Thread: my solar kiln finaly under way

The top one is, IMHO, the better, provided you've a long enough down-hill slope to build the air-heating bit.

I think I'd move the hot air exhaust to the bottom, the same as your bottom pic, and use those solar fans you mentioned inside the kiln - above or around the stack - to keep the air circulating around in there, minimising hot spots.

All pure speculation on my part. I'm sure that whatever you do, once you build it and run it for a while you'll be thinking "I wish I'd..."

i would buy second hand cool room panel these are great and can be brought for around $10m2, you will not need any framing or cladding as the panel is able to be free standing.

Primary school science says hot air rises - the best flow rate will be venting at the top - not the bottom. If you put the vent at the bottom, the slabs at the top will sit in a stew of hot wet air and you will cook the wood.

Technically speaking a kiln supplies heat 24/7 so a solar kiln has to store solar energy during the day and release it at night to continue drying. Common energy stores are rock or water reservoirs. This means your designs are not kilns but solar assisted drying sheds. If you put rocks in your ceiling cavity that would then be a solar kiln.

Both designs will dry timber faster than air drying but the hot air escape vent has to be at the top middle via a chimney or the top sides. Provided the shed is kept reasonable full and an adequate air flow is provided the timber itself will act as a partial heat store so although it sounds strange the fuller the shed the quicker and more evenly it will dry.

Both designs can be made more compact by using a false pair of opposite walls to deliver the warm air at the bottom middle of the wall, and vent via chimneys or the middle top of the other pair of walls.

If you want Any half decent decent kiln has continous major air recirculation movement inside the kiln. It is not unusual to use a 1 kW fan inside a kiln to keep the air recirculating. 2kWhr is a big solar collector or about 30 c an hour to run - that's $1314 a year to run - better have high value wood to warrant this.

Our woodies club have had a solar drying kiln since 1997 and I put information
Photos and details of the running of the kiln in the Australian Woodworker magazine February 1999.
We have used it since then and at present it has a load of Elm we milled about 3 week ago
If anyone would like more info or can't get to the magazine I will answer any
questions
Can post pictures ,take some new pic tomorrow.
Regards
David

I agree the second picture there will be a steam bath at the top of the stack so that needs altered. Also I dont agree you want to continually dry the stack . How the wood dries is the surface dries obviously but heat tends to push the moisture in. So you need a time when the wood rests and the central moisture tries to balance through the plank. .Just check your next steak. In commercial driers some even add steam to the chamber if the balance is out too far. A mate built a kiln on the side of his house, first year was a disaster, put wood in mid summer and it turned into corkscrews. Tried again starting in winter and that was a sucess.

Yar yar, post heaps of pics
Peter

Go to Incomac kilns and have a look at the design of their kilns.
you need a space at each side of the stack to allow air flow. Your top picture will not work.
your bottom picture will, but you need at least a meter gap between the timber and the wall on the left. Hot air rises, but hot moist air will not rise as fast. thus you need fans. To dry hardwood you need 1 meter per second of air flow.
I've been drying hardwood for over 30 years and the kiln compartments are fairly basic and easy to build. It is the control that is the hard part..

thaks glock.

i have heard teh 1m / second is that one cubic metre?

do you say the top pic wont work just due to the airflow?

its not really desighned to dry as fast as a kiln but just to speed up the drying process.

the main reason is we make a lot of log furniture and i need to dry pine logs (100mm diamiter adn under fast to stop the stain and so we can use them fast. i would also put slabs and boards threw it to eitehr dry them quicker or finish tehm off.

No. It is 1 meter per second of travel.

As in 1 lineal meter per second of air flow. An Anomometer is very handy to check this.

thats only 3.6 klm per hour just a little light breeze

where is the speed measured?

wouldnt the speed depend on the outlet size.

like 1m3 of air getting threw a .5m hole will be going a lot faster than 1m3 of air going threw a 2m3 hole.

or am i rong

Originally Posted by weisyboy

where is the speed measured?

everywhere in the kiln.

wouldnt the speed depend on the outlet size.

like 1m3 of air getting threw a .5m hole will be going a lot faster than 1m3 of air going threw a 2m3 hole.

or am i rong

It has nothing to do with volume. If you were to hold a air speed meter anywhere in the kiln where near where the wood is it should be 1 m/s or greater. It won't be possible to achieve this up against the floor or walls so that's why the wood should not be in those places. It also needs to be this speed between the slabs - that may mean a lot more speed elsewhere and careful stacking of the timber and internal baffling to direct air flow.

I don't agree with glock on the second design working, even if the timber is moved 1 m from the wall. The timber in the top part of the stack will not exchange much air and just sit there in a hot moist air and probably go mouldy. The escape vent has to be as high up as possible - preferably through the ceiling via chimneys.

It all depends how quickly you want the wood to dry, a proper solar kiln will dry 50 x 100 eucalypt in ~3 weeks. If you need it that fast then you will definitely need 1 m/s internal air speed, good spacing away from the walls, be well stacked, near full, and properly air baffled. If you want it in 3 months you can reduce these requirements.

yep now i understand

as for the vent at teh top.

if i put the outlet at teh top then the kiln would holld no heat overnight but i understand that the air up teh top as the desighn is is stationary and will become stale. if i was to put teh inlet at teh top would this create enough movement to stop this?

Originally Posted by weisyboy

yep now i understand

as for the vent at teh top.

if i put the outlet at teh top then the kiln would holld no heat overnight but i understand that the air up teh top as the desighn is is stationary and will become stale. if i was to put teh inlet at teh top would this create enough movement to stop this?

In design 2, if you arrange for the fans to turn off at night not much heat will escape. if you arrange flaps to fall over the exit events at night even less will escape.