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Solar Kiln - Fan size and other materials

8/19/14       
David Morris Member

This is a project I've been planning for a long time. I'm building a portable solar kiln for my back yard (on casters). It's about 4'6" long, 19" wide, with a front wall at about 2' high, and then a roof at a 33-degree angle. I think I have about 15 cubic feet of usable space, plus maybe another five behind the sloped roof.

I've done the framing and the interior walls. It's time to figure out a few things, and I'm hoping someone might take the time to help.

1. How big should the fan be? Any ideas where I might source it? I need to keep the cost down as low as possible, and would love to include a solar panel, as the kiln will be way at the end of the back yard.

2. How big should the vents at the bottom of the kiln be for air to escape?

3. How much space should I leave between the baffle and the rear wall for a kiln of this size? I'm thinking maybe 3-4", but I really have no idea?

Thanks in advance for any help you might offer.

8/22/14       #4: Solar Kiln - Fan size and other mat ...
David Morris Member

Ok, I decided to place the baffle about 3" from the back wall. I figure I can always make a bunch of small vents and close the ones I don't need. Still trying to figure out the fan, and would be grateful for any advice.

It occurs to me that a computer fan can circulate as much as 40 cfm, and my kiln is only about 15 cf. So I'm actually considering two small computer fans. Somebody talk me out of it if it's a bad idea.

Thanks.

8/22/14       #5: Solar Kiln - Fan size and other mat ...
Gene Wengert-Wood Doc

Is this for green or for air dried lumber?

What will be the size of the pile of lumber?

For fans, just get some three speed window fans from your discount store.

8/22/14       #6: Solar Kiln - Fan size and other mat ...
David Morris Member

Thank you, Dr. Wengert.

1. Right now I have some air dried lumber, but I'd like to do some green lumber at a future point as well.

2. The roof is about 15 sf, but I'm going to be limited in space. My maximum load will be about 100 bf. (It's just a hobby kiln.)

Regarding fans, I'm trying to stick with DC power. Any guidelines on cfm or anything?

Thank you again.

8/22/14       #7: Solar Kiln - Fan size and other mat ...
Dave Boyt  Member

Website: http://www.norwoodsawmills.com

In the upper right corner of the screen, type in "solar kiln" as a search item. Lots of threads pop up. The link below would be a good place to start for fans.

Photovoltaics would be fine, but even for a kiln your size, you'd want around 200 Watts capacity. Computer fans don't circulate enough air and won't stand up to the temperatures in the kiln. A couple of radiator fans from an auto salvage yard would be a much better choice.

Knowledge Base article

8/23/14       #8: Solar Kiln - Fan size and other mat ...
Gene Wengert-Wood Doc

To calculate cfm, we need to assume an air velocity of 100 fpm. Then multiply the velocity x sticker thickness in inches / 12 x lumber length x number of layers.

Example: 100 fpm x (3/4" / 12) x 8' x 20 layers = 1000 cfm. Then add 25%.

8/23/14       #9: Solar Kiln - Fan size and other mat ...
David Morris Member

Thank you both. Dr. Wengert's formula lines up with my estimates based on variation from the VT model. I'm getting this:

100 * (.75/12) * 4.5 * 12 = 300
Adding 25%, that's 375 cfm, if I did that correctly.

Still considering those computer fans, as the inside of a desktop computer while gaming can run around 160 degrees Fahrenheit, and they'd be easy to power. Radiator fans are an interesting idea as well.


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