Kevin Jenness
05-30-2016, 7:59 PM
I have been using my small solar kiln here in VT for a couple of years now. It has a maximum capacity of about 600 bd ft of 8/4 lumber, less for thinner material, and about 70 sq. ft. of single thickness tempered glass. The 2x4 walls are filled with 2 layers of 2" isocyanate foam board (about R22). I use a 10" attic fan on a thermostat and timer to move air through the stack. I mainly use it for conditioning lumber that has been previously air-dried or kiln-dried to equilibrium with the relative humidity found in covered outside storage here (11-14%). Some things I have learned:
Maximum temperature I have observed is 145 F and minimum RH about 10%. The RH rises in the night as the temperature drops, so there is a period of "relaxation" every day as opposed to most kilns where the conditions are more consistent. In the winter the temp may get up as high as 80F on a day when the outside temp is <32F . Wintertime drying is very slow but still happens. I set the thermostat to start the fan at 50F in the winter, more like 70F in the summer and set the timer to match the sun. In the summer it can take several weeks to get lumber down to 6-7% from air-dried depending on species and thickness. I keep the vents closed most of the time. I don't have a problem keeping the RH in the kiln low despite the fact that I built it fairly tight with OSB sheathing caulked at all interior joints and silicone weatherstripping on the doors. It is more of a challenge to keep the RH as high as I want it at times.
When drying green lumber it is important to monitor the drying rate to avoid problems like end and surface checking that occur early in the process . I use sample boards and a postal scale to do that. I have found that I need to cover part of the glass early in the cycle to avoid overdrying. When the MC gets down below 30% it may be time to open the vents, but the drying rate must still be watched. When the load is below 15% MC it becomes harder to squeeze the last bit of moisture from the core, so the vents are closed to keep the temperature high. When it gets down to the end, I open the vents again to try to equalize the load by keeping the RH around 30-35% for a few days if possible.
I am not in a production situation so I try to keep to a moderate schedule. My glass area/bd ft ratio is a bit higher than 1/10, but I am at a fairly high latitude (45 N). I would be interested to know what other folks with a similar setup have experienced and if they have any tips to offer for better operation.
Maximum temperature I have observed is 145 F and minimum RH about 10%. The RH rises in the night as the temperature drops, so there is a period of "relaxation" every day as opposed to most kilns where the conditions are more consistent. In the winter the temp may get up as high as 80F on a day when the outside temp is <32F . Wintertime drying is very slow but still happens. I set the thermostat to start the fan at 50F in the winter, more like 70F in the summer and set the timer to match the sun. In the summer it can take several weeks to get lumber down to 6-7% from air-dried depending on species and thickness. I keep the vents closed most of the time. I don't have a problem keeping the RH in the kiln low despite the fact that I built it fairly tight with OSB sheathing caulked at all interior joints and silicone weatherstripping on the doors. It is more of a challenge to keep the RH as high as I want it at times.
When drying green lumber it is important to monitor the drying rate to avoid problems like end and surface checking that occur early in the process . I use sample boards and a postal scale to do that. I have found that I need to cover part of the glass early in the cycle to avoid overdrying. When the MC gets down below 30% it may be time to open the vents, but the drying rate must still be watched. When the load is below 15% MC it becomes harder to squeeze the last bit of moisture from the core, so the vents are closed to keep the temperature high. When it gets down to the end, I open the vents again to try to equalize the load by keeping the RH around 30-35% for a few days if possible.
I am not in a production situation so I try to keep to a moderate schedule. My glass area/bd ft ratio is a bit higher than 1/10, but I am at a fairly high latitude (45 N). I would be interested to know what other folks with a similar setup have experienced and if they have any tips to offer for better operation.