wood drying kiln Manufacturing Supplier

Drying Method

1 、Atmospheric dryness
2 、Chamber dry
3 、Vacuum drying
4、High frequency vacuum drying
5、Natural air drying

With the development of technology, now more and more people are very familiar with vacuum drying system, Its principle is simple and easy to control, mainly consists of a well sealed chamber and a vacuum pumping system but the final performance and dying results are much better than traditional ways, lower water boiling temperature and shorter drying circle, let alone that it well solved the wood defects caused by high temperature.

For the vacuum chamber, it is really important and it directly related to the output of the kiln, a good vacuum chamber should be well sealed, and because there is atmospheric pressure difference, it should be very stable and strong. Now there are mainly two kinds of chamber, one is round and the other is square, Round shape is good to stand the pressure, but it is also easy to see, for the square slabs, the loading percentage will be much less than square one, To meet the standard, now the square chamber is usually made of stainless steel, while for round ones they use carbon steel to lower the cost.

1. What Is Kiln-Dried Wood And What Does It Do?
A conventional kilns is a specialized oven that is used to dry lumber. The wood is placed in the kilns and is heat to a specific temperature for a specific period of time.

Kiln Drying Your Own Wood Kiln-dried lumber can be expensive. Therefore, woodworkers may want to dry their own wood to help assure they are getting the MC they want.

This kiln drying process removes moisture from the Lumber, making it more stable and less prone to warping or cracking When made into furniture.

Of course, the amount of time the lumber needs to be in the kiln will depend on the type of lumber, dimensions, thickness, humidity, and moisture. It also depends on the type of drying kiln you are using.

There are many wood drying methods in the wood industry. Common kilns on the market are Vacuum drying kilns, High-Frequency Vacuum drying kilns, Air drying kilns, Conventional kilns,and Solar drying kilns ect. In the following sections, I’ll explain their differences.

There are three main types of lumber drying kilns on the market: Vacuum drying kilns, High-Frequency Vacuum drying kilns, and Solar drying kilns.

Solar Kilns:
A kiln drying method that uses direct sunlight to heat the drying medium, after which the hot air is concentrated and circulated between the collector and the pile to dry the wood. Solar energy is relatively cheap and is intermittent energy.

Vacuum Drying Kilns:
Vacuum drying of lumber is a method of drying under a vacuum and under conditions lower than atmospheric pressure.

Under vacuum conditions, the boiling point of water is lowered and evaporation is accelerated, thus speeding up the drying of wood at a lower temperature.

The water inside the wood constantly moves towards the surface and evaporates much faster than the water inside the wood moves.

Since it is difficult for air convection heat transfer under vacuum conditions, only heat conduction can be relied on to provide heat energy to the material.

Conventional vacuum drying methods are slow and inefficient in heat transfer, temperature control is difficult, and the heated material also produces internal and external temperature differences. so it still cannot solve the drying of thicker wood.

High-Frequency Vacuum Kiln Drying:
High-frequency vacuum drying is a joint drying method of high-frequency medium heating and vacuum low-temperature drying, which is a more rapid drying method with a lower waste material rate, especially suitable for drying hardwoods and heavy wood.

High-frequency heating is penetrating heating, the moisture or glue in the electric field is evenly heated at the same time, the part with more moisture absorbs more high-frequency electric field, and the place with less moisture only absorbs a small amount of high-frequency electric field, so that the power of high-frequency output is effectively used, and the medium such as wood or fiber does not have polarity, basically does not absorb electric field, so that if the high-frequency power is adjusted to match the object to be heated, the high-frequency Power is basically no waste of use.

High-frequency vacuum drying wood can penetrate even heating, effective energy saving, power off that stops, the electrode is basically not heating, the working environment is greatly improved; fast drying speed; more conducive to thicker objects heating; environmental protection without pollution; reduce labor intensity; kill insects, and sterilization, etc.

