Common wood drying methods and conditions

Solid wood flooring usually uses hard broad-leaved trees, and medium and high-grade solid wood flooring mainly uses high-quality imported materials. A more reasonable production method is to first process the logs into semi-finished products of floor slabs at the place of origin, and perform wax seals on the ends of the boards to prevent end cracking, and then sell them to the floor processing factory for artificial drying. Reprocessed into finished flooring.

Artificial drying is an important part of ensuring the quality of solid wood flooring. The purpose of drying is mainly to stabilize the wooden floor shape. It will not shrink, swell and deform during the future use, and also improve the processing performance, performance and durability of solid wood flooring. The key to the quality of the drying is that the final moisture content must reach an equilibrium moisture content slightly below the environment of use. It is about 10%-12% in the south of China, 8%-10% in the north, and 6%-8% in the northwest, and it must be dry and ensure that the non-warping, cracking and drying stress are basically relieved.

Dry quality and drying cost are generally mutually restricted, but with reasonable drying equipment, reasonable drying process and proper operation, the drying cost can be ensured while minimizing investment and drying costs. .

Due to the variety of wood drying methods and drying equipment, the products sold in the market are mixed. To help the wood flooring enterprises to purchase the drying equipment reasonably, this article is written for the reader's reference.

Commonly used wood drying methods include: steam heating, conventional drying, furnace gas heating, conventional drying, hot air drying, dehumidification drying, vacuum drying, and conventional drying of hot water and high temperature water circulation heating in recent years. The appropriate drying method must be selected according to the drying characteristics and requirements of the solid wood floor, as well as the size and conditions of the enterprise.

1.1 steam heating conventional drying

The method is a conventional drying method for drying wood by using a saturated water vapor having a gauge pressure of P ≤ 0.4 MPa and heating the drying air in the kiln by a heater. The advantage is that the process parameters are easy to control, the drying quality is generally good; the temperature and humidity can be adjusted in a wide range, and various materials can be dried; and the heat can be concentrated and the application is relatively common. The disadvantage is that steam boilers are needed, and the investment cost is high; and the latent heat of vaporization of steam is mainly utilized, and the condensed water is often unutilized, resulting in waste and increased cost due to the trouble of recycling. It is mainly suitable for the production of large-scale or ready-made steam for use, especially for dry softwood and soft hardwood or thin plate. For medium and small wooden flooring companies, this method is not suitable for expensive steam boilers.

1.2 furnace gas heating conventional drying

This is a conventional drying method for wood that is heated directly or indirectly with burning wood. The direct heating of the furnace gas will blacken the wood and easily cause fire. Therefore, there are almost no ways to dry the furnace gas without indirect heating: 1 simple fire type: the stove and the furnace gas pipeline are arranged in the underground of the fire room, and the pile is placed In the fire room, it is naturally circulated and dried by hot and humid air. This is a relatively primitive method of drying the soil, often causing the wood to bake or fire. It is not recommended. 2 small stove oven type: This is a small dryer with a wood waste burning furnace and its furnace gas pipe and simple fan on the side of a metal case similar to a container, which can hold wood 2-5m3. The drying method also has safety hazards, which are easy to roast wood or fire, and the air circulation is unreasonable and the drying is uneven. It is not recommended. 3 Modern furnace gas indirect heating and drying kiln: The kiln has a furnace tube heating tube set with reasonable design and arrangement. The other configuration is the same as that of the general drying kiln. In the case of such a kiln designed by experts, safety and reliability are guaranteed. The special burners of this type of kiln can be located outside the kiln or in the kiln. The former is relatively safer, but the cost and heat loss are relatively high.

Furnace gas heating conventional drying does not require a steam boiler, and uses wood waste energy, low investment costs, and low drying costs. However, it is difficult to adjust the temperature and humidity, and the quality of drying is not easy to guarantee. If the design of the drying kiln is not good, it is also easy to bake wood and fire. For medium and high-grade solid wood flooring, this drying method should be used with caution.

