Solar Drying Kiln Q&A

Basic questions and answers on vent sizing and temperature conditions in a solar kiln. January 18, 2011

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)
South wall-32x192
North wall-132x192
Floor- 96x192
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.

Forum Responses
(Sawing and Drying Forum)
From contributor X:
The Knowledge Base link below regarding kiln construction may help you out.

Kiln Construction

From contributor D:
You are right that solar kilns can be fun but they are probably not the right choice if planning and production are a concern. Also, some of the designs that use fans dry so slowly that the electric cost of running the fans is high.

From Gene Wengert, forum technical advisor:
First, Contributor D is correct about solar kilns in commercial operations. Next, the vents must be opened somewhat or else the moisture cannot get out (unless you built a leaky kiln). When drying 1000 BF of oak, you will lose 32 pints of water for each 1% MC loss, so with about 2% MC loss per day average, that would be eight gallons. Where will eight gallons of water go if it cannot exit through the vents? It cannot disappear.

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.

From the original questioner:
I thought faster drying had increased chances for checking, not slower. Am I incorrect on that?

From Gene Wengert, forum technical advisor:
Fast drying encourages surface checking. However, slow drying that is accompanied by oscillating conditions (which is almost always true for slow drying) will also result in surface checking.

From the original questioner:
You mentioned a 16 foot long stack of lumber. Should I not leave a 12" gap between the end of the stack and the side wall?

From Gene Wengert, forum technical advisor:
No gap is needed, but as lumber is not perfectly 16.0' long, some extra for those longer pieces is a good idea. Remember that any extra length will likely have a lot of air flow (checking) and will reduce the air flow in the rest of the stack (bad).

From contributor K:
Are you able to kill the bugs after the drying cycle in a solar kiln?

From the original questioner:
The temp in the solar kiln should reach high enough levels to kill any insects that are present in the spring-fall seasons and in guessing depending on your location possibly winter as well.

From Gene Wengert, forum technical advisor:
The basic standard is that 130 F or 133 F is required to kill insects and their eggs, etc. This is the temperature within the lumber and not the air temperature. Very few solar kilns will get hot enough to heat the lumber to 130 F throughout.