Question: When drying logs for firewood what is the optimum moisture content?
Answer: The optimum moisture content for drying logs is below 20%. Logs do not need to have uniform moisture content from the outside through to the centre of the log. There is a big difference when drying logs for firewood as compared to drying wood for the manufacture of joinery products, wooden flooring or furniture etc., In order to avoid degrade in these types of products more care and time must be taken when drying. For example, drying a 2” thick Oak board for joinery products etc., uniformly and without degrade can take five to six weeks. This would not be economical when drying logs for firewood. A log drying program taking 3.5 days would result in the outer layer of the log or first 25mm reaching a moisture content of 12% to 15%. However, the core of the log could still be around 20% to 25% moisture content, but as this is only a small area of the log the average moisture content of the log would be considerably lower than 20%.
Question: Can I use a converted shipping container as a drying kiln for logs?
Answer: A converted shipping container can be used for drying logs but this is not recommended as shipping containers are manufactured from mild steel and the high moisture and condensation within the unit together with the Tannic acid that is released during the drying of some timbers will quickly corrode the container and it will eventually need to be replaced. The average life of a container used for drying timber is around 3 – 4 years and the floor is usually the first area to suffer corrosion. Companies will find that they will have to replace the container several times during the period of the 20 year RHI contract which in turn reduces any monetary benefit gained from the scheme.
Question: Why is there such a difference in fuel consumption between kilns?
Answer: Firstly, it is necessary to understand a little about the moisture carrying capacity of air. A cubic metre of air at 20 deg. centigrade and 100% relative humidity will hold around 14 grams of water. If you were to heat the same cubic metre of air up to 70 deg. centigrade it would be holding around 400 grams of water at 100% relative humidity. It can be seen from these figures that the moisture carrying capacity of air varies greatly dependent upon temperature and relative humidity. Some kilns are poorly designed and the hot air is simply passed into the drying compartment over the logs and the same volume of air is then extracted and or vented. There is no control over the amount of moisture that the air is carrying. Our kilns are manufactured to work on a completely different principle and use an automatic relative humidity / moisture control system. The air in the drying compartment is circulated around the logs until it has achieved the correct relative humidity and is fully moisture laden. Once these conditions are met, the moisture laden air is vented and replaced by fresh dry air and the process is repeated.