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How to choose wood drying equipment

Before selecting wood drying equipment, we should first understand the characteristics and applicable scope of various drying methods, then select the corresponding drying methods and types according to the specific conditions of the enterprise itself, and then cooperate with the reasonable drying process, so as to achieve a significant reduction of equipment investment and drying cost on the premise of drying quality.


1. Comparison of drying methods


When choosing the type of drying kiln, we should consider the technical, technological reliability and economic rationality, and according to the tree species, specification and quantity of the dried wood, the requirements of the use of wood on the drying quality and the specific situation of the production unit.


Steam heated wood drying is a traditional and common drying method widely used at home and abroad. Its main advantages are stable technical performance, mature technology, easy adjustment and control of temperature and humidity; centralized heating, drying of various materials, wide range of application; if the drying process is reasonable, correct operation, guaranteed drying quality, large capacity of drying room. The disadvantage is the need for steam boilers. Equipment investment is high, drying cost is moderate or high.


Wood drying method heated by hot water has some advantages of steam drying. Wood processing residue can be used as fuel, and the drying cost is greatly reduced. Due to the large heat capacity of hot water, the air condition in the drying room is stable and easy to control. The main disadvantage is that the outlet temperature of atmospheric hot water is not high (about 95 ℃), and the drying cycle is slightly longer.


The main advantage of wood drying method with indirect heating of furnace gas is that it does not use boiler, and takes the processing residue of factory as energy source, so the drying cost is low. The disadvantages are that the temperature and humidity of the drying medium are not easy to adjust and control, the requirements for operation technology and sense of responsibility are high, and the labor intensity is relatively large. The reliability of this kind of equipment is closely related to the rationality of design. The "soil chamber" without reasonable design and construction is not only unevenly dry, long dry time, no dry quality, but also prone to fire.


The advantage of dehumidification drying method is that it can recover the latent heat of vaporization released by condensation of wet air, that is, it has little or no heat loss of exhaust gas; moreover, it is simple in technology, easy to operate and dry, and generally will not cause wood damage. The disadvantages are low drying temperature and long drying cycle. For thick plates or hard to dry materials, it is not easy to dry completely, and the drying cost is high.


The vacuum drying method puts the wood in a closed drying container, on the one hand, improves the temperature of the wood, on the other hand, reduces the pressure in the container, so that the moisture in the wood starts to vaporize and evaporate at a relatively low temperature, so as to achieve the purpose of drying the wood. The characteristics of this drying method are short drying cycle, good drying quality, high energy consumption, small volume of container, complex control of drying equipment and drying process. It is suitable for hard hardwood thick board with good air permeability or easy to shrink wood.


Microwave drying and high-frequency drying method have a series of advantages, such as internal and external heating at the same time, uniform heating, fast drying speed, short cycle, good drying quality, can maintain the natural color of wood, and is conducive to continuous automatic production. The disadvantages are high cost and imperfect equipment performance. Preliminary practice has proved that it is suitable for precious tree species (Redwood, etc.), timber and hard to dry timber.


Solar drying method is also an energy-saving wood drying method. It uses solar collectors to convert solar energy into heat to heat wood. The main characteristic of this drying method is to save energy, but the drying cycle is long, and it is affected by regional, seasonal and climatic conditions. Therefore, the use of this drying method has certain limitations.


2. Selection of drying equipment


In addition to the analysis of the above factors, the selection of wood drying method and kiln type should also consider the user's own characteristics, i.e. yield factor, dried wood factor, energy situation, equipment reliability and site conditions. In short, the final selection of wood drying method is the result of analysis and trade-off of various factors.


1. If the output factor is large, in order to reduce the equipment investment and the drying cost, in principle, a large capacity drying kiln should be built, so only the conventional steam drying or hot water drying methods are ideal.


For the small and medium-sized drying output, the wood processing residue is used as the energy source to dry in order to reduce the investment and drying cost. Small hot water heating drying, dehumidification or vacuum drying can also be selected according to the specific situation.


The size of the drying kiln should be determined according to the production capacity and the size of the dried wood (mainly the length). Secondly, it is also affected by the type of drying kiln, ventilation mode, chamber loading mode, etc. The size of drying kiln is directly related to the cost of kiln construction. Generally, the span should not be too large. The larger the span is, the greater the strength of the room and frame is required.


Other drying methods have lower thermal efficiency. Especially at the beginning of drying, a large amount of water needs to be removed, so a large amount of heat energy has to be consumed. In the later stage of drying, increasing the drying temperature is the main means to promote the movement of moisture in wood. If the low temperature drying method is used, the effect of drying the wood to a very low final moisture content (8-10%) is poor. At this time, conventional room drying is applicable. In addition, the movement speed of water in wood is also accelerated by the decrease of air pressure around. Therefore, the vacuum drying method can make the wood quickly reach a lower final moisture content.


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3. Energy situation


Wood processing enterprises produce a large number of processing residues every year, which are often used as the fuel of boilers to produce low-cost heat energy. In the case of no steam boiler or enough residual steam for wood drying, it is suitable to choose a wood drying chamber with hot water or indirect heating of furnace gas with wood processing residues as the energy source. The above two drying methods not only remove the waste of the factory, reduce the investment of drying equipment, reduce the energy consumption and drying cost, but also achieve energy self-sufficiency. In areas rich in natural gas and hydropower resources, direct combustion drying method with natural gas as energy source can be selected to dry coniferous wood, or dehumidification drying method can be selected to dry broad-leaved wood.


However, which kind of energy is chosen depends on the specific situation of the place of use. It is often the case that in theory, it is feasible to use one kind of energy, but in fact, there are many specific problems, so only another kind of energy can be used. For example, wood processing enterprises in urban areas are prohibited from discharging pollutants into the atmosphere. Therefore, only power supply can be used, which determines that the plant can only use special drying methods. It is worth mentioning that in the current era of advocating energy conservation and encouraging the development of new energy, it is of great significance to study the application of geothermal and solar energy in wood drying.


4. Reliability of wood drying equipment


The more complex the drying equipment is, the more complex the technology is and the more prone to failure. The reliability of the equipment mainly depends on the manufacturer's technology, equipment configuration and after-sales technical service. The first concern is the performance and quality of the equipment.


Generally speaking, the material of the circulating fan in the drying chamber shall be aluminum alloy die-casting, and its blade shall be symmetrical twisted wing type, with high strength, corrosion resistance and rate. Matching direct connected motors shall be class H insulated high temperature and moisture resistant. The air flow velocity of the circulating fan through the stack is 1.5-2.0m/s. If the air velocity is too low, it will affect the uniformity and drying speed of wood; if the air velocity is too high, it will increase the power consumption and drying cost unnecessarily.


Generally, the heater in the drying room shall be steel aluminum composite fin tube, and the heat dissipation area shall generally reach 2-5m2 / m3 of dried wood. It should be noted that when drying oak and other tree species, the wood will release a lot of corrosive acid substances, so it is particularly important to strengthen the maintenance of drying equipment. No matter which drying equipment is used, its service life is related to the maintenance level.


The loading capacity of drying equipment is also a problem that can not be ignored. The clear space size of the drying room shall be the specified real volume capacity. The ratio of the volume of wood loaded into the drying chamber to the volume of the drying chamber is called the loading factor or the loading factor. It depends on the stacking factor of the pile. The stacking coefficient of timber pile is the ratio of the solid volume of timber to the laminated volume of timber pile.


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