Basic facts about drying wood

Why dry the wood?

Freshly cut lumber contains a large amount of water. If it is not removed, the lumber cannot be used to produce a high-quality finished product. Properly seasoned lumber sells for a higher price and is much easier to work with than unseasoned lumber. When wood is properly seasoned, it machines better, glues better, and finishes better. Drying also improves the strength of the wood, kills infestations, preserves color, reduces weight, and controls shrinkage. Wood that is not dried under controlled conditions is prone to warping, staining, and other degradation that lowers its selling price.

With a TROCKNNER dryer, successful lumber drying is simple and affordable for operations of virtually any size.

How much water is in the wood?

Some species of wood are more than half water in terms of weight when freshly cut. The moisture content in lumber is generally expressed as a percentage of the dry weight. For example, if a freshly cut board weighs five pounds per inch of wood, then weighs 3 pounds per inch of wood after it has been dried in a dryer at 0% moisture content, that means it had two pounds of water in it. every inch of wood. Two pounds of water per inch of wood compared to the dry weight of wood of three pounds per inch of wood is a ratio of 2:3, so the wood has a moisture content of 2/3, or 67%. . That's similar to oak, for example, which is typically about 68% moisture content when fresh cut. It is truly astounding how much water must be removed from wood to make it suitable for finished products.

Let's take the example of a truckload of oak. Freshly cut oak weighs about 2.5 kilograms per inch of wood. So an 8,000-inch truckload of lumber weighs about 42,000 pounds, just under 20 tons. Once you remove enough water for the oak to have a moisture content of 6% to 8%, it weighs about 3 pounds per inch of wood. So that truck now weighs 12,700 kg, or about 13 tons. That means that to completely dry a truckload of 8,000 inches of oak, 7,058 kilos of water must be removed, almost seven tons! That is why it is so important to choose the right drying system and use the right method.

What is free water and bound water?

When trees grow, liquid water moves through the cells of the wood. This water is called free water because it exists in the form of water and can be relatively easily removed from the wood. Bound water is water that becomes part of the wood fiber itself and is more difficult to remove. When wood dries, the first thing that happens is free water evaporates until the wood drops to what is called Fiber Saturation. Fiber saturation is generally reached when the moisture content reaches approximately 28-30%. At that point, all the free water is gone and only the bound water remains. Wood does not shrink until it is below its fiber saturation point.

The difference between drying hard and soft wood?

This is a frequently asked question, but it is more important to consider the actual species of the wood than whether it is a hard or soft wood. The terms hardwood and softwood generally refer to whether the wood comes from a tree with leaves (hardwood) or a tree with needles (softwood). Some hardwoods are actually softer than many softwoods, so there is no general drying method that applies as a rule to all hardwoods or softwoods. Different species require drying at different temperatures and different speeds to produce the best results. Oak has to dry slowly or it will degrade badly. Pine must dry quickly or it will stain and mildew. That doesn't mean that all hardwoods should dry as slowly as oak, or that all softwoods can dry as quickly as pine. There is a suitable method that has been determined for almost all species of wood to produce the best results.

Does the wood stay dry?

Wood is always trying to balance itself with the air around it, so its moisture content can change a bit after it has dried. In dry air, the wood gives up water to the air until it dries, as it becomes equal with the air. When the air is more humid, the wood absorbs water from the air. Wood expands when it absorbs water and shrinks when it loses it.

Can drying cause wood to split or crack?

Wood shrinks as it dries, but shrinkage does not begin until the wood is below its fiber saturation point, about 28% moisture content. If the outer surface is below 28% while the center of the board is still above grain saturation, the outer will attempt to shrink while the center does not, and if this continues until the surface becomes too dry relative to the core, the wood will split or score.

Controlled wood drying allows to avoid cracking. However, when wood is air-dried, there is no control over the drying process, and the weather can easily cause cracks and cracks that result in loss.

Does the thickness of the wood affect the drying rate?

Yeah. The lumber industry generally refers to the thickness of lumber in terms of multiples of a quarter inch. Therefore, one-inch-thick lumber is called 4/4, one-and-a-half-inch lumber is 6/4, etc. Generally speaking, drying times are roughly proportional to thickness. That is, 8/4 wood usually takes a little more than twice as long to dry as 4/4.