High temperature technology has the potential to reduce current UK drying times for Sitka spruce by up to 80% while maintaining the quality of the end product.

This is the conclusion arising from trials conducted last autumn by Windsor Engineering of New Zealand in collaboration with Italian kiln manufacturer Nardi and the Building Research Establishment (BRE), which provided independent testing and assessment.

The results from these small scale trials are exciting and convincing – to the extent that we are now talking with sawmills about partnership arrangements to test the technology with commercial volumes.

High temperature drying is the main process used for kilning radiata pine in Australasia, but European – and especially UK – Sitka spruce is more delicate, and precise control of the drying process is required to produce a quality product.

Conventional kilns run at between 40-80 degrees C, with UK practice being to keep the temperature below 60 degrees C – taking around 100 hours to dry a charge. The upper limit of 60 degrees C is critical, since at this level the wood becomes prone to twisting, which is the main cause of degrade for spruce.

Using higher temperatures speeds up drying time, but the challenge is how to reach these higher temperatures without the timber becoming damaged as it passes through the 60 degrees C mark. Windsor offers systems that operate at up to 220 degrees C, and overcomes the problem of potential degrade through sophisticated control of temperature and humidity levels inside the kiln.

The trials were carried out according to a protocol designed by BRE and had four key objectives:

  • speeding up the rate of drying;

  • reducing degrade induced by the drying process;

  • reducing the variation in final moisture content;

  • and overcoming some of the inherent stresses that develop during the trees’ growth.

Timber from UK-grown Sitka spruce is different from that from the same species in other European countries and in North America – there are even small variations between different regions of the UK. A number of sawmills offered support for the trials, and in order to test timber from more than one region, packs were sourced from BSW’s Boat of Garten mill and James Jones & Sons‘ Lockerbie mill.

Control packs were dried using conventional kilns in Scotland, while the ‘experimental’ packs were wrapped and shipped to Italy, where they were kept in a cool store prior to drying.

Nardi’s research kiln was modified to simulate Windsor’s high temperature process which, although not allowing all the benefits of the Windsor system to be demonstrated, nonetheless produced results that closely mirrored the quality of the control packs dried using conventional methods while at the same time drying the timber in one quarter of the time.

A variety of drying schedules and a range of dry and wet bulb temperatures were used during the trials, and low-pressure steam was introduced at various times during the process to control humidity and condition the timber.

After drying, the experimental and control packs were shipped to BRE for analysis and comparison.

Controlling the amount of twist in the timber was one of the hardest issues to solve and it was not until the research period came to an end that the correct combination of drying conditions was discovered. Although there was insufficient time to develop this further, there are additional modifications that can be made to the drying process that will produce even better results.

These trials have proved that Windsor’s high temperature technology can produce kiln-dried Sitka spruce of the quality required by the UK market but in less time. This is a first step, but the results have enabled certain comparisons to be drawn that show the Windsor system requires less energy and less space for installation, while increasing productivity and reducing production costs.

The technology won’t suit everyone, but for larger softwood producers it has the potential to deliver increased profitability, improved timber quality and better customer service. The next stage is to test this on a commercial scale.