Many small sawmill operations look to lumber drying as a way to add value to their product and to gain entrance to new markets. Let’s face it; most purchasers of small volumes of lumber want a dried product.
If the small sawmill owner is going to sell lumber to local cabinet makers, custom home builders and local molding and millwork shops, they must be able to supply lumber at a moisture content below 12% (typically 6-8%). In most localities throughout North America this requires the use of a dry kiln. Moisture content is the ratio of the weight of water in wood to the weight of the actual wood material.
Types of Dry Kilns
Dry kilns are usually categorized by the way they are heated and how the humidity is controlled. The type, size and operating costs of a kiln will depend on the species dried, local energy costs and the amount of lumber to be dried at one time. Companies that produce 2,000-5,000 board feet of lumber a day are likely going to consider different technology and lower initial investment than companies producing 2,000-5,000 board feet an hour.
Let’s focus on kilns that are designed for smaller drying operations, say 1,000-10,000 board feet at a time. Also, keep in mind that most drying defects get their start during the first 1/3 moisture content loss, so stricter control is required if the lumber to be dried in the kiln is green versus air dried. Common types of kilns for small scale drying operations include conventional steam-heated, dehumidification and solar.
Conventional Kilns
The most common commercial kiln for large-scale operations is the conventional steam kiln. While they offer some of the best control and rapid drying, they are typically out of the capital investment range for most small drying operations.
Several sources are now available for small-scale, hot-water-heated conventional kilns. These kilns use hot water heaters (fired using natural gas or wood) to heat air which is moved throughout the kiln. These kilns require the ability to tightly control the temperature and humidity in the kiln. They also allow for the use of a drying schedule and consistent drying times throughout the year. They can require significant energy since humidity is vented from the kiln. Hot air is also vented, requiring further energy use. There are several suppliers of this type of kiln that utilize modified refrigerator trailers as the insulated kiln structure.
Direct-fired and Indirect-fired Kilns
For direct-fired kilns, the heat produced by combustion is blown directly into the kiln. Indirect-fired kilns use a heat transfer mechanism similar to a gas or electric furnace that heats air for homes. The air is heated and then blown into the drying chamber.
Typically, most small-scale kilns of this type have very poor control over the temperature and humidity. Therefore, they are usually used on already well air-dried lumber. I have visited many small drying operations over the years that use wood stoves as the primary heating source to treat well air-dried lumber. Most of these systems are homemade and are very simple. Drying times can be very uniform throughout the year provided that well air-dried lumber is supplied to the kiln. The biggest problem is getting the heated air uniformly distributed throughout the length of the kiln to provide uniform drying conditions. These kilns also require significant energy use since venting of humidity also leads to heat losses.
Dehumidification Kilns
Dehumidification kilns (DH) are similar to conventional kilns in the basic design of airflow and heating, but differ in the method that water is removed from the air. In a DH kiln, water in the air is condensed on the coils of a dehumidifier and is removed as liquid rather than vented out of the kiln. The lack of venting reduces the amount of energy required to heat the kiln atmosphere. Since the heat of vaporization is recovered through evaporation of water on the coils, this energy can be applied to drying the lumber.
Dehumidification kilns are generally more energy efficient than other kiln types and are used in both large and small-scale drying operations. They also have the advantage of being able to dry at lower temperatures. However, many small units are not capable of operating over 130oF. Thus, these kilns require longer drying times. New systems are currently available that allow higher temperature operation.
DH kilns can be acquired and installed by commercial companies, or components can be purchased and the kiln building constructed using locally obtained building materials. DH kilns provide consistent drying times throughout the year and can used for drying both green and air-dried material. Most units are capable of good temperature and humidity control.
Solar Kilns
Solar kilns use the energy from the sun as the main heating source for the kiln but also require electricity to run fans for air circulation through the lumber. Solar kilns may vary greatly in complexity and cost. The simpler kilns use passive solar collection where the air is heated by sunlight and moved to the lumber by fans. More complex solar kilns use active solar collection, heat storage capacity, and water or air is used as the transfer medium to move heat from the collector to the insulated drying compartment.
Passive solar kilns are relatively inexpensive and simple to construct; however, they provide the least amount of control over temperature and humidity. Typically the kilns are constructed such that the collector is designed to provide a specific amount of heat, typically such that the drying rate will never exceed the safe drying rate (prevents checks and splits).
The main problem with solar kilns is that the drying rate can be highly variable since it is dependent on the sunlight for heat. Drying times are significantly reduced in the winter and during rainy periods making it difficult to predict exactly when a load will reach its final desired moisture content.
A Word on Stress
Drying stresses occur in all wood due to the fact that wood dries from the
outside towards the center. The outside will begin to shrink prior to the interior. This process occurs in air-dried material and kiln-dried material regardless of the drying method used. However, in general terms, the faster the drying rate, the greater the stress. Also, the final use of the wood after drying will determine how much stress can be allowed in lumber.
Stresses can be removed from wood by adding humidity to the surface at the last stage of the drying process (stage with the highest temperature), commonly referred to as the conditioning phase. During this phase of the schedule, specific temperature and relative humidity conditions will be used for a period of time to correct for stress development.
Steam or water vapor sprays are used in the conditioning phase of the drying process. Samples are cut from boards to determine the amount of stress that has been relieved. For those kilns that do not reach the temperatures required for conditioning or do not have a method to introduce humidity, the drying rate of the lumber must be slowed to reduce stress build up.
In the solar kiln, stresses can be kept low by turning the fans off at night and allowing the kiln to cool, thus raising the humidity. The cycle of drying during the day and picking up humidity at night through the course of drying assists with keeping stress to a minimum.
