Selection of wood chip drying equipment for OSB production - mesh belt low temperature wood chip dryer

Introduction

The simple structures of a drum-type single-channel medium/high temperature flue gas particle dryer and a mesh-belt-type low temperature particle dryer for oriented particle board production are introduced. Based on the author's production practice, suggestions for particle dryer selection are put forward.

1 Drum type single channel flue gas dryer

2. Net-belt low-temperature wood shavings dryer

This type of dryer, originally used in other industries, was first introduced in Europe in 2010 by the Italian group IMAL & PAL when they built an oriented strand board (OSB) plant near Milan. The results have been remarkable. For example: 1) Because it uses hot water as the heat source, the wood particles are uncontaminated by smoke and dust, preserving the original wood color; 2) the drying process is low-temperature, compared to flue gas, resulting in relatively low shrinkage; 3) the low-temperature drying process flattens the particles, resulting in a smoother mat during subsequent laying. This is one of the reasons why this line can produce fine-grained OSB without the need to re-lay the larger surface particles onto the core layer; 4) Most crucially, and most importantly, its significant advantage is that, without specialized dust removal equipment, the exhaust gas dust content is ≤10 mg/m³, fully meeting emission standards in some regions of my country. However, its flammability is a perceived drawback. Analyzing the causes of fire, the basic conditions for fire are the presence of combustible materials and an accelerant. If both conditions are met, the temperature must reach the combustible's auto-ignition point or an open flame must be present. Clearly, the high temperature that caused the combustible to reach its auto-ignition point can be eliminated, and the remaining open flame is the sole cause of the fire. There are two main causes of open flames: overt open flames and covert open flames. While overt open flames are relatively easy to prevent, covert open flames are more difficult, as they are often caused by static electricity. Therefore, anti-static measures must be in place when using this type of dryer, as they can lead to serious consequences.

3 Conclusion

As a key process in the production of oriented strand board, particle drying is indispensable. For new production lines, the selection of particle dryers is an important issue for technicians. The author believes that the selection of particle dryers should be considered from the following five aspects:

1) Dryer safety. Fire accidents often occur in this process during the production of oriented strand board (OSB). Therefore, dryers are critical equipment that companies must pay close attention to. We've previously analyzed the fire prevention issues of low-temperature particle dryers. As long as anti-static measures are in place, fires are generally unlikely due to drying temperatures exceeding the wood's auto-ignition point. However, medium- and high-temperature particle dryers can catch fire due to high drying temperatures or the carbonization of fine particles deposited in dead corners within their internal structure.

2) The production capacity of the dryer must be considered with a margin. Most particleboard production lines in my country are generally in a state of overproduction after normal production, and some may overproduce by more than 30%. At this time, it will be found that the drying process is the bottleneck restricting the increase in production capacity. If it is modified, it will waste a lot of manpower, material resources and time.

3) Dryer energy consumption. Particle drying accounts for over 65% of the total energy consumption in OSB production. Calculations show that drying 1,000 kg of wet particles with an initial moisture content of 100% and a temperature of 20°C consumes approximately 3.2 GJ of heat, regardless of the dryer type. Energy savings can only be achieved by reducing dryer resistance and minimizing heat loss from insulation. Calculations show that dry air significantly outperforms rock wool in insulation. Electricity consumption for particle drying, under the same conditions, is primarily determined by the power of the main fan motor. The power rating also depends on system resistance. Higher resistance requires a higher motor power, resulting in higher electricity consumption.

4) The quality of wood shavings drying can be considered from the following three aspects: first, the cleanliness of the wood shavings, because flue gas drying will cause smoke pollution and cause the original color of the wood shavings to change to varying degrees; second, the degree of change in the flatness and warping of the wood shavings after drying. Of course, the smaller the warping, the better; third, the shrinkage rate of the wood shavings, of course, the smaller the better.

5) Dryer reliability is also crucial. If a dryer is unreliable and frequently breaks down, it will impact the normal operation of the entire production line, leading to high maintenance costs, increased product costs, and reduced product competitiveness. Having covered this point in this article, I believe industry colleagues will readily understand the implications.