Wood processing equipment Technical information
What is Hybrid Kiln Dryer by High-frequency heating?

Hybrid Drying - Heating of Lumber Core by High Frequency and Drying by Steam and Hot Air

■What Is "Good Drying?"

When drying wood, attention should also be given to the maintenance of its strength performance and dimensions without damaging its original characteristics, in addition to the reduction of the moisture content.

Drying methods that damage wood or degrade its quality may deviate from their original goals. It is essential to dry wood carefully and gently according to its properties.

Differences in Drying Quality by Drying Method

Wood with high levels of strength performance and dimensional stability is defined as one with few cracks on its surface and inside, a finished moisture content rate of 15% or lower, small differences in the moisture content between the "center and surface of wood" and the "center and edge"(in the longitudinal direction), or a small total moisture gradient, and little change in color (with the original flavor of wood maintained).

Sugi (240 × 135 × 400 (mm))
Right: Hybrid drying using high-frequency waves and vapor
Left: High-temperature drying for nine days

■Drying Mechanism and Differences in Drying Methods

It takes time to heat wood from the outside, and heating it consistently is difficult. To address these problems, this high-frequency method applies heat to the inside of wood to dry it.

How Drying Works

Hybrid drying is a combination of the advantages of general steam and hot-air drying (external heating) and high-frequency (internal) heating. In the dryer, steam and hot-air drying is conducted at a moderate temperature (approximately 70 to 90°C), while the center core of wood with a high moisture content are selectively heated using high-frequency waves to approximately 100°C.


Distribution of the internal temperatures of wood
(thermographic image)

A thermal gradient between the (external) temperature of hot air with steam and internal temperature of wood generates a significant difference in the pressure between the surface and inside of wood, which presses water in the center outwardly. This facilitates the drying and easily eliminates the moisture gradient, and the wood is dried consistently.

■About Hybrid Drying: "Hybrid Kiln Dryer by High-frequency Heating"

High-performance, steam and hot-air-type, high- and medium-temperature kilin dryer with the latest high-frequency heating machine and incorporated control device

  • ①Stacking of Lumber

    Lumber is stacked by sticker stacking to ensure that steam and hot-air drying can be performed more efficiently.

  • ②Insertion of Electrode Plates

    High-frequency electrode plates are inserted between the stacked lumber to perform high-frequency heating.

  • ③High-frequency Heating of Lumber Core

    High-frequency energy from a high-frequency generator is supplied for each drying kiln and trolley.

  • ④Combined Action of High Frequency and Hot-air with Steam

    The core of the lumber is heated by high frequency, and steam and hot-air drying is performed on the lumber exterior.

  • ⑤Application of High Frequency

    High-frequencies are applied to each stacking group to heat the lumber in rotation.

⑥Adjustment of Amount of High-frequency Power

The lumber is sorted into two to four stacks by estimated initial moisture content,
and the amount of water required for drying is calculated for each stack.

⑦Uniform Finish

To make the final moisture content uniform in all stacks,
the amount of high-frequency power and high-frequency heating time are adjusted for each stack.