The extended CDK cycle begins as cold, wet lumber entering the kiln on the near track from the left. As lumber moves through the kiln chambers, it is slowly preheated, heated and dried, and then slowly cooled and conditioned. This happens on both tracks as lumber enters from opposite ends. The wet lumber serves to cool and condition the dried lumber, while the dried lumber serves to preheat the incoming wet lumber. In the middle chamber, lumber is heated and dried on both tracks.
When it comes to adding drying capacity or upgrading existing kilns it is fair to say that CDK (continuous drying kiln) softwood drying technology is now the “norm” in most parts of the Southeastern USA. In this region new capital projects using CDK technology by far outnumber requests for batch kilns in the softwood market. Hardwood producers of course still use the batch kiln process.
The use of CDKs is now spreading to other regions with inquiries in the northwest USA and worldwide – New Zealand, Australia and now one being built for a mill in Canada. Windsor (licensed by Andy Pollard for access to his dual pass IP) and their associates KDS (of Asheville, NC) and Wellons (of Portland OR and Vancouver BC) are together offering CDK systems across the whole of the USA and now also in Canada.
Committed to lead the ongoing development of the CDK technology, the team has claimed a number of “firsts”:
• It was initially thought that the CDK process would not work with indirect steam heated kilns –the challenge was addressed and the first one was built in Bristol FL. There are now a number of these CDKs operating very successfully.
• The first conventional temperature (200oF DB) CDK designed for the specific purpose of drying high quality appearance grade pine was built in New Zealand. It dries Radiata pine and uses high pressure hot water (HPHW) as the heating medium – also a first.
• The first SPF drying CDK system in Canada is currently being manufactured. It is also a first in terms of it being a CDK operating in a “cold climate environment” and it is a direct gas fired system. This CDK system will be operating within a few months.
Unique CDK design aspects are being promoted with emphasis on pushing the boundaries in terms of energy efficiency (i.e. the lowest BTU usage per bdft dried) and drying quality (low MC standard deviation and minimal drying stress). This is done by correctly designing the ER (energy recovery) sections of the CDKs to guarantee the best heat/energy recovery and full equalizing and conditioning of the dried lumber. They have many other features in their design range such as powered vapor removal at the CDK ends, in-CDK and in-line MC monitoring and the pre-heating of wet bulb control air in indirect heated CDKs plus others.
Initially most CDKs involved the conversion of existing batch kilns to CDK systems but now inquiries include almost equal amounts of new and conversion CDKs. It is hard to ignore the energy usage and drying quality gains offered by a correctly designed CDK.