Wood is a hygroscopic material, meaning it exchanges water with the surrounding air, absorbing or desorbing it. This exchange is regulated by the air’s hygrometric conditions, such as temperature, relative humidity and atmospheric pressure. So, for a given climate, there’s a balance where wood is neither absorbing nor desorbing. This is called the equilibrium moisture content of wood. Under normal conditions, such as furniture in a house, the equilibrium moisture content of wood is much lower than the initial moisture content of the wood. This is one of the reasons why lumber must be dried before it hits places like Ikea.

The natural drying of wood is a slow process. To quicken things up, several industrial dryer technologies have been developed, including the air-conditioned hot-air dryer, which is one of the most widely used kiln configurations across the country. The aim of this type of dryer is quite simple: to maintain hygrometric conditions conducive to rapid drying. In other words, we want to control the temperature and relative humidity inside the dryer. We also want to ensure good air circulation to accelerate drying and rapidly evacuate water evaporated from the wood.

To achieve climate control, several components are required. Firstly, since wood dries faster at higher temperatures, a heating system is required which can be direct or indirect. The former is equipped with a burner in which fuel (biomass, natural gas or oil) is burned to produce hot combustion gases. These gases are fed directly into the dryer to heat the air. The amount of material burned can be controlled to reduce or increase the temperature of the combustion gases. In the indirect heating system, a burner is used in conjunction with a boiler to heat a fluid (steam, thermal oil). This hot fluid then circulates through coils inside the dryer to heat the air in contact with them. The amount of fluid circulating through these coils can be controlled to regulate the rate at which the air in the dryer heats up. This process is very similar to how in some baseboard heaters, a home’s heated water is circulated through these heaters to warm up the home.

As the wood dries, the relative humidity of the air in the kiln increases. Without proper venting, the air would become saturated and lumber drying would come to a complete halt. Several components are therefore required to control the relative humidity of the air in the dryer. The first is the presence of vents that open to evacuate warm, humid air from the kiln and inject cooler, drier air. Heat exchangers are complementary and optional systems that recover part of the energy evacuated by the vents.

Generally, the use of vents is sufficient to control humidity when drying softwood lumber, but certain situations require the use of humidification systems when relative humidity is too low in the dryer. This may be the case during conditioning and balancing periods, which are sometimes used at the end of drying cycles. Humidification systems include steam injection systems and water atomizers, which spray liquid water as a fine mist into the dryer. Humidification systems are generally optional in the softwood lumber industry.

Finally, all air-conditioned hot-air dryers are equipped with a ventilation system. Its first function is to ensure a uniform climate inside the dryer. The second is to promote rapid drying of the lumber. The faster the air circulates over the surface of the wood, the more efficient the exchange of water between the wood and the air. This is particularly true when the moisture content of the wood is above the fiber saturation point, i.e., around 30% moisture. Fans can be configured transversely or longitudinally, single or double pass, but their main functions remain the same in all cases.

All these systems, together with a computer or programmable controller, allow you to control conditions in the dryer for a desired drying time. At this point, it’s important to remember that the conditions to be maintained in the dryer are not arbitrary. The choice of drying conditions is the result of a compromise between productivity and quality. The kiln has been the standard way of drying in the lumber industry. That’s why it’s so important to monitor its operation to ensure that its various components are under control. The slightest deviation will have a direct impact on productivity and/or lumber quality.