Benefits Of Using Wood Drying Kiln
1）Improve The Dimensional Stability Of Wood Products And Prevent Wood From One-Type Cracking And Deformation
Wood exposed to air for a long time will occur wet expansion and dry shrinkage phenomenon, and the uneven wet expansion and dry shrinkage of wood, often cause wood cracking and deformation, affecting its use, and resulting in waste.

If you use wet wood or no good drying wood manufacturing products (such as doors, windows, flooring, furniture, etc.) just do a good job seems to be good, but after a period of time, as the wood dries out will occur door frame skew, floor warping, mortise and tenon release or board cracking and other phenomena, resulting in great losses.

If the production unit is before use, the wood will be dried to the moisture content required for use, you can ensure the stability of the structure of wood products so that the appearance of beautiful, and durable.

2）Reduce The Loss Of Drop, Save Wood Resources, To Prevent Wood Mold, Decay And Insects.
If the logs are not dried in time after sawing or improper drying process, they may make the sawn wood cracking, deformation, discoloration, and other defects, while when the moisture content of the wood is between 20% and 100%, it is easy to produce mold, leading to decay and insect infestation.

Only when the wood is dried to a moisture content of less than 20% or stored in water can these pests and diseases be eliminated. China’s annual loss due to wood deformation, cracking, decay, insect infestation and loss of wood to use (refers to the manufacture of wood products) is very serious.

3）Improve The Mechanical Strength Of Wood, Improve Processing Performance
When the moisture content of wood is lower than the fiber and saturation point, the strength of wood will increase with the reduction of wood moisture content.

For example, when the moisture content of pine is reduced from 30% to 18%, its static flexural strength will increase from 50 MPa to 110 MPa. In addition, drying the wood, can improve the cutting and processing conditions, improve the strength of wood structural parts, gluing strength, and surface decoration quality of wood products.

The thermal conductivity and conductive properties of wood are changed with its moisture content, to improve the thermal insulation and insulation of wood, it is also necessary to reduce the moisture content to reduce the thermal and electrical conductivity.

4）Reduce The Quality And Improve The Transportability

The mass of newly made sawn timber can be reduced by 30% to 50% or more after drying.

Therefore, if the logs can be concentrated in the forest area or near the port of wood import, concentrated sawn timber drying to the transport (20%), and then transported to the user’s area and then dried to the required final moisture content, can reduce freight costs, but also reduce wood cracking, deformation, etc.

① In a drying kiln, wet air driven by a fan absorbs the heat energy of the heat carrier (usually water vapor) in it as it passes through the heater and subsequently transfers the heat energy to the wet wood to be dried as it flows through the pile.

When the wet wood is heated, the surface water vaporizes and is absorbed by the medium when the partial pressure of water vapor in the medium is lower than the partial pressure of water vapor on the surface of the wood.

The higher the temperature of the drying medium, the lower the relative humidity (i.e., the lower the partial pressure of water vapor), and the higher the flow rate (within a certain limit), the faster the removal of water from the wood.

②Higher media flow rate can weaken the influence of the boundary layer on the wood surface and accelerate the heat and moisture transfer process between the media and the wood surface.

③As the moisture is continuously discharged from the surface, a gradient of internal high and external low water content and water vapor partial pressure gradient is gradually established inside the wood.

Under the action of these two gradients, the moisture gradually moves from inside to outside in a liquid or gaseous state or alternates between the two states.

④ In the whole drying process, control the temperature and humidity at each stage according to the predetermined benchmark, and reasonably adjust the relationship between the movement of water in wood from the central region to the surface layer and the speed of evaporation from the surface layer to the medium.

So as to get the suitable drying speed and avoid excessive internal stress in wood, thus avoiding drying defects, achieving the purpose of saving energy and time and ensuring drying quality.

Solid wood furniture refers to pure solid wood furniture, that is, all materials are unprocessed natural materials, and furniture made of no man-made board is not used. Because there are no two exactly the same trees and two exactly the same materials, each product will have its unique characteristics.

The natural characteristics of wood, such as mineral lines, changes in color and texture, needle knots, resin bags, and other natural marks, all make furniture 

more natural and aesthetic. At the same time, the quality of wood is also an important factor to determine the quality of solid wood furniture. What factors will affect the quality of wood?

