The DIY/home enthusiast is likely to want to operate a lumber drying kiln to dry green lumber so that it can be used more quickly after being cut down. Lumber drying kilns can be used to dry wood so it can be used as firewood more quickly. However, this is really not a cost effective use of a lumber drying kiln.
Lumber kilns are also used commercially to produce charcoal; however, again to produce a small amount of charcoal for personal use there are quicker and cheaper methods than operating a lumber kiln. So, if you’re thinking of using a lumber drying kiln for lumber to do woodwork with, here are some pointers you need to be aware of when operating lumber kilns.
Whether you’re working as a DIY enthusiast or even turning your hobby into a small business making furniture, having a regular and reliable supply of lumber will be important to you. This means having dried lumber available to you with consistent levels of moisture that also retains all of the natural handling properties when working with it.
This means you want to be confident that if you’re making a cabinet, once the door is fitted it won’t suddenly warp and not fit due to a moisture content variation when placed in a different location. Also properly dried lumber, so that the moisture content is below 20% of that at the time it was cut, will be less susceptible to fungal and insect attacks; and by removing moisture from the lumber it will be more receptive to preservative treatments, if the lumber will eventually be used outside and exposed to the elements.
Softwood and Drying Kilns
Generally speaking softwoods, such as the pines commonly used in the construction industry, will dry quite quickly. So when drying softwoods attention needs to be made as to the kiln temperature and the time the lumber has been inside the kiln, as over drying it will quickly render a softwood unusable.
A particular problem with softwoods dried in kilns is that they can be prone to warping if they are cut. This is because the re-sawing releases stresses that arise during the drying process. The secret here is to cut the timber to the required lengths etc before putting it into the lumber drying kiln.
This way, a temperature of 1500C can be used with a through flow of air at 300 ms-1 which, if the lumber is stacked properly, can dry out a consignment of pine construction lumber within 12 hours. This drying time is of course subject to the dimensions of the cut lumber, for thickly sawn lumber the time can drift out to 20-24 hours.
To kiln dry a hardwood is a more complex process, requiring substantially more time with 6 weeks or more not being uncommon. Refractory woods, like oak, that have been recently cut need particularly delicate handling through the drying process. A typical lumber drying kiln temperature for hardwoods is between 40 and 600C with airflow of little more than 100 ms-1.
The airflow through the kiln must give an even distribution of both heat and humidity, which can be facilitated by carefully stacking the lumber. To reduce the risk of stresses arising in the lumber, such as described above for the softwoods in a lumber drying kiln, the lumber also undergoes a conditioning process, whereby the kiln temperature and humidity are lowered gently in a controlled way, rather than simply being switched off.