" " wisno wood furniture finishing: Kiln dry

Wednesday, April 13, 2011

Kiln dry

Kiln dry or wood oven is facility to dry the wood. It is a basic equipment in the woodworking industry. Now almost all of wood and wood products have to pass the dry kiln before are processed and used in the woodworking industry. The wood has to be dry before go to the production process. The wet wood when it is used will cause of many problems in the wood product resulted (see our previous article: the problem caused since of wet wood) . In the past, the wood drying process may still be done naturally by put the wood on some places and let them dry naturally. The wood is hygroscopic, it will adjust its moisture content according with its environment humidity. By this way the wood will dry if it is put in the outside room, but of course it will take long time. The drying process is also highly depend on the environment condition. In the cold and humid weather the drying process will run slow and it will risk to get problem with fungi or insect. With the kiln dry, the drying of the wood can be better controlled and scheduled to fulfill the woodworking requirement. 

How much is the wood moisture content should be? 

The wood moisture content depends on the humidity conditions in the place where the wood product will be used. The moisture content of a wood product as far as possible has to close to the wood moisture content that balancing with air humidity at the place where the product is used. Then to find the recommended wood moisture content we should see the data of the average humidity of the place where the product will be used. From this data then we can look at the graph of wood moisture content vs. relative humidity to find the proper moisture content of the wood product at the region.
Usually the standard of wood moisture content is set in 10%. Normally the moisture content 10% is quite safe for wood products in most places in the world even though there some places may with extreme weather may need some special wood moisture content. 

The kiln dry 

Kiln dry actually is an enclosed room that is equipped with an air heater and circulation control. The timber from the the plantation or forest are cut to make board in special size and placed in the room where the drying process is done. Usually it takes several week to let the wood dry, depend on the type of the wood and the size of the wood board.

Here are the things that must be considered in the uses of kiln dry.
  • The room temperature
The room temperature setting is the key to control the drying process of the wood. The high ambient temperatures will lower the air humidity, while the low air humidity will speed the water releases from the timber. By adjust the room temperature we can control the drying process of the wood. The drying process of the wood should not be too fast, since the drying of wood will be followed by volume shrinkage. "Too fast volume change" of the wood will potentially damage the wood, such as the wood crack or warp.
The room temperature should be set so that the drying occurs quickly but without causing damage to the timber. Usually the temperature is set at relatively low at the beginning of drying when the wood is still wet. Then when the wood is quite dry, the room temperature can be set in the higher to fasten the drying process. The temperature setting is also should be set depend on the type of wood. The wood with big volume shrinkage usually require slowly drying, while the low volume shrinkage wood can be dried faster.
  •  The air circulation
The air in the room has to be circulated to prevent the air saturation. The saturated air will not be able to receive the moisture released from the wood, and it means that the drying process is stop. To keep the drying process run, the saturated air should be removed from the room and replaced with fresh air. Good air circulation will also help to make the air temperature in the room more uniform to make the drying process can be run by more equitably.
  •  The arrangement of the wood plank.
The wood from the trees are usually cut in board form before go to the oven. In the oven room the board should be well arranged to let the air can blow freely around them. The distance of each pieces wood cannot be too tightly since it can make substandard air circulation and make the drying process is not doing well.

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