1.Temperature is the main factor affecting the drying speed of wood. With the increase of temperature, the water pressure in wood increases, and the viscosity of liquid free water decreases, which is conducive to promoting the flow and diffusion of water in wood; At the same time, the moisture solubility of the drying medium is improved and the evaporation rate of water on the wood surface is accelerated. However, it should be noted that if the temperature is too high, it will cause wood cracking and deformation, and reduce mechanical strength and discoloration, which should be propel controlled.

2. Humidity relative humidity is an important factor affecting the drying speed of wood. When the temperature and air velocity are the same, the higher the relative humidity is, the greater the partial pressure of water vapor in the medium is, the less water on the wood surface is easy to evaporate into the medium, and the slower the drying speed.

When the relative humidity is low, the surface moisture evaporates quickly, the surface moisture content decreases, the moisture content gradient increases, the moisture diffusion increases, and the drying speed is fast. However, if the relative humidity is too low, it will cause cracking, honeycomb, and other drying defects, and even aggravate them.

3. Air circulation rate is another factor affecting wood drying rate. High-speed airflow can destroy the saturated vapor boundary layer on the wood surface, so as to improve the heat and mass transfer conditions between the medium and wood and speed up the drying speed.

For difficult to dry wood or when the moisture content of wood is low, the internal moisture movement of wood determines the drying speed; Increasing the flow rate of a large medium to accelerate the evaporation rate of surface water has no practical significance. On the contrary, it will increase the moisture content gradient and increase the risk of drying defects. Therefore, difficult to dry wood and does not need a large medium circulation speed.

The above three factors are external factors that can be artificially controlled. If properly controlled, the drying speed can be accelerated on the premise of ensuring the quality of wood drying.

For example, when drying coniferous wood or soft broad-leaved wood sheet, due to the easy movement of water in the wood, the dry bulb temperature can be appropriately increased, the medium humidity can be reduced, and the air circulation flow rate can be increased to speed up the drying speed;

However, when drying mahogany wood or thick board, lower temperature, higher humidity, and lower air circulation speed should be adopted to avoid drying defects.

4. Wood species and structural characteristics the wood of different species has different structures, and the size and number of pits and the size of micropores on the pit membrane are very different. Therefore, the difficulty of water moving along the above path is different, that is, wood species are the main internal factor affecting the drying speed.

Due to a large amount of filler in the ducts and pits and the small diameter of microscopes on the pit membrane, the drying speed of ring hole hard hardwood (such as acid branch wood) is significantly lower than that of loose hole hardwood; In the same tree species, with the increase of density, the water flow resistance in the large capillary increases, and the water diffusion path in the cell wall is prolonged, which is difficult to dry.

5. The conventional drying process of wood thickness can be approximately regarded as a one-dimensional heat and mass transfer process along the direction of wood thickness. With the increase of thickness, the heat and mass transfer retention distance becomes longer, the resistance increases, and the drying speed decreases significantly

6. Below the fiber saturation point of wood moisture content, with the decrease of moisture content, the transverse diffusion coefficient of absorbed water decreases, while the diffusion coefficient of water vapor in the cell cavity increases. Due to the small proportion of water vapor diffusion in the cell cavity during drying, the lower the moisture content, the longer the water diffusion path, so the lower the moisture content, the more difficult it is to dry.

7. There are more contents in the heartwood cells of hardwood and sapwood, and most of the pits in the heartwood of conifer are blocked, so the heartwood is more difficult to dry than sapwood.

8. The wood ray in the direction of wood texture is conducive to water conduction. The water conduction along the radial direction of wood is about 15% – 20% greater than that along the chord direction. Therefore, the drying speed of chord cut boards is usually faster than that of a radial cut board.

Although the internal causes cannot be controlled, as long as we make good use of drying equipment and technology according to the characteristics of wood, we can also improve the drying speed, reduce unnecessary losses and improve the drying effect on the premise of maintaining the wood properties.