Conventional drying is a method in which normal pressur […]
Conventional drying is a method in which normal pressure humid air is used as the drying medium, steam, hot water, furnace gas or hot oil is used as the heating medium, and the air is indirectly heated, and the air heats the wood by convection to achieve the purpose of drying. In conventional drying, most of the 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 drying medium temperature 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, but high temperature drying is prone to drying defects, darkening of material color, surface hardening, and difficult processing.
Dehumidification drying, like conventional drying, also uses atmospheric 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 pollution to the environment. However, dehumidification drying usually has low temperature, long drying cycle, and high power consumption due to electric heating, which affects its popularization and application.
Solar drying is to use the heat energy of solar radiation to heat the air, and use the hot air to circulate between the collector and the wood pile to dry the wood. Solar drying generally has two types: greenhouse type and collector type. The former integrates the collector and the drying chamber, and the latter separates the collector and the drying chamber. The collector-type solar drying room has a flexible layout, 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 a clean and cheap energy source, it is an intermittent energy source that is greatly affected by the climate. It has a long drying cycle and a large investment per unit volume, so the promotion of solar energy is limited.
Vacuum drying is the drying of wood under atmospheric pressure. The drying medium can be moist air, but most of them are superheated steam. During vacuum drying, the water vapor pressure difference between the inside and outside of the wood increases, which accelerates the speed of moisture migration in the wood, so the drying speed is significantly higher than that of conventional drying, usually 3-7 times faster than conventional drying. At the same time, due to the low boiling point of water in vacuum state, it can achieve high drying rate, short drying cycle and good drying quality at low drying temperature, especially suitable for drying thick hard broad-leaved wood. Because the vacuum drying system is complicated, the investment is large, the power consumption is high, and the vacuum drying capacity is generally small, otherwise it is difficult to maintain the vacuum degree.
Both high-frequency drying and microwave drying 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, and friction occurs between the water molecules to generate heat, so that the wood is 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, a long wavelength, and a deep penetration depth to wood, which is suitable for drying thick wood with large sections. The frequency of microwave drying is higher than that of high frequency (also known as ultra-high frequency) but the wavelength is shorter, and its drying efficiency is faster than that of high frequency, but the penetration depth of wood is not as deep as that of 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, followed by uniform temperature in the wood, low drying stress and good quality. 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 large or the drying process is not properly controlled, which is prone to internal cracking and carbonization. In addition, microwave drying is not ideal for drying wood with large thickness or high moisture content.
Because microwave and high-frequency drying have outstanding advantages in solving the drying of large-section pith core, and microwave and high-frequency drying equipment has been relatively complete, the drying process has gradually matured, and its industrial application is similar to that of vacuum drying. 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 chamber built by the soil method. The advantages are low investment and low drying cost. Its main disadvantage is that the smoke and dust pollutes the environment seriously, is prone to fire, and the drying quality is not easy to guarantee, which is easy to cause losses.
In addition to this, there is the wax cooking process that has caused widespread controversy in the rosewood industry. To be precise, the wax cooking process is a drying process but not exactly a drying process. It is a process technology to stabilize the wood property and prevent cracking in the drying process of wood. Not every kind of wood needs to boil paraffin wax. The paraffin troughs are different, which can dry the wood, but because the thickness and density of the dried wood are different, the required troughs are different, so the cost during use may be high. Some. It is currently rarely used in the field of solid wood, because the technical difficulty is relatively high, and it is still being continuously improved.