Investigation of vacuum and steam treatments to heat treat and sanitize ash logs and ash firewood

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Virginia Tech. Department of Wood Science and Forest Products


The goal of this project was to investigate the efficacy of vacuum/steam technology to sanitize low quality ash logs and ash firewood. It is difficult to heat treat the saw logs and firewood because of the relatively large cross sectional dimension and its irregularity. The combination of steam condensation and vacuum technology is one of most effective heat transfer mechanism because the steam carries large amount of heat and the condensation is fast to release the energy to the wood. Also, under the pressure difference, steam can migrate quickly into the wood. The vacuum/steam system consists of a vacuum source (vacuum pump), controlling device, flexible container and a steam generator. At low temperature, steam is created using a hot water heater rather than a boiler. The treatment system creates a vacuum in the container and at the same time produces the saturated steam. Monitoring devices were attached to the equipment to record and control the process. The white ash logs and firewood were harvested in the Montgomery county, Virginia. Ash log diameters ranged from 6.5 to 11 inches on the small end. The logs were cut into 6 foot lengths. They were treated to determine the time and energy consumption. After vacuum was drawn to 300 or 500 mmHg inside the container, steam was injected into the container. The steaming continued until 56°C was reached at the center of the logs. A total fifteen logs were treated to document the treating times. The treating time for all the logs varied from 5.5 to 14.5 hours which includes a vacuum and holding time of 30 minutes. The six feet logs were cut into 16 inches, plus or minus 2 inches bolts and then split into firewood, rarely larger than 6 inches on the wider side. The treating time for firewood varied from 80 to 137 minutes which includes vacuum and a holding time at temperature of 30 minutes. Energy consumed was about 0.154 to 0.309 kwh to treat one pound of log and 0.111 to 0.219 kwh to treat one pound of firewood using this process. Steam and vacuum can be used to efficiently heat treat ash firewood and firewood logs to kill wood boring forest pests.