Halving the time it takes to kiln dry softwoods is the promise held by drying techniques used in Australasia and currently under trial in Europe.
Last year, Italian kiln manufacturer Nardi announced a marketing collaboration with Windsor Engineering Group of New Zealand.
Nardi is the world’s largest manufacturer of kilns and its expertise covers drying of softwoods and hardwoods, heat treatment of pallets to ISPM 15 requirements and steaming timber such as beech for aesthetic effect. Windsor gains particularly from being able to offer Nardi’s hardwood drying knowledge and systems to complement its existing services that are primarily aimed at the softwood industry.
For Nardi and its UK/Irish agent Woodtech Machinery, the partnership provides a link to southern hemisphere developments in kiln drying, and in particular to add Windsor’s high temperature drying methods to the available options for customers.
Conventional kilns run at between 40-80oC, with UK practice being to keep the temperature below 60oC – taking around 100 hours to dry a charge of softwood. The upper limit of 60oC is critical, since at this level the wood becomes prone to twisting, which is the main cause of degrade for spruce.
Above 80oC the wood ‘plasticises’, at which point water can be removed with no degrade. Using higher temperatures speeds drying time, but the challenge is how to reach these temperatures without the timber becoming damaged as it passes through the 60oC mark.
Three ranges
Windsor has developed three ranges of kilning systems, operating at 80-110oC (accelerated conventional temperature), 120-160oC (high temperature) and 160-220oC (ultra high temperature) – the latter drying a charge of pine in an eight-hour cycle. The problem of potential degrade at these temperatures is overcome by sophisticated control over the temperature/humidity balance inside the kiln.
Windsor’s method comprises three phases: pre-steaming; drying; and conditioning. Steaming in the kiln is achieved using a low-pressure steam bath rather than spray lines. This ensures that a saturated steam environment is created in the chamber, so that the wood temperature increases without drying taking place.
When hot air comes into contact with the timber during kiln heat-up, any existing degrade such as checking can worsen due to the surface drying before the core. However, if steam is condensed on the hot timber the surface moisture content rises, which lessens the degrade by relieving the stress on the surface. Condensing steam also raises the temperature of the wood, and when this exceeds 80oC plasticisation occurs, which “relaxes” the wood and further reduces stress.
When the wood becomes plasticised it is not susceptible to twisting or checking, and water can be removed without degrade. Once the required dry bulb temperature in the kiln has been reached, the wet bulb temperature depression can be increased – and only at this point does the wood start to dry.
The final conditioning process, for which Windsor manufactures a steam generator, relaxes the stresses caused by drying and removes or reduces degrade – resulting in top quality KD timber in reduced times.
High temperature drying is the main process used for drying radiata pine but, as Woodtech Machinery’s managing director Justin Peckham, said: “Windsor’s experience suggests that the technology could also be used to dry Sitka spruce – even UK-grown Sitka, which is fast-growing, more ‘delicate’ and requires careful drying.
As a result of discussions with major producers and their kiln operators, trials are being set up in Italy, using Windsor’s high temperature technology to dry UK-grown Sitka and determine the time-savings that can be achieved. UK mills are supplying the wood for the trials, which it is hoped will be overseen by BRE.
“We await the results of these trials with great interest,” said Mr Peckham, “but Windsor is confident that it can reduce drying times for UK and European softwoods by 50% or more without increasing the level of degrade. On top of this, Windsor expects that it will be able to dry Sitka more consistently and to lower moisture contents than conventional techniques allow without increasing degrade to unacceptable levels.”
