Humidify Your Kiln
FPInnovations – Forintek Division recently completed a study on the use of low-pressure steam and cold water spray during the drying of mountain pine beetle (MPB)-infected lumber. Lumber produced from lodgepole pine logs attacked by the MPB infestation exhibits very low initial moisture content (MC). Depending on the time elapsed since attack; the initial MC can be significantly lower than the fibre saturation point (FSP ~30%). Lumber exhibiting 19% MC or less is considered ready for the dimension lumber market and does not necessarily need to be kiln-dried. However, phytosanitary regulations from the Canadian Food Inspection Agency (CFIA) require that lumber products destined for export be heat-treated (wood core temperature of 56° Celsius (133°F) for 30 minutes) before delivery to customers.
December 2, 2011 By Liping Cai Luiz C. Oliveira and Peter Garrahan – FPInnovations Forintek Division
Heat-treatment options listed in the CFIA guidelines (PI-07, Option D) require minimum kiln residence times of 12 hours for schedules based on dry-bulb temperatures. In many cases, mills using current drying schedules frequently experience over-drying because of longer kiln residence times required to meet these CFIA regulations. Other generic treatment options, such as Options A and B, within PI-07 are shorter but call for wet-bulb temperatures of 60°C (140°F) or higher. In kilns without artificial humidification, it is often impossible to achieve a wet-bulb temperature this high when processing low-initial-MC material. For the significant volume of MPB lumber with an initial MC close to or below 19%, the use of low-pressure steam or cold water spray could allow this lumber to be heat-treated without causing further drying.
Potential benefits from the use of low-pressure steam were confirmed by both laboratory and industrial tests conducted by the Forintek Division. Use of cold water spray was examined through laboratory experiments. To produce the water mist as fine as possible, a pressure of 1,000 psi was used for the pump throughout the experiments in this study. The mill trial was carried out at West Fraser in Quesnel, B.C. (Figure 1) and the laboratory experiments were completed at the Forintek Division’s Vancouver laboratory. Furthermore, Forintek has also developed a model to predict phytosanitary heating time for MPB lumber. The results of the laboratory and industrial tests support the following conclusions:
- Forintek’s heat treating experiments show that the use of low-pressure steam or cold water spray systems reduces the time required for the core of the lumber to reach the required temperature of 56°C (133°F) by 12.7% or 13.9% respectively (Figure 2).
- Kilns equipped with either a low-pressure steam or cold water spray humidification system can meet CFIA requirements for Option A or B (from PI-07).
- Compared to the control runs, warp was reduced when either a low-pressure steam or cold water spray system was used.
- No significant differences on dry/wet-bulb temperature profiles and dried lumber quality were observed between kilns humidified with cold water spray versus low-pressure steam. Thus, a lower cost, cold water spray system can be used to control the relative humidity during heat-treatment.
- A schedule, for material with an initial MC of 20% or less, is presented in Table 1 that meets the requirements of the CFIA’s Option A. As seen in the table, the kiln residence time is reduced to seven hours.
- By using this schedule, the requirements of Option A are met and the kiln residence time is reduced from 12 hours (Option D) to seven hours, which is a reduction of 42%. As a result, the energy consumption can be reduced, kiln productivity is increased, and the grade recovery can be potentially increased due to the reduction of over-drying.
Improved Lumber Quality
This study demonstrated the potential to improve lumber quality since over-drying can certainly be minimized and possibly eliminated. By heat-treating SPF lumber without over-drying, mills can greatly improve grade recovery as a result of the reduction in warp and improved final moisture content uniformity. Assuming a conservative value loss due to over-drying of $10/Mfbm, a typical SPF mill with an annual production of about 250 MMfbm, with 80% of the total volume being MPB lumber, has a potential opportunity to increase revenue by about $1.5 to $2 million per year. In addition to the significant potential benefit due to improved grade recovery, mills will also benefit from increased planer productivity since they will be processing straighter lumber.
When using low-pressure steam or cold water spray for mills, the main challenge is kiln corrosion. The extra steam or moisture can condense on metal surfaces and may increase kiln corrosion. Thus, the following operational procedures are suggested during operation of the low-pressure steam or cold water spray system:
- Check all steam/water systems to ensure that they have drains to the outside.
- Locate and eliminate cool spots that can cause condensation in kilns.
- Unblock all floor drains.
- When cold water spray is used, it is suggested to supply only the amount of water that is required for obtaining the expected relative humidity. To assist kiln operators in using a cold water spray system properly, a computer program to estimate the volume of water required during drying has been developed by Forintek.
- Consider applying a coating to all steel and iron components in the kiln.
Another challenge for the use of cold water spray system is nozzle clogging. The following is recommend for any operation using cold water spray:
- High-pressure systems, operating at 800 to 1,000 psi create a finer mist that is more readily absorbed by the air.
- Frequently inspect nozzles. Open plugged spray holes or replace damaged nozzles.
- Depending on the source and quality of the water available, some pre-conditioning might be necessary to prevent the nozzles from clogging.
- Regularly examine the valves that control the flow of water into the spray line.
- Make sure that the spray nozzles are directed toward an open area in the plenum to avoid droplets forming and water accumulating on walls or other kiln components.
If using steam to humidify, a low-pressure steam supply of 10 to 15 psi is recommended. Steam at higher pressure has a higher temperature and tends to push the dry-bulb temperature up at the same time as raising the wet bulb. If a low-pressure steam supply is not available, a stand-alone, gas or oil-fired, or even electrically heated steam generator, can be installed. There are also steam generators that will take the heat from high-pressure steam or hot oil through a heat exchanger to produce live steam.
This study showed the benefits associated with supplying artificial humidification during the heat treatment of low-MC material; however, this equipment has other potential applications. The ability to positively control the humidity in the kiln provides the kiln operator more options in scheduling. In particular, it can provide the ability to equalize and condition lumber, which is often desirable when dealing with certain value-added products.
The results of this research constitute a significant contribution of Forintek’s overall strategy to assist Canadian sawmills in their efforts to maximize value recovery from the MPB-infected wood resource. Please feel free to contact us for further details on this or other work related to the processing of MPB affected material. Special thanks to the staff and management at West Fraser’s Quesnel mill for their co-operation, their assistance, and use of their facilities.
Contact info for authors: Peter Garrahan, FPInnovations – Forintek, Ottawa, Ont. Ph. 613-523-1232. E-mail: firstname.lastname@example.org; Liping Cai and Luiz Oliveira, FPInnovations – Vancouver, B.C. Ph. 604-224-3221. E-mail: email@example.com and firstname.lastname@example.org
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