How wood drying with a kiln works

Published Thursday 23rd October 2014

Wood drying is a crucial part of using timber as a construction material, but for many outside the industry the process and rationale behind this are something of a mystery. 

In this guide, we'll look into the drying process and offer an in-depth exploration as to why kilning is such a vital step when it comes to seasoning your wood. 

Wood and Moisture 

Before we delve into the processes used to get the moisture out of wood, it's well worth exploring why we do this in the first place.

Whether you're using wood for a small scale woodworking project or as a heavy duty construction material – the moisture content of your timber can have a profound affect on its performance.

Before, during and after it's in place – your timber will continue to absorb or lose moisture until it reaches the same level as the surrounding environment. This process, also referred to as equilibration, can cause a range of issues – most commonly involving the wood shrinking unequally or becoming damaged if the process occurs too quickly.

The type of timber you're using can also play a part in this process, with hardwoods typically being more difficult to dry due to their decreased permeability. This is one of the reasons why softwoods continue to dominate the commercial wood supply around the world.

Obviously, this is far from ideal and in the worst case scenarios can lead to costly delays on construction projects. To avoid this, there's several ways that the moisture content of your wood can be dramatically decreased before it sees any use.

If drying takes place shortly after felling, it'll also help protect the timber against decay, fungal infections and infestations from certain types of insect. There's also a range of beneficial side effects to the drying process, including (but not limited to):

·         Decreased weight and subsequently lower handling costs

·         Increased strength when compared to fresh timber

·         Better insulation properties

·         Enhanced suitability for machine work, finishes, glues and paints.

Techniques

By drying the wood ahead of time, you'll ensure that any major structural changes (like shrinkage) will take place in the drying process – rather than in-situ. There's two main techniques for carrying out this process, namely:

Air Drying: Air drying simply involves storing your sawn timber in a clean, dry, temperate and shady area, stacking it on raised foundations and separating each level. This allows nature to take its course and gradually, the wood's moisture content will become level with that of the surrounding area.

How quickly this process occurs will depend on several factors, including the climate of the surrounding area and to what extent the stack is exposed to air. The effectiveness of the technique can also be improved by coating the planks with substances like oil or wrapping them in a material that still allows some moisture movement.

Kilning: Kilns are basically a type of oven created within a thermally insulated chamber. They've been in use for thousands of years and are used to tackle a wide range of tasks including cooking, creating cement and smelting ores.

While air drying might do the trick for small-scale jobs and woodworking projects – the overwhelming majority of the world's commercial timber is dried in industrial kilns. The kilning process is superior to air drying in several ways, including:

·         More effective moisture removal

·         Substantially shorter drying times

·         Greater control over degrading

·         Obliterating fungi and insect infestations

·         No seasonal or climactic variations in drying time

How do Kilns Work?

The most common type of kilns are ones that utilise compartments. These are filled with batches of wood and air is circulated until the desired moisture content is reached. As you might expect, these can cater for various species, thickness and quantities of timber. They're also able to safely tackle tough refractory hardwoods, which are susceptible to cracking or splitting during the drying process.

These are made up of four main elements:

Heating: The compartments in a kiln are typically heated by steam heat exchangers and pipes that can be configured in various ways. Alternatively, large flue pipes can be used to pass gas from a furnace directly into the compartment. In some cases, electricity or gas can be employed directly for heating.

Chambers: These have traditionally been built using brick masonry or slabs of hollow cement. However, in modern kilns, materials like aluminium or sheet metal are used in conjunction with thermal insulation like glass wool or polyurethane foam.

Humidification: As the amount of moisture in the air needs to be strictly regulated, steam spray pipes are often used to pump live steam into compartments. Many kilns will also utilise a ventilation system to control the humidity and extract evaporated water from timber that's being processed.

Air Circulation: Air is used both to transfer heat into the compartment and extract evaporated moisture from it. Usually, this is achieved by circulating air via fans or blowers that are fixed inside the chamber or externally.

Getting the Job Done

To ensure timber is dried in the quickest possible time without causing damage or degradation to the load, schedules will be created based on the species, thickness, consistency and intended use of the wood.

As the costs involved in processing timber on an industrial scale can be substantial, an unbelievable amount of research has gone into developing effective schedules for virtually every scenario.

Hi-tech computer programs, measurement tools and auto-shutoff systems are also commonly used to ensure the kiln dries each load at its maximum possible effectiveness.

And You?

Kilns are a complex topic and we've barely scratched the surface with the above. If you've got any more questions about kilning or want to share your interesting kiln facts, be sure to give us a shout on Twitter.

And if you're looking for kilning services or want to investigate other preservation treatments don't hesitate to get in touch today.