Kiln dry is a room for drying wood that equipped with tools and facilities for manipulating the air temperature, air humidity and the air flow. The drying process is fully controlled by arrange the air circulation, humidity and temperature. The wood can be dried in shorter time, without any problem with the environmental conditions such as: humid or wet weather. Almost all of commercial wood is dried in a kiln dry.
Pre drier kiln.
It is the kiln that is used to dry the wet wood to the moisture content about 25%. It can be used to do the pre-drying before the wood go to the kiln dry to achieve the standard low moisture content (moisture content 6% to 10%). The pre-drying process can be done with air drying or sheds drying but the commercial pre-drier requires more complex facility than the air drying. Pre-drier kiln in commercial operations is a large building equipped with: heating tool, fans and vents to keep the ideal conditions for drying process during the whole year. Automatic controls were installed to monitor the temperature of the air. The room temperatures ranges from 75°F to100°F, relative humidity ranged from 60% to 90%.
Pre-drying is used by the woodworking industry to prepare the wood to dry before entering the kiln. Pre-drier kiln will give the maximum benefit to dry the hard wood. Buildings and facilities that is built could be a substantial capital investment, but it will reduce the energy consumption in the kiln dry.
The advantages of pre-drying are:
- Give the brighter color of the wood.
- Give more uniform wood MC.
- Minimize the defects due to the drying process.
- Reduce the drying time in the kin dry.
Dehumidifier kiln dry.
It is a low temperature kiln dry, but capable to dry the wood to the maximum level. Dehumidifier kiln dry is able to dry the wood to the moisture content at around 5%. It operates at relatively low temperatures, around 160°F and small heat energy is used. The drying process will run slower compared with the kiln dry with high temperatures. The dehumidifier kiln dry is using the water condensation (the same principle with the conditioning system) to retrieve of water vapor in the kiln chamber. The explanation can be viewed in this diagram below:
dehumidification kiln dry.
picture is from :http://fennerschool-associated.anu.edu.au/fpt/drying/dehum.drying.html
- The air from the drying chamber is pumped to the evaporator coil on a cold, where the water in the air is condensed.
- The condensed water is removed from the system.
- The heat from the air and water condensation evaporates the refrigerant in the evaporator coil.
- The refrigerant then passed through a compressor where it re-absorbs more heat.
- Refrigerant heat is passed back to the condenser where heat is transferred back and the cool refrigerant is used to condense water from the air.
Vacuum Kiln dry.
Vacuum kiln dry using a low pressure to accelerate the evaporation of water. Vacuum kilns can reduce the drying times than the conventional or dehumidification kilns. A major limitation of the vacuum kilns caused by the chambers sizes. The small chamber limits the kilns capacity. It cannot dry large quantities at one time.
It still heat to speed the drying process, where it quite difficult to be facilitated. Some systems use electric blankets in contact with each piece of lumber, while some use heat coils or microwaves.
All of these systems are much more expensive to run when compared to dehumidification or conventional kilns. Operating costs and initial cost are much higher than the dehumidification kiln and steam kiln dry. The handling costs are much higher because of the smaller loads. The vacuum drying only can be considered when the thicker hardwoods are dried.
Solar dry kiln
The kiln is used the heat energy for drying comes from solar collectors. Depending to the chosen design, humid air can be removed through vents or condensed in the solar collectors at night. Drying times is dependent upon the weather, and thus unpredictable. In hot climates they can degrade lumber due to excessive drying. In colder climates they are unreliable and slow. Solar kilns often use electric fans to circulate air through the lumber and it become expensive due to the long time drying time.
The solar kiln dry may be relatively inexpensive in some tropical area, where the weather is hot with more sunlight is much more available.
Steam kiln dry
It is the facilitation for drying wood is widely used commercially in the woodworking industry. It is a conventional kiln dry that uses the hot air to drying the wood. The air with higher temperature will hold more water. In a steam kiln drying, the steam generating boiler is built to be used to heat the air during the drying process. The fans are used to circulate the air at high speed until 400 f.p.m. The temperature of air drying can reach 180 °F and can be adjusted by adjusting the flow of the hot air. Temperature and humidity can be fully controlled in accordance with the schedule for drying, size, condition and species of wood. Heat to the boiler can be supplied from gas, oil or wood waste-fired.
Although drying of wood products will add value to their products and reduce transportation costs, but the drying operation can be expensive operation in terms of energy used. Many timber mills use the wood waste as fuel for boilers to reduce costs.
High temperature dry kiln (kiln dry with high temperatures)
It is conventional kiln dry which operates at high temperature. It works to dry the wood with a high speed and is widely used for drying the soft wood where appearance is not a problem. The temperatures at about 200 °F to 240 °F and the speed at 800 f.p.m. can be set to the air drier. The ventilation openings are usually closed because the relative humidity control is not important.
In the high temperature kiln, the wood can be dried in large quantities in just few days. Soft wood that are used for interior such as sengon, jabon, or other cheap wood can be dried with this dry kiln. However, only a few hardwood species is be able to be dried in this way.
Stress on the wood
The woods will shrinkage when it dries. Outer surface of the wood will shrink faster than the inside and causing the pressure in the timber. Depreciation will also occur at different rates related to the direction of wood grains. The variation of the shrinkage can create problems in the wood panel such as: warping, checking, splitting, etc.
When the pressure is quite big and can cause damage to the wood, then the operator can do some manipulations to reduce the pressure on the timber. Wetting the wood surface with water in to some species and reduce the air drying temperature can be done to control the drying process. In some cases of hardwood, the drying can occur faster than softwood. In both cases, the surface of the wood will swell slightly, to eliminate the pressure at the wood surfaces. For some uses, such as wood for construction where appearance is not important, then some damage at the surface could be not an issue.