Where Acclimation Begins: Mills and Moisture

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Photos courtesy of NWFA

In our industry, when we think of moisture and wood, we think of acclimation of the raw or prefinished hardwood flooring product and how long it needs to acclimate to someone’s home or office before installers can install it or sand and finishers can sand and finish that raw product. We know the acclimation process is not based on how many days the product needs to sit within the home or office, but on measurements of relative humidity and temperature in the building, and the moisture content in the product and substrate that the product will be installed over. But what about before it becomes a final usable product? How do the mills get the wood to a moisture content that is ready to be installed in someone’s home or business?

Let’s start with how mills get the wood dried. The short answer is wood is dried in a kiln. So what is a kiln? A kiln is a big insulated box that has enough room to hold a great amount of lumber where it will be dried to a designated moisture content. A kiln also kills any insects and fungi that may be living on, or in, the wood. There are several different types of kilns, but for this article I will give you a summary of two kilns that commonly are used in flooring mills.

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Stacks of green lumber are placed outside in the yard to begin the air-drying process.

stacking green-1The most common kiln is a conventional kiln. This kiln uses a boiler that is heated either by wood cut-offs from the milling process, or by gas. When heated, the boiler produces steam that travels through the pipes, like a radiator, that run into the top of the kiln chamber. From there, fans blow the hot steamed air around the kiln chamber where the moisture in the wood is forced out and then vented to the outside.

The second type is a dehumidification kiln. This kiln is different from the conventional kiln because it does not vent any of its air. It recycles heated air in the kiln that circulates around the wood. The moisture in the wood is forced out, which then is cooled over refrigerated coils. The vapor then condenses to liquid, which passes through a pipe to a drain.

Stacks of lumber are loaded into the kiln to continue the drying process.
Stacks of lumber are loaded into the kiln to continue the drying process.

System of Drying Lumber

Stacks of lumber are loaded into the kiln to continue the drying process.
Stacks of lumber are loaded into the kiln to continue the drying process.

No matter what kiln is being used, the system of drying lumber is pretty much the same. The lumber that is used for flooring is shipped on flatbed semi-trucks to the mill in a 4/4” thick plank form known as “green lumber.” Green lumber means the planks of wood still are wet, and have not yet begun the drying process. Once at the mill, the yard operator will unload the truck using an industrial-sized forklift, and they then will tag each stack with the month and day it arrived. Then the stacks of lumber are placed outside in the yard to begin the air-drying process. Each row of lumber in the stack is spaced with stickers or boards to leave an air gap so the lumber can start a natural slow drying process by letting the outside air flow through the sticker stacks for a defined period of time (usually no less than 120 days). The target moisture content of the wood at this point normally is somewhere between 15 to 20 percent.

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Once target moisture content is achieved, the stacks are pulled from the kilns and stored indoors to be milled and processed into hardwood flooring.

After 120 days in the yard, the operator will choose stacks of lumber and prepare sample boards to be used to monitor moisture readings throughout the rest of the drying process. Next, they will load the stacks of lumber into the kiln. Depending on kiln size, they can hold anywhere from 300 board feet to 300,000 board feet. The initial moisture content reading from this air-dried lumber gives the kiln operator a starting temperature and humidity level point to begin the drying process.

Throughout the drying process, which can take several days, computers and/or gauges monitor the sample pieces of wood, which alert the kiln operator to raise or lower temperature and humidity. The objective is to get the lumber dried down to a moisture content between six and nine percent. Once target moisture content is achieved, the yard operator will pull the stacks of dried lumber from the kilns and store them indoors to be milled and processed into hardwood flooring.

Computers and/or gauges monitor the sample pieces  of wood
Computers and/or gauges monitor the sample pieces of wood, which alert the kiln operator to raise or lower temperature and humidity.

The mill operators also monitor the moisture content of the wood flooring through the entire milling process, from the time they receive it, through the milling of the product, and all the way to the point where the product is packaged and ready to ship by using handheld or in-line moisture meters.

As you can see, it is a long process from start to finish. Throughout the milling process to the end product, the moisture content is monitored to make sure that it will be a stable product. Now it is up to the installers to take it from here and acclimate the product to the customer’s home or office living condition to make sure this floor will last for years to come.

Terry Patton is the technical service manager for the National Wood Flooring Association (NWFA) in St. Louis. He can be reached at terry.patton@nwfa.org.

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