John TenEyck
09-24-2015, 8:45 PM
I mill much of the lumber I use for my furniture and cabinet work. Milling your own lumber can save you a lot of money if you have a source of free logs - I do. But milling is only half the battle; you still have to dry the wood before you can use it. Air drying to 6 - 8% MC is a very long term proposition and I'm not a patient man so I wanted a means to finish dry lumber after air drying outside to the EMC of my area of around 12 - 14%. I decided on a dehumidification kiln after reading an article in Fine Woodworking (1991). A dehumidification kiln is pretty simple to build at relatively low cost, and is safe because it operates at low temperature. I modified the design, dimensions, and some of the components outlined in the FWW article to suit my needs and the components I could source.
Here's a photo of the kiln as I'm loading it with some very nice red oak I milled last Fall. The kiln is made from OSB, plywood, and 2 x 4's. The internal seams are all caulked and interior surfaces were painted with a couple of coats of oil based paint. Overall it's about 10 ft long so it can handle 8' long boards, and can hold about 275 BF.
322074
A closer view of the drying chamber shows the vertical slots from in the rear air plenum where the dehumidified air exits and flows through the stack of wood.
322076
Here's a photo after the kiln has been fully loaded and the dehumidifier installed in the right chamber. Note the catch pan under the cooling cools and hose that ducts out of the kiln to a collection bucket. That allows me to measure the amount of water removed every day.
322078
Above the dehumidifier are 3, 100W light bulbs that provide the primary heat source. That's all it takes to heat the chamber to it's operating temperature because it's so well insulated with the 1-1/2" foam board. Behind the dehumidifier is a squirrel cage blower that puts out about 500 CFM.
322081
The fan blows heated and dehumidified air into the rear plenum, which flows through the stack and returns to the right chamber. Here's a photo of the control system.
322084
On the far right is a high limit temperature switch. Just to the left and almost hidden from sight is the analog humidistat, which is only thing I adjust during a drying run. To the left is the primary power switch, and then switches for Heat, the Dehumidifier, and the Humidifier. Top left is the digital temperature control which is always set to 110 F. In the center below the switches is the wiring rats nest, which is all done through a DPDT relay. Just below that is an alarm below that will ring if the high limit temperature switch is tripped. At the lower right is the ultrasonic humidifier. It's used for reconditioning the lumber at the end of the drying cycle, but I've found there's no case hardening if I follow the drying rate schedule given in the FWW article so I never use it.
The door seals to the chamber against a foam seal to create an air tight seal.
322086
Drying takes about 2 weeks with a full load of wood at about 14% MC, and costs perhaps $20 in electricity. Operation is as simple as checking how much water is removed every day, and adjusting the humidistat to stay on but not exceed the drying rate schedule. If I have to leave town for a few days, I just let the kiln idle at the last setting and then resume lowering the humidistat when I get back. As long as you don't try to dry faster than the schedule allows you really can't do anything wrong.
I built the kiln 12 years ago at a cost of about $300. I've dried at least 3000 BF of lumber in it over the years.
John
Here's a photo of the kiln as I'm loading it with some very nice red oak I milled last Fall. The kiln is made from OSB, plywood, and 2 x 4's. The internal seams are all caulked and interior surfaces were painted with a couple of coats of oil based paint. Overall it's about 10 ft long so it can handle 8' long boards, and can hold about 275 BF.
322074
A closer view of the drying chamber shows the vertical slots from in the rear air plenum where the dehumidified air exits and flows through the stack of wood.
322076
Here's a photo after the kiln has been fully loaded and the dehumidifier installed in the right chamber. Note the catch pan under the cooling cools and hose that ducts out of the kiln to a collection bucket. That allows me to measure the amount of water removed every day.
322078
Above the dehumidifier are 3, 100W light bulbs that provide the primary heat source. That's all it takes to heat the chamber to it's operating temperature because it's so well insulated with the 1-1/2" foam board. Behind the dehumidifier is a squirrel cage blower that puts out about 500 CFM.
322081
The fan blows heated and dehumidified air into the rear plenum, which flows through the stack and returns to the right chamber. Here's a photo of the control system.
322084
On the far right is a high limit temperature switch. Just to the left and almost hidden from sight is the analog humidistat, which is only thing I adjust during a drying run. To the left is the primary power switch, and then switches for Heat, the Dehumidifier, and the Humidifier. Top left is the digital temperature control which is always set to 110 F. In the center below the switches is the wiring rats nest, which is all done through a DPDT relay. Just below that is an alarm below that will ring if the high limit temperature switch is tripped. At the lower right is the ultrasonic humidifier. It's used for reconditioning the lumber at the end of the drying cycle, but I've found there's no case hardening if I follow the drying rate schedule given in the FWW article so I never use it.
The door seals to the chamber against a foam seal to create an air tight seal.
322086
Drying takes about 2 weeks with a full load of wood at about 14% MC, and costs perhaps $20 in electricity. Operation is as simple as checking how much water is removed every day, and adjusting the humidistat to stay on but not exceed the drying rate schedule. If I have to leave town for a few days, I just let the kiln idle at the last setting and then resume lowering the humidistat when I get back. As long as you don't try to dry faster than the schedule allows you really can't do anything wrong.
I built the kiln 12 years ago at a cost of about $300. I've dried at least 3000 BF of lumber in it over the years.
John