1.3 hot air drying

The hot blast stove for heating is located outside the kiln, and is connected to the drying kiln by inlet and return pipes. The air drying medium is heated by the return air duct to the hot blast furnace outside the kiln, and then sent back to the kiln by the air blower, which is a conventional drying of the air flow double cycle. The shortcomings of this method are low heat utilization rate; large power consumption; uneven distribution of temperature and humidity fields in the kiln, difficult adjustment of process parameters, slow and uneven drying speed; easy return to the return air duct and blower; small capacity of the drying kiln The output is higher than the cost, and it is not recommended.

1.4 Dehumidifying and drying

A method of drying wood by gradually dehumidifying and drying the air drying medium in the kiln with a dehumidifier (or heat pump). The dehumidifier can only work in the set temperature range to have the dehumidification effect. The working temperature of the ordinary dehumidifying dryer does not exceed 55 ° C, and the working temperature of the medium temperature dehumidifying dryer does not exceed 65 ° C. The advantage of this method is that the use of electrical energy is not polluted; it is easy to operate, and the drying quality is generally good. The disadvantage is that the use of electric energy drying has high cost; the drying time is long and the productivity is low; the dehumidifier is prone to failure, the maintenance cost is high, and the service life is short; there is no humidity control device, and the drying stress cannot be eliminated. Since the drying power mainly relies on lowering the relative humidity of the drying medium instead of increasing the temperature, the effect is more obvious in the stage of high water content or when the drying is easy to plant, but the effect is not obvious in the stage of low moisture content or dry and difficult dry species. However, drying dry tree species by this method or when the water content is high, it is too slow to dry, sometimes causing wood mold. Wood flooring companies are advised not to use unless the production is low and there are special requirements for environmental protection.

1.5 vacuum drying

The dried wood is placed in a sealed container, and after preheating, the air in the container is taken out to dry the wood under a certain negative pressure condition. The method is quick to dry and the quality of drying is not easy to guarantee; the load of the dryer is small, the equipment is complicated, the investment cost is high, and the electric energy is used, and the drying cost is high. Only for a small amount of difficult dry materials, vacuum drying is feasible when the conventional drying time is too long. The general floor company recommends not to use it.

1.6 Conventional drying of hot water or high temperature water circulation heating

This method was developed by the author in 1996-1997 and was manufactured and operated by Nanjing Yunsong Drying Equipment Co., Ltd. It has become a very popular wood drying method in China, especially suitable for small and medium-sized enterprises to dry hard hardwood. The hot water circulation heating adopts an improved heat pipe normal pressure steam and water boiler matched with the drying kiln, and can burn the processing residue such as wood chips, shavings or block wood waste. The hot water with t=95-100 °C is used as the heating medium for circulating heating. The reasonably designed heating system can make the drying temperature reach 70-75 °C. This is the proper drying temperature range for hardwoods. The atmospheric pressure steam with P=0-0.04MPa can meet the process requirements of the humidity control treatment. Semi-automatic control of temperature and humidity and automatic detection of moisture content. The hot water has high heat content, stable temperature, easy control of the process, and guaranteed drying quality. The atmospheric pressure hot water boiler has low cost, is safe and reliable, and is easy to operate. Hot water circulation heating not only saves water but also recovers hot water waste heat, and simplifies soft water treatment equipment. The use of wood waste energy, not only waste utilization, greatly reduces drying costs, but also greatly reduces environmental pollution. In short, the method has the advantages of both steam heating and furnace gas heating, and at the same time discards the shortcomings of both, and is the most ideal drying method for solid wood flooring and furniture materials, especially suitable for small and medium-sized enterprises.

The high-temperature water circulation heating adopts the B-class wood-fired waste heat pipe hot water boiler. The hot water circulation system operates under the pressure of gauge pressure P=0.3MPa, and controls the water temperature t≤115°C, which can make the drying temperature in the kiln reach 95°C, suitable for Dry softwood and soft hardwood. The key of this method lies in the pressure-preserving measures of the system. It must be ensured that the saturation temperature of hot water is about 30 °C higher than the actual temperature, so as to avoid cavitation of the circulating hot water pump.

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