Wood drying is the process with the largest energy consumption in the furniture production process. It is also one of the processes with the highest technical content and occupies an important position in the whole wood customization process.
Hotel furniture customization is a high-grade, personalized hotel space customization that has particularly high requirements for quality and craftsmanship. During the production process, drying high-quality imported wood is an important method and process to ensure and improve wood quality, reduce wood consumption, and improve wood utilization. The quality of wood drying technology is the determining factor in the service life of solid wood custom furniture products.
A. Reasons for wood dryness:
Wood is a material with a loose porous structure. These pores can absorb and release water. Wood has moisture absorption and release properties. Untreated wood is extremely unstable in terms of material and performance. In addition, wood has the characteristic of swelling when wet and shrinking when dry. Therefore, it is difficult for wood and its products to achieve natural and regular shapes.
Wood contains a certain amount of moisture. The amount of moisture in wood varies with tree species, age, and season of felling. To ensure the quality and extend the service life of wood and wood products, appropriate measures must be taken to reduce the moisture (moisture content) in the wood to a certain level. To reduce the moisture content of wood, the temperature of the wood must be increased so that the moisture in the wood evaporates and moves outward. In the air with a certain flow speed, the moisture quickly leaves the wood to achieve the purpose of drying.
B. Definition of wood drying:
To ensure the quality of the wood being dried, the humidity of the drying medium (such as the humid air commonly used at present) must also be controlled to obtain the effect of drying the wood quickly and with high quality. This process is called wood drying. Since the above method uses convection heat transfer to dry the wood from the outside, it is also called convection drying. In a nutshell, wood drying is the process by which moisture is removed from wood by evaporation or vaporization.
C. Purpose of drying wood for customized furniture:
1. Prevent parts from deforming and cracking.
When the moisture in the wood is expelled into the air, it will cause the volume of the wood to shrink. If the shrinkage is uneven, the wood will crack or warp. If the wood is dried to a degree that is suitable for the use environment or the state required by the use, the volume and size of the wood can be kept relatively stable and durable.
2. Prevent wood deterioration, decay, and insect infestation, and extend the service life of wood products.
If wet wood is piled in the open air for a long time, it will often rot or be infested by insects if appropriate measures are not taken. When the moisture content of wood is reduced to less than 20%, the damage and damage caused by fungi and pests can be greatly reduced. Therefore, in production units, wood is generally dried to a moisture content of about 8-12%. This not only ensures the inherent properties and strength of the wood but also improves the wood's corrosion resistance.
3. Improve the mechanical strength of wood and wood products and improve the physical properties of wood.
When the moisture content of wood is below the fiber saturation point, the physical and mechanical strength of the wood will increase as it decreases; at the same time, the wood is easy to saw and plane, reducing the loss of woodworking machinery.
4. Reduce the weight of wood and help improve the carrying capacity of vehicles.
The moisture content of newly cut wood even exceeds its weight. After short-term storage and natural drying, its moisture content is still very high. After the wood is dried in a chamber, its weight can be reduced by about 30-50%, which is beneficial to improving the carrying capacity of vehicles.
5. Improve the quality of wood and furniture products
Scientific practice has proven that wood drying is an indispensable process in production. After drying, the size of the parts and product structure of the wood or furniture products after drying are stable, the product quality is guaranteed, and the grade is high; the dried wood can ensure the quality of the wood products, improve the performance of the wood, and extend the service life, thus Save wood raw materials.
D. Methods of drying wood
Wood drying is an important link to ensure and improve wood quality, reduce wood loss, and increase wood moisture content. At present, artificial drying methods include conventional drying, high-temperature drying, dehumidification drying, solar drying, vacuum drying, high-frequency and microwave drying, and flue gas drying. Among all artificial drying methods, conventional drying occupies a dominant position in the wood drying industry at home and abroad due to its long history, mature technology, guaranteed drying quality, and easy implementation of large-scale industrial drying. It accounts for more than 80% of the total in our country.
Conventional drying is a method that uses normal pressure moist air as the drying medium, steam, hot water, furnace gas, or hot oil as the heating medium to indirectly heat the air, and the air heats the wood in a convection manner to achieve the purpose of drying. In conventional drying, most drying chambers use steam as the heat medium, which is generally referred to as steam drying.
The difference between high-temperature drying and conventional drying is that the temperature of the drying medium is higher. The drying medium can be moist air or superheated steam. The advantages of high-temperature drying are fast drying speed, good dimensional stability, and short cycle time. However, high-temperature drying can easily cause drying defects, darken the material color, harden the surface, and make it difficult to process.
