Drying Wood Using Air

Richard Jones has spent his entire life as a professional woodworker and has dedicated himself to researching the technical details of wood in great depth, this material being the woodworker’s most important resource. The result is “Cut & Dried: A Woodworker’s Guide to Timber Technology” (from which the information below is an excerpt). In this book, Richard explores every aspect of the tree and its wood, from how it grows to how it is then cut, dried and delivered to your workshop.

In section 6.4 the drying and rewetting of wood was illustrated by using a sponge or towel to represent wood. An extension of this analogy serves as a preliminary introduction to terminology about the wood-seasoning process.
Let’s say, for the sake of discussion, that you soak a very large and thick bath towel in a water bath. Lift up the sopping towel and wring it out as thoroughly as possible. Let us also assume you have the unlikely physical ability to wring out every drop of loose (free) water in the towel so the only water left is that bound within its fibres. This towel now stands for wood commonly and erroneously described as being at fibre saturation point (FSP), although the comments on FSP made in section 6.5 should be borne in mind. Fold the towel up three or four times into a long large sausage and hang it over a washing line. It’s a cool, dull, still, overcast day with, perhaps, intermittent, very light drizzle.

The towel will barely dry any further in the described weather conditions until either a breeze starts, the sun comes out or both changes happen together. It’s common knowledge that even if the sun doesn’t come out but a breeze starts the bundled-up towel will dry. Similarly, if there’s no breeze but the sun comes out the additional warmth causes water to evaporate from the towel’s fibres. In both cases described, the towel will eventually dry through. Put the two factors together, i.e., warmth and moving air, and the towel dries more rapidly than it will with either just a breeze or just extra warmth. Within the bundled-up drying towel there is a moisture gradient: As the towel dries it remains wetter in the middle of the bundle than near the surface. Assuming drying continues, the moisture content within the towel gradually evens out until it has an equal moisture content all through.

Without really knowing any science or terminology we know how to dry clothes quickly. Options include hanging them on a washing line on a warm, lightly breezy, sunny day, putting them out on a dull but dry and windy day, or hanging them over a warm radiator, and so on. Clothes fully opened and pegged on a line dry much quicker than clothes bunched up tightly.

What applies to drying clothes has similarities to the conditions that will dry wood. To dry wood quicker, heat air and move the hot air over it, although with wood, when it has dried to approximately 20 percent MC, the primary drivers for further drying are air temperature and humidity, not the speed at which the air passes over the wood. Thin boards dry faster than thick boards, which is analogous to opening clothes out to dry rather than leaving them bunched up. Fast drying of wood with very hot dry air will certainly accomplish the task, but it usually comes with an unacceptable price, i.e., degradation of one sort or another such as splitting, surface checking, case-hardening18, collapse (aka core collapse), honeycombing etc., making the wood unusable and unsellable. It’s imperative to control the speed at which wood dries in order to produce an acceptable end product.

The air’s RH must be low enough to absorb more water vapour. Air at 100 percent RH cannot absorb any more water vapour. Wet wood in RH conditions like this is comparable to my earlier description of hanging washing out to dry on a cool, damp, intermittently drizzly day – the clothes dry very slowly.

Warm air transfers heat to the wood causing the moisture in it to evaporate into the air. Again, the RH of the air must be low enough to absorb the water vapour given off by the wood. Drying kilns add warm air to the drying chamber, which transfers heat energy to both the wood and the water within it. The difference in temperature between the introduced dry air and the wet wood is often, but not always, quite small at the beginning of the kilning process. Water in the wood converts to vapour and evaporates through the wood surface into the air introduced into the drying chamber. The air temperature within a kiln is high, e.g., at stages in the wood drying process temperatures of 65.5° C (150° F) or more are used. At this temperature if the air stays at 70 percent RH it will eventually dry wood to approximately 10.5 percent MC (see figure 8.2).

