PubDate：2014-06-20 Views：796
Hard broad-leaved wood is usually used for solid wood flooring, and high-quality imported materials are mainly used for medium and high-end solid wood flooring. A more reasonable production method is to first process the logs into semi-finished floor slabs at the place of production, and wax the end of the board to prevent end cracks, and then sell it to the floor processing plant for manual drying. Reprocessed into finished flooring.
Artificial drying is an important part of ensuring the quality of solid wood flooring. The main purpose of drying is to stabilize the shape of the wooden floor. It will no longer shrink, swell and deform in the future use process, and also improve the processing performance, use performance and durability of the solid wood floor. The key to drying quality is that the final moisture content must be slightly lower than the equilibrium moisture content of the use environment. It is about 10%-12% in the south of my country, about 8%-10% in the north, and about 6%-8% in the northwest. It must be dried thoroughly to ensure that it is not warped, cracked, and the drying stress is basically relieved.
Drying quality and drying cost are generally mutually restricted, but if there are reasonable drying equipment, reasonable drying process and correct operation, you can ensure the drying quality while minimizing the investment cost and drying cost. .
As there are many types of wood drying methods and drying equipment, and the products on the market are uneven, this article is hereby written for readers' reference in order to help wood flooring companies purchase drying equipment reasonably.
Commonly used wood drying methods are: steam heating conventional drying, furnace gas heating conventional drying, hot air drying, dehumidification drying, vacuum drying, and the popular conventional drying of hot water and high temperature water circulation heating in recent years. It is necessary to choose a suitable drying method according to the drying characteristics and requirements of solid wood flooring, as well as the scale and conditions of the enterprise.
1.1 Steam heating conventional drying
This method is a traditional drying method that uses saturated steam with a gauge pressure P≤0.4MPa to heat the drying kiln through a heater to circulate the air drying medium to dry the wood. Its advantages are that the process parameters are easy to control, and the drying quality is generally good; the temperature and humidity can be controlled in a wide range, and various materials can be dried; and it can be used for centralized heating, and the application is more common. The disadvantage is that a steam boiler is required, and the investment cost is relatively high; and the latent heat of vaporization of steam is mainly used. The condensed water is often unused due to troublesome recovery, causing waste and increasing costs. It is mainly suitable for enterprises with large production scale or ready-made steam available, especially suitable for dry coniferous wood and soft broad-leaved wood or thin board and other easy-to-dry wood species. For small and medium-sized wood flooring companies, this method is not very applicable because of the need for expensive steam boilers.
1.2 Furnace gas heating conventional drying
This is a conventional drying method for wood that is directly or indirectly heated by burning waste wood. Direct heating furnace gas drying will darken wood and easily catch fire, so indirect heating furnace gas drying is almost no longer used. There are many ways: ①Simple fire-kang type: stove and furnace gas pipes are arranged underground in the fire-kang room, and the wood piles are placed In the fire-kang room, the hot and humid air flow naturally circulates for drying. This is a relatively primitive method of drying, which often burns the wood or causes fire. It is not recommended. ②Small stove oven type: This is a small dryer equipped with a wood waste burning furnace and its furnace gas duct and a simple fan on one side inside a metal shell similar to a container, which can hold 2-5m3 of wood. This method of drying also has potential safety hazards, and it is easy to bake wood or cause fire, and because of its unreasonable air circulation, the drying is uneven. It is not recommended. ③Modern furnace gas indirect heating drying kiln: There is a reasonable design and layout of the furnace gas heating tube group in the kiln, and other configurations are the same as general drying kilns. If it is this kind of kiln designed by experts, safety and reliability are guaranteed. The dedicated combustion furnace of this type of kiln can be set outside the kiln or inside the kiln. The former is relatively safer, but the cost and heat loss are relatively high.
Conventional drying of furnace gas heating does not require a steam boiler and uses wood waste energy, so the investment cost is low, and the drying cost is low. However, because the temperature and humidity are difficult to adjust and control, the drying quality is not easy to guarantee. If the design of the drying kiln is not good, it is easy to dry the wood and cause fire. For medium and high-grade solid wood floors, 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 air inlet and return pipes. The air drying medium is introduced into the hot air furnace outside the kiln through the return air pipe for heating, and then sent back to the kiln by the blower, which is a conventional drying with double air circulation. The shortcomings of this method are low utilization rate of heat energy; high power consumption; uneven distribution of temperature and humidity in the kiln; difficult adjustment of process parameters; slow and uneven drying speed; easy corrosion of return air ducts and blowers; small drying kiln capacity. The output is higher than the cost, so it is not recommended.
1.4 Dehumidification and drying
It is a method of using a dehumidifier (or called a heat pump) to gradually dehumidify the air drying medium in the kiln and heat it appropriately to dry the wood. The dehumidifier can only work in the set temperature range to have the dehumidification effect. The working temperature of ordinary dehumidifying dryer does not exceed 55℃, and the working temperature of medium temperature dehumidifying dryer does not exceed 65℃. The advantage of this method is that there is no pollution when using electric energy; it is easy to operate and the drying quality is generally good. Disadvantages are the high cost of using electrical energy for drying; long drying time and low productivity; the dehumidifier is prone to failure, high maintenance costs, and short service life; there is no humidity control device, which can not eliminate drying stress. Because its drying power mainly relies on reducing the relative humidity of the drying medium rather than increasing the temperature, its effect is more obvious in the high moisture content stage or when drying easy-to-dry tree species, but not obvious in the low moisture content stage or when drying difficult-to-dry tree species. However, using this method to dry easy-to-dry tree species or when the moisture content is high, it is too slow to dry, and sometimes it will cause the wood to grow mildew. It is recommended that wood flooring companies do not use it unless the output is very low and there are special requirements for environmental protection.
1.5 Vacuum drying
The dried wood is placed in a sealed container, and the air in the container is drawn out after preheating, so that the wood is dried under a certain negative pressure. This method dries quickly, and the drying quality is not easy to guarantee; the dryer has a small loading capacity, the equipment is more complicated, and the investment cost is high; electric energy is used, and the drying cost is high. Only for a small amount of difficult-to-dry materials, when the conventional drying time is too long, vacuum drying is feasible. General technical flooring companies suggest not to use it.
1.6 Conventional drying with hot water or high temperature water circulation heating
This method is the first research and development by the author in 1996-1997, and is a technical achievement made 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 atmospheric steam and water boiler matched with the drying kiln, which can burn wood chips, shavings or lump wood waste and other processing residues. Use hot water with t=95-100℃ as the heating medium for circulating heating, and the reasonably designed heating system can make the drying temperature reach 70-75℃. This is the suitable drying temperature range for hard hardwood. The atmospheric steam with P=0-0.04MPa can meet the process requirements of humidity control treatment. It adopts 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. Atmospheric hot water boilers are low in cost, safe and reliable, and easy to operate. The hot water circulation heating not only saves water but also recovers all the waste heat of the hot water, and can simplify the soft water treatment equipment. Using wood waste energy, not only waste utilization, greatly reduces drying costs, but also greatly reduces environmental pollution. In short, this method has the advantages of both steam heating and furnace gas heating, and at the same time abandons the shortcomings of both. It 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 a B-grade wood-burning waste heat pipe hot water boiler. The hot water circulation system operates under a gauge pressure of P=0.3MPa, and controls the water temperature t≤115°C to make the drying temperature in the kiln reach 95°C, which is suitable Dry coniferous and soft hardwood. The key to this method is the pressure-keeping measures of the system, which must ensure that the saturation temperature of the hot water is about 30°C higher than the actual temperature to avoid cavitation in the circulating hot water pump.
