Hardwood Plywood Series: The Drying Equation

The moisture content (MC) of any wood product can impact many factors: mass, decay susceptibility, permeability, strength, electrical properties, heat transfer properties, adhesion and dimensional stability. 

Veneer dryer

Specifically, the inner-ply MC of plywood is important during the manufacturing process for several reasons—wood changes dimensions when it dries or absorbs water. Therefore, to produce a stable and quality product, the dryer has to reduce the MC to the right level prior to further processing. Also, in order for the adhesives to bond properly, the wood has to have the proper MC.

So what does the dryer do and what key factors of the wood dictate the process? This is an excellent question, and to demonstrate, we'll use an equation developed and presented in the paper "Modeling Drying Time during Veneer Drying and Comparison with Experimental Study." 

As we will show, there is plenty involved when drying a section of wood. Though the below equation may look daunting, the following descriptions of each of the terms provides a general understanding of what happens as the wood dries. It should be noted that many manufacturers adopt different equations based on their specific product mix and properties.

 

t
The total time needed to dry the wood to the final moisture content (MC).

Mo
The initial MC of the wood. Coming out of the peeler lathe, the wood has an MC by weight of around 75%. However, the weight needs to be under 10% for proper glue adhesion and dimensional stability. The higher the Mo, the longer the dryer has to run and expend energy.

Mi
The final MC. As the wood conveys through the dryer, hot air jets blow across the surface. The greater the difference between Mi and Mo, the more time and energy is required. This is also part of a squared term, so a large difference between Mi and Mo has a significant impact on the time.

ρ
The weight density of the dry wood. The higher the density, the more time and energy required.  Naturally, you will have varying densities depending on the type of wood you are drying.

λ
The heat needed to evaporate water. Essentially, this is a physics constant that represents how much heat is required to evaporate a pound of water.

L
The thickness of the wood. This is a squared term, so if the thickness is doubled, the drying takes four times longer.

K
The thermal conductivity of dry wood. In other words, this is a constant representing the resistance of wood to conduct heat. The dryer has to overcome this resistance to get enough heat into the wood so that moisture can evaporate.

T2
The temperature of the wood surface when heated by the hot air jets. A higher T2 will dry the wood faster. However, this has practical limits. If you have excessive amounts of a T2, the wood will become damaged, but having a low amount of T2 will detrimentally increase the drying time.

T212
The boiling temperature of water.

The critical term in all this is "t," or the time it takes to dry the wood. This time figures into how long it takes to produce the plywood from beginning to end, so when going through the drying process, time is the key factor for a smooth-running production line and a constant supply of quality products.

To learn a bit more of the exact process Timber Products Company goes through when drying wood, watch this informative video:

 

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