For woodworkers who decided to build a wood drying kiln. There is information: principles of building a small wood drying kiln, fans, heaters and boiler for your own kiln, control of the drying process and wood drying kiln controller with a mode of flexible technology.

Wood drying kiln controller

Wood drying kiln controller was created for woodworkers and firms that have built wood drying kilns with their hands.

Automatic control system for wood drying kiln The controller has the following features:
--- controller can work with drying chambers of different size and construction principles with using both a water boiler and a boiler with air-agent
--- controller has a sufficiently low cost
--- work with controller can semi-skilled personnel with no special programming knowledge
--- controller can operate any external devices electromechanical and electromagnetic type
--- easy to connect and maintain

--- Wood drying kiln controller provides:
- measurement of temperature and humidity of air inside the drying chamber
- remote humidity measurement of wood in five points of stacking
- drying of lumber in an automatic mode
- drying of lumber in a semi-automatic mode
- voltages and signals to control the operation of actuators, valves, and other peripherals
- control signals for operate of reversible fans

In automatic mode you specify the kind of wood, thickness of lumber, softness of technology, final moisture content of lumber.
Species of wood - pine, oak, beech, birch, spruce, ash, linden, alder, maple, poplar, hornbeam, sycamore, aspen.
The controller measures the moisture content of wood at different points stacking every two hours, calculates the average moisture content and will maintain the temperature and humidity of air required by technology.
If the actual temperature of air in the drying kiln is different from the required, controller compensates for this lack by changing the air humidity in the kiln by a certain law. Thus, the controller seeks to create optimal conditions for drying on the actual temperature in the drying kiln, when the temperature required by the technology is unattainable.
In semi-automatic mode you make wood moisture measurements at different points of stacking and set the desired temperature and humidity of air in the wood drying kiln.

This version of the controller has been developed and manufactured since 2007. You can easily verify this by going to the Canadian site on the link:
Publication date is at the bottom of the page.

   - The controller (consists of three modules)
   - Two electronic thermometer (dry and wet)
   - A set of cables for remote measurement of moisture content of wood (3 or 5 units)
   - A set of documents on the system and detailed connection instructions
This kit costs 400 $.
Cost of electromechanical actuators $20 - $25 in firms of Ukraine.
Italian technology applied to automatic mode.

Wood drying kiln plan

Drying chamber

    To successfully build a wood drying kiln you must understand the basic principles which you should use for a specific project of your own kiln. For instance, we use the widespread kiln plan with the transverse direction of air movement through lumber stacking. Volume drying lumber - 15m³ (stacking - 2.0m × 2,5m × 6,0m). Principles of building this small wood drying kiln is well suited for very small kilns for a few cubes as well as for larger chambers. For example, you can easily get a small kiln for 9 m³ by resizing the stacking to 2.0m × 2.0m × 4.5m.
Principles of building are especially important if you remake already built premises to the drying kiln for wood.

The width of the lumber stacking largely determines the aerodynamics of the drying kiln, the type and power of used fans and scheme of air humidity control.
For small kilns best suited plan with a narrow stacking.
Stacking height determines the way of kiln loading , and is usually (2.0 - 3.0)m for a small wood drying kilns... more about lumber stacking here

Air humidity in the drying kiln is a very important parameter. It is much more important than the temperature, so the accurate monitoring and the accurate humidity control is very important.
Energy. Remember that one m³ drying green wood you have to spend about 270 kW of energy. It does not matter what way you pass energy to the lumber - boiler, electric heating elements, microwaves, infrared radiation, or something else. In any case it is necessary to transform the moisture from one aggregate state to another.
Wood readily gives away the moisture to 30% moisture when water molecules are in the intercellular space. Therefore, to reduce the cost and time of the drying process in a wood drying kiln is better to pre-dry the lumber in a natural way, and then to dry in the kiln.
The less moisture content of wood, the harder wood gives out a moisture.

Aerodynamics. The main task - to make a plan so that the air moving through the stacking uniformly... more information here

Fans can be axial and centrifugal. An axial fan motor is located in the kiln and must be the tropical version. You can remake the conventional engine in tropical version, if you decide to make their own fans, using suitable impellers.
A centrifugal fan motor is out of the kiln. Centrifugal fans have much more noise than the axis ones. They usually require the use of air ducts...more info here (calculation of the parameters and a choice of the fans)
Plan for small wood drying kiln with centrifugal fan without air ducts here.

Solid fuel boiler with air agent can heat the chamber faster than water boiler but gives greater temperature fluctuations in the kiln. For drying of softwood boiler with air agent is well suited.
If you want to dry and hardwoods, the best is a water heating system or a combined system... more information here

Air humidity control in the wood drying kiln. When you build a drying kiln, pay special attention to humidity control. Air humidity is an important parameter at the process of the wood drying.
Air may move only in one direction during the drying process or periodically change direction to opposite. For the non-reversible direction, the scheme of reducing of air humidity is much easier, cheaper, and has many other advantages for small wood drying kiln.
If you are going to dry only softwoods (pine, etc.), an air humidifier is not a compulsory thing.
If you are going to dry and hardwoods (oak, etc.), an air humidifier is needed... more information here

When building a wood drying kiln try to apply widespread materials. They are readily available and low cost.


