Timber Drying - Fundamentals Concepts and Definitions

Introduction

The aim of drying is "the removal of the water (sap) as economically and with as little damage as possible”.

How much water do we have to remove on drying? Well how much is present in timber when it is removed from the forest?

Green Moisture Contents (% oven dry basis)

Species  Heartwood    Sapwood

Yellow Birch         64              68

Am. Beech           58              79

American Elm       92              84

Douglas Fir           40             116

Western Hemlock 93            167

Sitka Spruce          50            131

In the above table note the variations in moisture content between species and between heartwood and sapwood. There will also be a variation with the time of year when felled. The drying “load” on a timber kiln may also vary depending on where and for how long a stack of sawn timber has been stored.

 

Wood Drying: Fundamental Concepts and Definitions

In order to understand and better control the wood drying proces it is necessary to be familiar with a number of concepts and definitions:

Free water

This is the liquid water in the cavities of wood (the “lumens”). The removal or replacement of this water does not cause shrinkage and swelling or the material.

Bound water

This is the water contained intimately within the structure of the cell walls of the wood. It is the removal or replacement of this water which causes shrinkage and swelling of the material

Fibre Saturation Point (FSP)

This is defined as the point at which the cell walls are full of water, but there is no water in the lumen.  In practice it is very difficult to determine the FSP accurately.  Importantly within a piece of wood it is possible that some regions may be above FSP (and so not inclined to shrink) at the same time as others are well below FSP (and so may be shrinking or attempting to shrink).

Equilibrium Moisture Content (EMC)

Wood will always adjust its moisture content so as to be in equilibrium with its surroundings (but not at the same moisture content as it’s surroundings). In fact the “equilibrium moisture content” of wood depends on the “drying potential” of the surrounding air – this is determined by both the “relative humidity” of the air and its temperature. Problems commonly occur when timber adjusts it’s EMC to new conditions! Hence the need to produce products at a moisture content appropriate to the service conditions.

Note:

The percentage Relative Humidity = 100 times the amount of water vapour in a certain volume of air divided by the maximum amount of water vapour the air can hold at that temperature.

Charts exist which enable the EMC to be determined for combinations of relative humidity and temperature.

An understanding of the above fundamental concepts is critical in understanding how and why timber is optimally dried to maximise financial profit.

 

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Contents

Reasons to Dry Timber: An Introduction to Timber Drying

 

Timber Drying - Fundamentals Concepts and Definitions

 

Factors controlling the Drying of Wood

 

The Structures of Softwoods and Hardwoods and their effect on Wood Drying

 

An Introduction to the Air Seasoning of Timber

 

Layout of a Timber Drying Yard

 

Design of Stacks in the Timber Drying Yard

 

Kiln Drying of Timber

 

Types of Kiln Drying Equipment

 

Benefits of Kiln Dried Timber production compared to Air Seasoning Timber

 

Using a Dehumidifier to Dry Wood

 

High Temperature Timber Drying

 

Solar Kilns for Drying Timber

 

Drying Defects in Sawn Timber

 

Case Hardening of Timber

 

Avoiding Case Hardening by Monitoring the Drying of Timber. Also Collapse & Staining of Timber