First let me apologize for the amount of information and questions that will be following. I have done some searching and have not found enough information to answer the following to my liking. I am making a solar kiln with the following dimensions that will be facing the south.
Location: (middle MN)
Collector area is about 172 sq ft at a 45 degree angle
Species to be dried - red oak..
The inside will be flat black with insulation (r-13) in the walls and floor. The collector material is clear Tuftex (polycarb). With the information that I have found my kiln load should be about 1700bf.
The questions I have are as follows:
1. What is the optimum sq ft of venting for this kiln?
2. What will the effects of using less then the optimum amount of venting?
3. What are the effects of using less then the 150 fpm air velocity?
4. If the kiln load is under the 1700 bf rating of the kiln can using less air flow and less venting compensate for the difference or is the only way to compensate is by covering part of the collector?
5. If using a second layer of collector (one on the outside and one on the inside) increases the kilns efficiency by ~35% will this increase need to be compensated with more venting and air flow and how would this effect lesser bf loads?
I would rather error on the side of slower drying then drying too fast and end up with defects. If anything that I have stated seems incorrect please let me know.
(Sawing and Drying Forum)
From contributor X:
The Knowledge Base link below regarding kiln construction may help you out.
The vents are never opened wide, as this results in too much fresh air and the loss of most of the air that has just been solar heated. The kiln will be cool and will not be able to dry under 12% MC. In fact, why not just use air drying if the vents are wide open?
I discussed the proper venting when I developed the Virginia Tech kiln in 1978. What is required is that you watch the daily drying rate (by weight, as electric moisture meters do not work above 30% MC). If drying is too slow on a sunny day, then the vents are closed too far.
Now to your questions: with 16' lumber and 6' wide load (which seems to be your kiln size), you will get about 80 to 85 BF per layer. Each layer of lumber, with 3/4" sticks will be 1-7/8". So, for 1700 BF, your stack will be 22 layers high or 41". For 1700 BF, it will take almost 14 gallons of water per day to lose 2% MC. Slower drying has a risk of stain and checking. So, that is quite a bit of venting indeed. You need 150 fpm velocity to get uniform drying. Lower air flow will mean some areas will be drying too slowly.
If you only have 1000 BF, you must cover the collector (down to 100 sq ft) for oak, as you will have too much energy in the kiln. Energy is what makes the wood dry. The design is such that when the kiln is loaded full with 4/4 oak, it will dry safely. With only one layer, you will not retain the solar heat and so the kiln will dry more slowly (more than 35% slower) and in fact may not reach 6-8% MC. If you take samples and measure the MC daily, you have no risk of drying too fast and getting defects, as you will be under safe drying control.