For efficient drying of wood does not necessarily support in the kiln is very high temperature. Certainly remarkable, if the drying kiln can maintain a temperature of 70 °, but if your kiln can maintain (50-55) °, do not worry. For most of the drying process will be sufficient
In case of the second method of control, significance of temperature decreases. Pay more attention to controlling the humidity of air in the drying chamber, as the humidity in this case will have more influence on the process of drying wood than the temperature.

Control of wood drying

In order to successfully drying wood is executed, we consider some basic concepts and principles of control of the drying process of wood.

Eq Humidity Equilibrium moisture content of air - is the equilibrium moisture content, which will get the wood, located in this air.
For example, if the humidity of wood 30%, and the equilibrium moisture content of air 15%, the wood will dry to 15% and will stop.
If the humidity of wood 8%, and the equilibrium moisture content of air 15%, the wood will attain humidity 15% and will stop.

Gradient of drying - the ratio of the humidity of wood to the equilibrium moisture content of air.
For example, if the humidity of wood 30%, and the equilibrium moisture content of air 15%, the gradient of drying 2.
Gradient of drying - one of the main parameters that affect the speed and quality of drying.

The graph shows the dependence of equilibrium moisture content of air (15 and 18)% of its temperature and relative humidity.
It is clearly seen that the same equilibrium moisture content of air can be obtained with its various parameters.
For example, the equilibrium moisture content of air 15% can be obtained at a temperature of 65° and humidity 85%, and at a temperature of 40° and humidity 80%.

Now consider two methods of control the process of drying wood.

Method 1.
Based on the chosen technology, the control system of wood drying maintains the desired temperature and humidity of air independently.
Then, for insufficient temperature, gradient of drying decreases and speed of drying slows down, for excess of a temperature, gradient increases and the quality deteriorates.
Even more drastic dependence of the speed and quality of wood drying on the humidity of air in the chamber. Especially at the initial stage, when the humidity of wood is high. Then, even with a slight excess of humidity of air, speed of drying very much slows down, and when there is insufficient humidity of air seriously eroded the quality of the wood.

Method 2.
At the second method control system of the drying chamber besides temperature and relative humidity of air also focuses on the equilibrium moisture content of air in the drying chamber and at a gradient of drying. The parameters of temperature and humidity of air in the chamber are interconnected. The insufficiency of one of the parameters is compensated by another change.

In modern drying chambers to determine the equilibrium moisture content of air using a special thin wood plate, which quickly becomes an equilibrium moisture content. But with no less success and you can use an ordinary psychrometer, determine the equilibrium moisture content on the graphics or tables, published on the Internet.
It is important to understand that the temperature and humidity in the chamber for drying wood are interdependent parameters, complex effect on the drying process.

Practical technologies

During operating the kilns were tested different technologies.
Most woodworkers like low - temperature technologies.
For an example, one of the low - temperature technologies used in the modern drying chambers. Kind of wood - pine. Thickness - 30mm.

Step 1. Heating of lumber.
Heating to a temperature of 50° at 7° per hour.

Step 2. Excerpt.
Exposure at a temperature of 50° for 5 hours .

Step 3. Drying of wood from the initial moisture content to 30% humidity.
Constant temperature 50°. Equilibrium moisture content of air 16%.

Step 4. Drying of wood from humidity 30% to 15% humidity.
Temperature increases in proportion to decrease the wood humidity of 50° to 60 °.
Drying gradient increases in proportion to the moisture of wood from 2,7 to 3,3.

Step 5. Drying of wood from humidity 15% to 8% humidity.
Constant temperature 60°.
Gradient drying constant 3,3.

Step 6. Air conditioning.
Constant temperature 60 °.
Equilibrium moisture content of air 6%.
Conditioning time of 13 hours.

Step 7. Cooling.
Heater - off.
Humidifier - off.

It is not necessary to blindly copy some technology. For example, for pine 30mm nobody carries out a stage of conditioning. When lumber reaches final humidity, stops heating, closes air moisture shutter and begins the process of cooling. If you are well learned the basic concepts of process control of drying wood, you can modify any technology to your needs and the capabilities of your drying kiln.

Get creative when building of the drying kiln.

drying-kiln-001      drying-kiln-002     For instance. Existed a building that we had to remake for wood drying. In this building we have done two drying kilns. The client wanted to be able to use the air flow in the reverse direction. We could not lay pipes for air humidity control upwards (high roof) or through side walls, so we combined pipes and put them through back walls of kilns.
We applied widespread plastic tubes with low cost and everybody was satisfied.

Partnership and collaboration

Anatoly, Chisinau, Republic of Moldova.     Professionally engaged in designing new and upgrading existing wood drying chambers. Chef supervision and consultation at all stages of work. Help with procurement of necessary equipment. Over the past five years of our partnership in the his account for more than fifty drying kilns.

- Vladimir, Chernigov, Ukraine -

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тел. +380-462-675316
Skype: vladimir79001