Characteristics of Timbers Dried Using Kiln Drying and Radio Frequency-Vacuum Drying Systems

Zairul Amin Rabidin,Gan Kee Seng,Mohd Jamil Abdul Wahab,H.F Abdul Amir,P.S Khiew

doi:10.1051/matecconf/201710810001

Zairul Amin Rabidin, Gan Kee Seng + Show 3 more

Open Access

https://doi.org/10.1051/matecconf/201710810001

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Abstract

Heavy hardwoods are difficult-to-dry timbers as they are prone to checking and internal stresses when dried using a conventional kiln drying system. These timbers are usually dried naturally to reach 15% to 19% moisture content with an acceptable defects. Besides long drying time, timbers at these moisture contents are not suitable for indoor applications since they will further dry and causing, for example, jointing and lamination failures. Drying to a lower moisture content could only be achieved in artificial drying kilns such as conventional kiln, dehumidification kiln, solar kiln, radio frequency-vacuum, etc. The objective of this study was to evaluate the characteristics of 30 mm and 50 mm thick kekatong ( Cynometra spp.) timber dried using kiln drying (KD) and radio frequency-vacuum drying (RFV) system. The investigation involved drying time, moisture content (MC) variations between and within boards, drying defects, shrinkage, and drying stress. Drying defects include checks (surface, end, and internal checks) and warping (bowing, cuping, spring, and twisting). The results showed that RFV drying time was reduced to 50% compared to the KD. RFV dried boards demonstrated a more uniform MC between and within boards. Shrinkage in width and thickness, as well as tangential/radial and volumetric shrinkages were substantially less in RFV boards. The amount of cupping, bowing and spring were very low and negligible in all drying runs. There was no twisting observed in all drying methods. The number of stress-free RFV board was higher than KD. With proper procedure, the RFV technology could be used for drying heavy hardwoods which are difficult to dry in conventional kilns due to excessive drying times and degradation.

Highlights

Sawn timbers have to be dried before they can be efficiently processed and used
Each board was cut into 990 mm and 1,700 mm long which was the length required for kiln drying (KD) and radio frequency-vacuum drying (RFV), respectively
The physical properties and drying characteristics of 30 mm and 55 thick kekatong boards dried using KD and RFV system are summarized in Table 2 and Table 3, respectively

Summary

Introduction

Sawn timbers have to be dried before they can be efficiently processed and used. Timber which has not been dried will gradually dry during service. The primary reason for drying of wood is to reduce dimensional changes. Excessive dimensional changes will cause distortion and checking in wood, causing both appearance and structural issues in furniture and other wooden products. To minimize extreme changes in MC, and subsequently restrict the movement of wood in service, wood should be dried to its equilibrium moisture content. Timbers for furniture and other interior woodwork should be dried to a targeted MC of 15 to 19% (depending on the timber species), which can be achieved by air drying. Air drying is the cheapest method of drying timber, it is a long process and the MC could not be reduced to 12%

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Conclusion