Why is lumber kiln dried

This type of kiln requires large amounts of energy; therefore, it is neither economical nor efficient compared to dehumidification kilns. Figure 1. Schematic of a dry kiln. USDA, Today, dehumidification kilns are among the most commonly used kilns in the wood products industry. An advantage of using dehumidification kilns is the continuous recycling of heat within the kiln rather than the discharging of heat from the kiln as in the case of conventional kilns.

The majority of water is condensed on the coils of the dehumidifier and removed as liquid rather than being ventilated to the outside of the kiln. Although dehumidification kilns use electricity, which is more expensive than gas, they are still more economical than conventional kilns because they recycle heat and also are more environmentally friendly. The air dehumidification kiln reaches a temperature of 95 to oF, and the hot air is circulated over the wood.

The hot, moist air is then cooled by passing over cold refrigeration coils. The evaporated moisture condenses into liquid form and is drained as cool water.

Figure 2 illustrates the working principles of the dehumidification kiln. Figure 2. Dehumidification drying system. The less commonly used vacuum kiln is 3 to 4 times more expensive than either conventional or dehumidification kilns because of limited drying capacity in the chamber. However, the main advantage of this system is a very high-drying speed.

Kiln schedules are used to determine the temperature and relative humidity needed in the kiln to dry specific wood products at a satisfactory rate without causing objectionable drying defects. A typical kiln schedule is a series of temperatures and relative humidities, which are applied at various stages of drying as can be seen in Table 1.

In general, the schedule should be developed so that drying stresses do not exceed the strength of the wood at any given temperature and moisture content. Schedules vary by species, thickness, grade and intended final use of the material.

For example, a typical hardwood schedule would begin at to oF and to percent relative humidity when lumber is green. Temperatures could reach up to to oF by the time lumber has a moisture content of 10 to 15 percent. Softwood kiln schedules differ from hardwood schedules because the kiln temperature and relative humidity are decided at predetermined times rather than at moisture content levels. Hopefully, that can help you to understand those different terms.

Please let us know if you have any questions below in the comments section! Paul Cuenin is a former Associate at Thompson Mahogany. He enjoys technology, photography and marketing. Troy, Yes if you are going to be building furniture for use in a climate controlled environment you definitly want to go with Kiln Dried Lumber. Glad to provide some good info for you. Think of a sponge.

When a sponge is wet, it holds moisture in each hole or cell. As it dries, it not only releases that moisture, it shrinks in size. If it has been trimmed to a functional shape, like a rectangle, it may twist or curl as it dries.

It does not, though, lose its capacity to reabsorb water when it is present. In the same way, wood may have much of its moisture removed during the kiln drying process, but it does not lose the ability to reabsorb moisture that is in its environment. Even when wood has been kiln-dried, it may have a documented moisture content level, but at any step of the process after the kiln—transportation, manufacturing, storage or installation—it is possible for wood, even manufactured wood products, to lose additional moisture in a dry environment or re-absorb ambient moisture , altering its moisture content level and possibly even its dimensions.

Any completed wood project or product can be jeopardized by moisture imbalances in the wood used. Continuing with our wood flooring example, it is just as important that the flooring installer allows each board bundle to acclimate to the job site environment before installation begins as it is for the manufacturer to make sure the moisture content is accurately monitored before it leaves the plant.

An accurate wood moisture meter can provide quick, accurate and reliable monitoring of wood flooring to ensure that the installed floor will not suffer from moisture-related problems like those identified above. With accurate and current wood moisture content measurement, the flooring can be installed with assurance, finished for long-lasting, and durable use. Some woodworkers feel air-dried lumber is nicer to work, and that kiln dried lumber is a bit more brittle. It is a matter of personal choice.