Thread: my solar kiln finaly under way

This is my little kiln, the other is 3 metres longer, same principle and design

Originally Posted by nifty

Cooling off and relieving stresses has nothing to do with case hardening. Put simply, case hardening happens when you attempt to dry too quickly which happens easier in a dehumidifier or vaccum kiln than a solar kiln. It can most commonly happen when your kiln is not filled to its capacity, the fuller they are the better they function. When dried too quickly boards will lose their moisture on the outside faster instead of an even release throughout the entire board, like cooking steak on the bbq, therefore capture moisture inturnally, unable to be released. The only real time a solar kiln can do his is when an extrnal means of heating is used as well as solar. As far as I know, its best to air dry for at least 6 months before placing into any kiln to allow timber "condition" or begin its process of releasing moisture evenly. Cooling off at night means nothing also, a true solar kilns job at night is to release the moist air,therefore creating condensation and removing moisture and off you go again the following day.

See this article and other kiln articles on the same site:
"www.woodweb.com/knowledge_base/Solar_Kiln_Designs_2.html" by Gene Wengert and Luiz Carlos Oliveira. These guys are gurus in the field of solar kiln design from Virginia Tech in the US but have researched kiln designs from around the world including CSIRO designs for Aussie conditions.

Specifically;
"Drying Stresses (Casehardening)
It is a natural event for stresses to develop within the wood during drying as the wood shrinks. These drying stresses do not cause much problem if the dried lumber is to be used without much remanufacturing. But if the lumber is to be ripped into narrower pieces or resawn into thinner lumber, these drying stresses will cause lumber to pinch the saw, cup during planning, and change immediately in size and shape when being machined. In greenhouse and semi-greenhouse designs drying green lumber, relative humidities in the dryer will reach 100% during the nighttime. This high humidity apparently partially or totally relieves these drying stresses. However, other designs, where this high nighttime humidity is not achieved, will require that the lumber receive a high humidity treatment at the end of drying. Hot water spray atomization has been used with some success, but generally a several hour steaming treatment is more common. (It is because of the stress problem that supplemental heat is generally not suggested and that the simpler greenhouse and semi-greenhouse designs are preferred.)"

Have read this before Bob and it does make alot of sense if resawing is required after drying, I find it very common over here in the west with Marri, Ive seen it bounce back to 14% within 24 hours of drying to 10% in 17mm flooring. My kilns are a Gene Wengert design from long ago that he designed for drying white oak flooring for someone.

Nifty, in your design, it looks like the air is just recirculated, just wondering if there is means to vent the moisture laiden air and draw in fresh air? I cant seem to see anything to indicate venting. If this is how the design is meant to be, How does it dry?
Peter

Nifty, I like the door mechanism, makes for easy access. What is behind the plastic curtain, more timber?

There is nothing other than the fan shelf you see in the other photo, these pics were taken while unloading. My kilns dont draw new air, they have to heat the air up thats already in there and then constantly circulate it. At nite thate warm moist air is drawn through the 2 plastic skins on the door which causes the moisture drawn from the timber to condensate and run out down the bottom. Kinda simple but effective.The doors are just held up by hooks on the arms, you let 1 side down and then unhook the other and let it go and it comes down like a parachute. On my big kiln there is a winch at the back coz shes pretty heavy