Dehumidification drying is the same as conventional drying. It also uses normal-pressure humid air as the drying medium, and air convection heats the wood. It has the advantages of energy saving, good drying quality, and no environmental pollution. However, dehumidification drying usually has a low temperature, a long drying cycle, relies on electric heating, and has high power consumption, which affects its promotion and application.
Solar drying uses the thermal energy of solar radiation to heat the air and uses the hot air to circulate between the collector and the wood pile to dry the wood. There are generally two types of solar drying: greenhouse type and collector type. The former integrates the collector and drying chamber, while the latter arranges the collector and drying chamber separately. The layout of the collector-type solar drying room is flexible, the collector area can be large, and the corresponding drying room capacity is also larger than that of the greenhouse type. Although solar energy is clean and cheap energy, it is an intermittent energy source that is greatly affected by climate. It has a long drying cycle and requires a large investment per unit volume, so promoting solar energy is limited.
Vacuum drying is the drying of wood under atmospheric pressure. The drying medium can be moist air, but most of it is superheated steam. During vacuum drying, the water vapor pressure difference between the inside and outside of the wood increases, which accelerates the migration rate of moisture in the wood. Therefore, the drying speed is significantly higher than conventional drying, usually 3-7 times faster than conventional drying. At the same time, due to the low boiling point of water under vacuum, it can achieve a higher drying rate at a low drying temperature, a short drying cycle, and good drying quality. It is especially suitable for drying thick hard broad-leaf wood. Due to the complexity of the vacuum drying system, large investment, and high power consumption, the vacuum drying capacity is generally relatively small, otherwise, it will be difficult to maintain the vacuum degree.
High-frequency drying and microwave drying both use wet wood as the dielectric. Under the action of the alternating electromagnetic field, the water molecules in the wood rotate at high speed and frequently. Friction occurs between the water molecules and heat is generated, causing the wood to be heated from the inside to the outside at the same time. dry. The characteristics of these two drying methods are fast drying speed, uniform temperature field in the wood, small residual stress, and good drying quality. The difference between high-frequency and microwave drying is that the former has a low frequency, longer wavelength, deeper penetration into the wood, and is suitable for drying thick wood with large cross-sections. The frequency of microwave drying is higher than high frequency (also called ultra-high frequency) but the wavelength is shorter. Its drying efficiency is faster than high frequency, but the penetration depth of wood is not as deep as high-frequency drying.
The advantages of high-frequency and microwave drying are that the drying speed is very fast, usually dozens or even hundreds of times faster than conventional drying. Secondly, the temperature within the wood is uniform, the drying stress is small, and the quality is good. However, the disadvantages of these two drying methods are large investment and high power consumption. At the same time, if the power selection is different, the power is too high or the drying process is improperly controlled, internal cracking and carbonization may easily occur. In addition, microwave drying is not ideal for drying wood with large thicknesses or high moisture content.
Since microwave and high-frequency drying have outstanding advantages in drying large-section pith square timbers, and microwave and high-frequency drying equipment have been relatively complete, the drying process has gradually matured. Its industrial application is similar to that of vacuum drying, and Usually vacuum-microwave, vacuum-high frequency combined drying.
Flue gas drying is the primary stage of conventional furnace gas drying and generally refers to a small drying room built by indigenous methods. The advantages are low investment and low drying cost. Its main disadvantage is that smoke and dust seriously pollute the environment, fires are prone to occur, and the drying quality is not easy to guarantee, and it is easy to cause losses.
In addition, there is the wax boiling process that has caused widespread controversy in the mahogany industry. To be precise, the wax boiling process is a drying process but not entirely a drying process. It is a technology used to stabilize wood properties and prevent cracking during wood drying. Not every kind of wood needs to be boiled with paraffin. Paraffin wax has different grooves, which can dry the wood. However, because the thickness and density of the dried wood are different, the grooves required are different, so the cost during use may be high. Some. It is currently rarely used in the field of solid wood drying because the technology is relatively difficult and is still being improved.
E. Wood drying equipment:
The company's drying is based on the principle of wood drying, using electric heating and humidification, and proceeds through four stages: the preheating stage, drying and dehumidification stage, intermediate treatment drying stage, and final treatment stage. The drying process is from the outside to the inside.
Wood drying principles:
1. Ensure wood drying quality requirements;
2. Consume less energy (save energy and reduce costs);
3. Control the temperature, humidity, and airflow speed of the drying medium;
4. Increase drying speed.
Wood with a thickness of more than 10CM must be processed by powerful microwave drying. The drying principle is permeable drying, which is a drying process from the inside to the outside.
