a simple prototype

[solarbird]

Right then. A very simple prototype air-splitter and filter, with the filter on one output only.





It's as basic as it looks. It's a plastic-lined cardboard box with an intake and two outputs, one of which leads outside, the other of which leads to a small, easily acquired, cheaply available HEPA filter with charcoal prefilter.

I ran a very small test load of two bath towels in the washing machine, then in the dryer, to see whether we'd get much of anything out of it, and we do. Air coming out the filter is at 125F - very warm, but not hot enough to scald. Even on such a small, short test load, it bumped relative humidity at the sensor outside the laundry room by three points, and temperature up by about half a degree C. This all despite the window being open for additional ventilation while we wait out a COVID isolation protocol.

Obviously, this minimalist prototype effort lacks any sort of shutter system, but for the conditions of the test that's fine. This is in no small part about discovering how much this kind of maximalist filter slows exhaust into the room/heat and moisture reclamation, and how often it ends up needing to be cleaned.

I do think HEPA is over the top here, but it was convenient and nicely sized for the application. Also, for those who would have reason for HEPA-type filtration, it'd good to find that it does in fact allow for meaningful airflow, at least so far, with the relatively low pressure of an electric dryer. I will try this with the charcoal pre-filter, then, depending upon outcomes, replace the charcoal with a simpler plastic mesh (much akin to a typical lint screen) and see how that affects things.

There is no extra occlusion to the output vent hose, though that's certainly something which could be done. Basically, I don't want to create out-of-spec backpressure for the dryer, and in particular, I don't want to occlude outbound airflow in the event of complete filter blockage. This approach means that even were the filter to become completely blocked the dryer would behave normally, which means it can be completely blocked come warmer weather without having to resort to disassembly.

At the moment, my anticipated benefit case for this is mostly in very cold and/or dry weather, when inside humidity drops into the low to mid 30s. While it is in no way a constant supply like a humidifier, I think it could help delay the drying out of the building as a whole, via humidity retention in various humidity mass objects like couches, carpets, and so on.

Comments

[kathmandu]

Thank you for posting this! I was wondering how it worked.

[solarbird]

This is the one I built today. The previous one was basically a hole cut into that exhaust duct - a big hole, but just a hole - wrapped with filter cloth. Compared to that, this is actual engineering. xD

[solarbird]

Oh yeah, something you can't see: I used heavy plastic tape and taped up the sides including the round duct holes. Two layers, one on the inside, one from the outside. I then cut out the holes again, leaving a large margin of now very stiff plastic tape all around - a good 10mm flexible margin of tape, essentially. I did this so that this would form a flexible plastic seal, smaller than the wire frame of the duct, that I could basically screw the duct into! So it's not just held in by friction, but also by actual material. Does that make sense?

[kathmandu]

Yes, makes sense, thank you for explaining.

I remain impressed by your engineering prowess.

[solarbird]

Here, I drew a quick ... diagramme, I suppose? lol ... of the side.

[peristaltor]

Interesting.

I've been toying with an idea for a cross-flow system that would use exhaust to pre-heat inflow. It's bigger, though, and would probably require more… effort. (And I still don't know if the dryer would compensate for the additional heat, or just overheat and save no power at all.)

[solarbird]

That's an interesting question. I mean, they do have temperature control via thermostats, so I'd think they should compensate just fine. You'll lose some efficiency over the heat transferal, of course. And plumbing it into the heating chamber intake might be a bit of a trick. But I think in theory you'd be okay.

I mean, the problem will be the trapped lint fragments clogging the heat exchanger. The finer the fins, the more transfer you'll get, but the worse the clogging will be. That's why I abandoned that approach.

But make it a big exchanger and that would take some time.