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A firewood kiln is an insulated chamber into which heat is introduced for the purpose of drying the firewood. Firewood kilns are a convenient way of prevailing over weather unpredictability. Firewood which has been dried will produce more heat and less condensates, such as creosote, which are one of the leading causes of chimney fires.
When a firewood kiln is used, cut firewood is placed in baskets. These baskets are generally made from fencing or industrial strength wire and angle iron, and are positioned on pallets or in racks in the chamber. Once all the wood is loaded, the chamber is heated. The firewood kiln is equipped with fans which circulate the air in the chamber so that moisture is dispelled out through vents.
Since temperature directly influences the drying process, it is important that the temperature in the firewood kiln is kept high and the relative humidity at a constant. Drying times are dependent on the type and amount of wood, as well as thickness. Softwood, such as pine, is easier to process than hardwood and takes less time to thoroughly dry.
It is important to note that intense heat can sometimes have an effect on the quality of kiln dried firewood. Temperature and humidity must be monitored throughout the process. Other than knots, which naturally occur, defects such as rupturing and collapsing of the wood can sometimes result due to uneven drying or shrinkage of the wood.
Firewood kilns which are not commercially produced are usually constructed out of shipping containers or sheds that have been converted for the sole purpose of drying wood. Shipping containers are preferred due to their portability and durability. Made of stainless steel, these containers will not rust. During the conversion process, the container is insulated and equipped with fans and vents for proper air circulation. The doors of the container are also lined so that, when closed, they create a hermetic seal.
Wood kilns use either direct or indirect sources for introducing heat into the chamber. Some firewood kilns use heat exchangers which are powered by the steam produced during the drying process, which is considered an indirect source. Other kilns use direct sources for heat production by utilizing natural gas or electricity. Kilns generally reach a minimum temperature of 160°F (71°C) and can potentially achieve heating temperatures up to 200°F (93°C) which can result in lowering the wood moisture content to expected levels of 10 to 20%.
Firewood that is kiln dried is free of bugs and mold, which allows for clean storage. Since it is properly seasoned, kiln dried wood burns clean, leaving little ash, less particulate matter, and no creosote behind. Additionally, the wood burns hotter and longer when it has been properly dried. The disadvantages of firewood kilns are generally considered to involve the amount of time needed for the actual drying and monitoring of the process, the space required for the kiln set-up, and the cost of operation.
I have done a lot of research about firewood kilns and one question I have left is what is the ideal humidity to be running in the kiln?
I would plan on the vent fan being humidity controlled, as obviously not enough venting will cause high humidity and too much vent will bring the temperature down or require more heat to keep the same temperature.
I am ideally looking for a humidity range ideal for firewood drying, e.g., 20-35 percent (example only)? So I could have the vent open and the fan going when the humidity is above 35 percent and close the vent to warm it up again when the humidity drops below 20 percent.
I'm just wanting to know what to aim for, and any help is appreciated.
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