Editing Kiln (section)

== Wood-drying kiln ==
{{Main|Wood drying#Kiln drying}}

[[Green wood]] coming straight from the felled tree has far too high a moisture content to be commercially useful and will rot, warp and split. Both [[hardwood]]s and [[softwood]] must be left to [[wood drying|dry out]] until the moisture content is between 18% and 8%. This can be a long process unless accelerated by use of a kiln. A variety of kiln technologies exist today: conventional, dehumidification, solar, vacuum and radio frequency.

'''Conventional wood dry kilns'''{{sfn|Rasmussen|1988}} are either package-type (side-loader) or track-type (tram) construction. Most [[hardwood]] lumber kilns are side-loader kilns in which fork trucks are used to load lumber packages into the kiln. Most [[softwood]] kilns are track types in which the timber is loaded on kiln/track cars for loading the kiln. Modern high-temperature, high-air-velocity conventional kilns can typically dry {{convert|1|in|mm|adj=mid|-thick|order=flip}} [[green wood]] in 10&nbsp;hours down to a moisture content of 18%. However, 25-mm-thick [[green wood|green]] red oak requires about 28 days to dry down to a moisture content of 8%.{{citation needed|date=May 2024}}

Heat is typically introduced via steam running through fin/tube heat exchangers controlled by on/off pneumatic valves. Humidity is removed by a system of vents, the specific layout of which are usually particular to a given manufacturer. In general, cool dry air is introduced at one end of the kiln while warm moist air is expelled at the other. Hardwood conventional kilns also require the introduction of humidity via either steam spray or cold water misting systems to keep the relative humidity inside the kiln from dropping too low during the drying cycle. Fan directions are typically reversed periodically to ensure even drying of larger kiln charges.{{citation needed|date=May 2024}}

Most softwood kilns operate below {{convert|115|C|abbr=on|-1}} temperature. Hardwood kiln drying schedules typically keep the dry bulb temperature below {{convert|80|C|abbr=on|sigfig=2}}. Difficult-to-dry species might not exceed {{convert|60|C|abbr=on|sigfig=2}}.

'''Dehumidification kilns''' are similar to other kilns in basic construction and drying times are usually comparable. Heat comes primarily from an integral dehumidification unit that also removes humidity. Auxiliary heat is often provided early in the schedule to supplement the dehumidifier.

'''{{visible anchor|Solar kilns}}''' are conventional kilns, typically built by hobbyists to keep initial investment costs low. Heat is provided via solar radiation, while internal air circulation is typically passive.

'''Vacuum and radio frequency kilns''' reduce the air pressure to attempt to speed up the drying process. A variety of these vacuum technologies exist, varying primarily in the method heat is introduced into the wood charge. Hot water platten vacuum kilns use aluminum heating plates with the water circulating within as the heat source, and typically operate at significantly reduced absolute pressure. Discontinuous and SSV (super-heated steam) use atmosphere pressure to introduce heat into the kiln charge. The entire kiln charge comes up to full atmospheric pressure, the air in the chamber is then heated and finally a vacuum is pulled as the charge cools. SSV run at partial-atmospheres, typically around 1/3 of full atmospheric pressure, in a hybrid of vacuum and conventional kiln technology (SSV kilns are significantly more popular in Europe where the locally harvested wood is easier to dry than the North American woods.) RF/V (radio frequency + vacuum) kilns use microwave radiation to heat the kiln charge, and typically have the highest operating cost due to the heat of vaporization being provided by electricity rather than local fossil fuel or waste wood sources.{{citation needed|date=May 2024}}

The economics of different wood drying technologies are based on the total energy, capital, insurance/risk, environmental impacts, labor, maintenance, and product degradation costs. These costs, which can be a significant part of plant costs, involve the differential impact of the presence of drying equipment in a specific plant. Every piece of equipment from the green trimmer to the infeed system at the planer mill is part of the "drying system". The true costs of the drying system can only be determined when comparing the total plant costs and risks with and without drying.{{citation needed|date=May 2024}}

Kiln dried firewood was pioneered during the 1980s, and was later adopted extensively in Europe due to the economic and practical benefits of selling wood with a lower moisture content (with optimal moisture levels of under 20% being much easier to achieve).<ref>Maviglio, S. 1986. From stump to stove in three days. Yankee. 50(12): 95-96 (December).</ref><ref>[http://www.fpl.fs.fed.us/documnts/fplrn/fplrn254.pdf fpl.fs.fed.us]</ref><ref>{{Cite web |url=https://www.certainlywood.co.uk/facts-and-answers/wood-facts-about-logs |title=Important information and facts about our firewood |publisher=www.certainlywood.co.uk|access-date=2016-10-27}}</ref><ref>{{Cite web|date=2021-12-06|title=Efficient Kiln-Dried Firewood Heating|url=https://www.lektowoodfuels.co.uk/blogs/news/why-heating-your-home-efficiently-is-easier-with-firewood|access-date=2022-02-11|website=Lekto Woodfuels}}</ref><ref>{{Cite web |last=Firewood.co.uk |title=Wet Firewood Ban - Ready to Burn {{!}} Not Sure What This Means, or Why it Matters? |url=https://www.firewood.co.uk/firewood/firewood-knowledge-base/ready-to-burn/ }}</ref>

The total (harmful) air emissions produced by wood kilns, including their heat source, can be significant. Typically, the higher the temperature at which the kiln operates, the larger the quantity of emissions that are produced (per mass unit of water removed). This is especially true in the drying of thin veneers and high-temperature drying of softwoods.{{citation needed|date=May 2024}}