are supported with a wire cage in the inside and can be cleaned easy by blowing air in.
It can be seen here, but not many details. Service of wood gas-generator - YouTube
He has no cyclone, the filter is directly after the gasifier in a way Göran describes in post no. 18, so no gas stream is directed on the fabric. A bit care is necessary not to get too high temp. The fabric gets damaged then.
If I remember right Max once suggested very fine metal mesh instead of fabric. As a filter cake forms on the fabric, its filter ability will increase because of this.
Every fabric filter must stay dry, above the dew point of the gas and no dry filter will survive if glowing char will come in with the gas.
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When Jacob stopped by last fall I watched him clean his hay filter. Seemed like a overly messy job. I thought at that time that cutting the bottom out of a clothes dryer drum, narrowing the diameter, cutting the drum and using ratchet straps to suck that to a tighter diameter and tacking the base back on and then filling that inner canister with hay would be better. The canister could be pulled out intact and cleaned and the metal drum would further mitigate heat. Seems like that same dryer drum canister would be covered with welding blanket and then cleaned with compressed air like in the video. It would be similar to the Ben Peterson filter or what I imagine I would see in the Mikkonen filter.
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Thanks Tilman, I’ll try to find what he uses for fabric.
Some pictures from the new book on engine wear on some tractors.
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A point of pressure differential will cause wet gas to condense.
Its not just a filter problem.
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Max is talking about a very wet wood gas. That is filtered first by a slyclone filter action to remove char/ashes first. Then cool the gases down, with using a condensation tank vertical cooling tubes that collect more soot as the waterdrop out of the gases though condensation on the tubes. It is self washing set up. Now you have a much lower moisture content in the gases stream but still to high to go through a PAPER FILTER. By heating the gases stream up to 300 °f or more will dry the gases back to a hot drier vapor that will pass through the paper filter with out water saturation and ruining it.
If you have ever gotten your paper car filter wet and try to use it with dirty air coming in to it, it plugs up fast.
And yes differentials pressures and velocities all play into this also. The key is lots of surface area filtering capabilities.
Now a charcoal gasifer gases are much drier and easier to filter out the soot and ashes. No potential tars to deal with with engine grade charcoal.
Bob
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Some questions Göran, was it a fire blanket or welding blanket that you bought?
What do you think about making a frame of 2cm square, and then loose square bags of fiberglass 2cm x 45cm x depth.
Thought I would put the blanket on the 2mmx18mm irons, and let it go up through the frame and seal the top.
So there is room for 6 filters in this construction.
I do not understand how you got the blanket tight around your center tube?
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That looks great, Jan.
I sent you a pm before, did you got it?
Yes, I got it, thank you so much for the information, however, I have a hard time testing blankets where I live, there are no such shops at 60-100 km.
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Hi Jan, here’s some ideas for you, and others ofcourse,
I borrowed some ideas from Svedlunds filters, and SMP, this gives some benefits, like easy to see if one bag leaks, easy to change out single bags, or even plug one temporarily with a plate.
I haven’t gone deeper into case construction, it should be insulated to avoid condensation, be easy to open for cleaning and check up, and gas should enter in a way, it doesn’t directly hit the fabric cloth’s, (think dropbox)
I think this should make the fastening of the bags easy and tight, just fold them in the frame opening, maybe with a little red silicone in between.
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And here’s some ideas i had before in an attempt to improve the bag filter type that has become popular around here, the idea is to mount it horizontal and have the filter turnable inside the housing to avoid soot build-up on top.
This could be done with a hand crank, or automatically.
The seals i think could be made of “graphite string” available for water pumps.
And ofcourse i have to over-complicate things, this is what i thought about building, when I decided to make a fabric/cloth filter.
The windshield wiper motor could be controlled by a adjustable under-pressure switch, or just by a intermittent wiper-controller.
This i think should slowly turning the filter, to dump the soot-caking.
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Thank you Göran,
The first pictures are about as I thought too, it seems to be a little easier to make and get tight than the other idea you have, however, that bag is easier to make in the second picture, Hmm.
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I got home some fiberglass fabric, so thinking about what to do, I think Tilman had a good idea with the pipes, thought I would make 4 “pieces that go through a lid to the fabric tube attachment, and then 2” pipe pieces at the bottom on the fabric tube, with rebar in between, which keeps the tube pieces apart?.
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Why would the gas come back up through the pipes instead of the area around them? Unless I misunderstand what I’m seeing then you will need to drill out a plate to seal that area, along the lines of a fire tube boiler and extend the bottom of the container down as more of a drop box. It would be good to have a way to bleed the heat out from around those tubes as well.
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