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Explaining kiln drying

Eric W

Eric Wong, digital analyst at Algorex

When I try to explain kiln drying to my friends and family, the responses I get are usually, “so it’s basically a big oven?” And they would be correct in the most basic description of the process. Wood is naturally wet and if we made wood products without drying, the dimension and shape of the wood product will start to deform as it dissipates moisture to equalize with the surrounding environment. Whole buildings may shift if they were made of wet wood and the building is in Arizona. Most sawmills have on-site kilns that dry their lumber before they go through the planer that require multiple factors such as kiln operators, an air-tight kiln, and various sensors (temperature, moisture content, etc.). But why is this process so complex? In this post, we will break it down into three categories: variability in nature, quality control/compliance, and optimization.

If you look closely at an end face of a piece of lumber, you’ll notice that there are rings. And if you look even closer (with a loupe), you may notice holes of varying sizes depending on the species. These holes denote the vessels or “pipelines” in the wood that carries water and nutrients throughout the tree. Free water is the water that is within these vessels and bound water refers to the water that is bound within the tree’s smaller capillaries. Drying’s main goal is to remove this water and it takes much more effort to remove the bound water than the free water. To do this, an environment needs to be created so that water can dissipate into the atmosphere. This environment needs to be dry and hot. And as water is removed, it becomes steam which humidifies the atmosphere. Like when you bake some vegetables and are met with a face full of steam when you crack open the oven door. If you ever travelled to a place that is hot and humid, you’d feel like it’s way harder to cool off. That’s because as humans, we sweat to cool off through the mechanism of evaporation. If there’s too much water in the air, the water won’t have anywhere to go. In the industry, we use “wet-bulb depression” as a measure of how much water there is in the atmosphere. For example, if you’re in a dry environment, a regular thermometer would read your “dry-bulb” temperature and a thermometer that is covered in a damp cloth would read your “wet-bulb” temperature. It’s this balance and control of the environment that that adds to the complexity of drying which some refer to it as an art.

Just as the saying goes that no two human beings are born the same, no two trees are grown the same either. Which is why we have processes such as silviculture which is the management of forest and woodlands to meet its landowners’ needs such as timber or wildlife habitat. Aside from the most important aspect of forestry management, which is sustainability, forestry management breed and grow trees that provide the most fibre in each growth period. Forestry management also prune the branches as trees grow to reduce the number of knots, the dark circles or wedges you see in lumber, and encourage the tree to grow taller. All this effort is to reduce the natural variability that trees have so that sawmills can better streamline their operations including kiln drying. We can compare this process to that of cooking time variability. Imagine that you have a thicker piece of chicken roasting on a pan versus a thinner slice of chicken. Of course, the thinner piece would start burning before you even start warming the middle of the other piece of chicken. That’s what it is like when there’s a lot of variability in lumber drying. Sawmills don’t want over-dried or under-dried lumber going to their customers.

Most kiln dryers have a compliance requirement that requires most of the turnout of lumber to be within a certain range of moisture content. If on a given audit, the kiln doesn’t meet the compliance, that kiln could be out of operation which could be a big loss of productivity and potential loss of revenue for the sawmill. Therefore, kiln operators must go off what they see on their screens or stop the kiln to perform periodic checks to see what’s going on in the kiln. It’s a whole song and dance that needs to be performed harmoniously to dry lumber properly.

There are bottlenecks in many processes and operations and kilns are a common culprit in sawmill operations. It’s like how in the kitchen, it’s not advisable to cook something faster in the oven by cranking the temperature higher. Once the lumber is in the kiln, it must dry for hours, days, or even weeks. If a sawmill can optimize their time so that there’s no downtime between batches or nail their drying schedule so that the total drying time is shorter, then their overall throughput is higher. And the name of the game in the sawmilling industry these days is optimization if they want to remain competitive.

A lot goes into kiln drying but we can go back to our kitchen analogy again; yes, it is a big oven. But the food is going to be served to guests who, if the food is bad, will go to another restaurant in a heartbeat. The guests also don’t want to be kept waiting even though lumber doesn’t go bad when it’s cold. So, keep in mind that most wood products that you see had to go through some sort of drying process when you see them.

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