If the air becomes too humid to dry the wood effectively, one of two things must happen for the wood to continue drying. First, further raising the temperature of the air in the kiln reduces its RH. Hotter air is capable of holding additional moisture released from the wood. Second, moving the humid air out of the drying chamber and replacing it with drier air will continue the drying process. Raising the temperature of the air already in the chamber is the cheapest option, but too high a temperature may lead to faults in the wood, particularly in some species more than others, e.g., surface checking as described earlier.

As timber dries, a moisture gradient develops inside the wood much like the earlier-described folded-up towel hanging over a washing line. In a wood-drying kiln where air temperatures are artificially high, generally the greater the temperature of the air acting on the wood, the steeper the moisture gradient within it, and moisture moves out of the wood faster. This also leads to faster evaporation of moisture from the surface of wood. Conversely, when wood is air dried and therefore experiences normal weather conditions, or if the wood is in service in a typical environment found in habitable buildings, RH is the primary controller of the steepness of the wood’s moisture gradient – air temperature in these circumstances has only a small effect.

For green wet wood to dry, as freshly milled boards or planks, for example, air must be moving to carry moisture away from the wood’s surface; this creates a place for the water deeper in the wood to migrate to, where it will also be carried away by the flow of air. If only a small volume of stagnant dry air surrounds wet wood, that air quickly becomes fully saturated with evaporated water. At that point no further drying can occur until that pocket of air moves away and is replaced by drier air.

Moving air carries moisture away from the wood’s exterior, thus drying the wood. But the air molecules adjacent to the wood surface stick to it. Air molecules just above the surface collide with the stuck air molecules and their movement is disrupted and slowed down. In turn, these air molecules impede the flow of air molecules just above them. As distance from the wood surface increases, the collisions diminish until air movement is unimpeded and becomes free flowing. In effect there is a thin layer of viscous “fluid” near the surface where velocity changes from zero at the surface to free flowing some distance away from it. “Engineers call this layer the boundary layer because it occurs on the boundary of the fluid.”19 (Benson, 2009, p 1) Within the boundary layer next to the wood the air is wetter (because it’s picked up moisture from the wood) and travels slower than the air above the boundary layer – it tends to hold the moisture taken from the wood close to the wood’s surface. A faster-moving air stream reduces the effect of the boundary layer and it sweeps away the damp air with its high-vapour pressure. The damp air is replaced with new drier air, i.e. air with a lower vapour pressure better able to absorb further moisture from the wood.

Whether wood is air dried or kiln dried the air entering the wood stack from one end has a lower RH than the air leaving the stack at the far end. Moving air leaving a stack of drying wood is cooler than the air entering it. The air cools as it transfers heat to the wood, thus enabling the drying process. If the air continuously passes through a stack of wood in one direction, the wood at the “upwind” end of the stack always dries faster than the wood at the “downwind” end. This results in unevenly dried planks of wood where the downwind end of a stack might be 3 percent or 4 percent wetter than the upwind side. In more extreme cases, the difference in moisture content between the upwind and downwind side of a stack may be 8 percent to 10 percent MC if the wood is very wet at the start of the drying process – in this case one possible result is the stack of wood may lean toward the drier side. This effect is more evident in wide stacks of wood, e.g., greater than about 2 metres (~6′), than in narrow stacks. Natural changes in wind direction and speed cancel out this effect in stacks of air-dried wood. It is only if a kiln operator is drying a wide stack of wood, or some particularly difficult to dry woods, that there is a real need to regularly alternate the air flow direction within the chamber. To achieve this, the fan blade rotation is reversed at evenly spaced intervals, e.g., every two hours, four hours, 12 hours etc. This upwind and downwind disparity in the drying ability of moving air in a stack of wood limits the size of a stickered pile of planks. This is especially the case with air drying where the yard owner really has less control over temperature, wind speed or wind direction. However, it should be noted that air velocity in either a kiln or in an air-drying wood pile is most important at the initial drying stage of wet wood because of its role in carrying away moisture from the wood surface. As the wood dries the significance of air movement gradually diminishes until the wood reaches about 20 percent MC. At this MC the primary critical factors for further drying are humidity and temperature, with the importance of air movement reducing significantly the drier the wood becomes.