Operating a Logosol WDU (Wood Drying Unit) kiln.

We are quite often asked by people researching our WDU and kiln system about the process and operation of system. The instructions included with the WDU are international in nature, so we thought we would elaborate on the process here with a little additional localised info.

The Sauno method (summary)

The Sauno method is a process in two steps, designed to be a time and energy efficient process, drying the timber at relatively low temperatures; therefore minimising cracking and warping of the timber for a high yield.

  1. The first step is the steaming process at 60°-75° depending on the timber species, dimension, and other factors which are discussed later. This steaming process opens the pores of the wood and re-saturates the timber as much as possible to facilitate even drying in the next step.
  2. The second step is the drying process. It happens at a temperature of 40°-50° and the moisture is slowly evaporated from the centre of the timber outward. The risk for drying cracks is in that way reduced to a minimum.

The Sauno process can take anywhere from two weeks depending on various factors and requirements.

Preparation

It is not essential the timber to be dried in the kiln is all the same dimensions, species, or moisture content at the beginning of the process. However, watch out for some timbers with high levels of dark tannins that may discolour lighter timber as the moisture is circulated around the kiln. It might be best to not mix these to avoid staining.

Stacking

Do a little research on stacking timber in small kilns.

  • Short lengths of timber, often called “stickers”, are used to separate each layers of timber and allow adequate air-flow between the layers and around the kiln.
  • Choose a timber for stickers that will not stain the timber being dried. Ensure stickers are thick enough to allow good air-flow between layers. At least 25mm thick. A little thicker stickers can allow a you to get your moisture meter between layers to take moisture level measurements from within the stack at lots of locations.
  • Don’t crowd the kiln by overloading it with too much timber. Allow some space at each end (between the ends of the boards and the end walls) to enable air to circulate up and down the stack. Make sure there is at least 30cm gap between the end of the stack and the WDU.

Do you have a moisture meter?

A wood moisture meter measures the MC (Moisture Content) of the timber. A good moisture meter is invaluable in checking the progress of the drying process and help you understand how your timber is drying: is it drying evenly, is all the timber in the kiln at the same MC level, have you reached your desired MC?

Determine appropriate temperature settings and durations

These somewhat generic curve chart shows the average time for the steaming process and the drying process with different assumptions. A fast increase of the ventilation will shorten the time for the drying process, but it will increase the risk for cracks. Some factors to consider:

  • In cases where it is difficult to calculate the time for the steaming process is it always better to better to add a day or two than to end the steaming process too early.
  • The steaming process will last as long for pre-dried wood as for green wood, because the wood has to be steamed all the way to the centre of the timber in both cases.
  • When drying timber of mixed thicknesses, both the steaming process and the drying process should be calculated for the thickest dimension.
  • You may have purchased a Logosol cabinet or built your own. Regardless of the approach you took, remember your kiln is unique. It will be in a different location and environment than other kiln owners, be different dimensions, may be more or less effectively insulated, and it will be meeting your specific needs. Keep this in mind when determining the appropriate settings and durations, monitor your first few loads closely, take notes, and be prepared to do a bit of process adjustment to get the best results. Be conservative with your first few “learning” loads in case something goes wrong and the load is at risk.
  • As your kiln is unique, the above charts and temp/durations in the process below are good suggestions, but the final settings will be up to your judgement of variables, measurements, and experience developed.

Steaming

  1. Close the kiln, ventilation valves and floor level drain valve. Keep the overflow drain open during the steaming process.
  2. With the vents and drain valve closed, moisture from the timber will circulate around the kiln and any dryer timber will be saturated with moisture from the wetter timber. Add some water to the floor of the kiln to ensure the overall level of moisture in the kiln is high enough to ensure all timber regardless of starting MC level can be fully saturated.
  3. Adjust the thermostat switch to 50°C. and the power on the kiln. Let the kiln work under these conditions for about 24 hours. The temperature in the kiln should now have reached 50-60°C.
  4. You can now change the thermostat step by step with a few degrees until the temperature reaches 70° C or a few degrees above that level. Try to avoid to set the thermostat on a too high level. It can force the thermo fuse to cut out and stop the heating process until the temperature falls to 30° until the aggregate starts heating again.
  5. Let the steaming process go on for about 4 days. Timber with a thickness of 75 mm or more will need 5-8 days. It is important to keep water on the floor during the entire steaming process because otherwise the timber can start cracking. Add water if necessary.
  6. The WDU is thermostatically controlled to the set temperature with the sensor located in the WDU. As airflow can vary between loads and cabinet designs, monitor internal temperature with a thermometer readable from the outside of the cabinet. The sensor of the thermometer should be placed on the short side opposite to the WDU in the cabinet, and at a height of 0.5m above the chamber floor. Make compensatory adjustments to the settings in case the average actual cabinet temp differs from the set temp.

Drying

  1. Open the floor level drain to get rid of the water.
  2. For softwood: Adjust the thermostat to 40 and adjust the thermostat step by step until you reach about 50°C in the kiln
  3. For hardwood. Adjust the thermostat to 30 and adjust the thermostat step by step until you reach about 40°C in the kiln (really dense wood may need even lower temperature to completely avoid cracking). Err on the side of lower temperature if unsure; at least for your initial loads until you develop experience with different species.
  4. Open the two ventilating valves about 5-8 mm. The vent on the WDU is the intake vent, and the vent on the end of the cabinet is the exhaust valve. Check every day that you can feel some damp on the edge of the exhaust valve. If not, close the valve a little. If there is damp around the valve, you will know that the humidity in the kiln is high enough to admit the wood to dry from the interior of the wood. The wood will need at least two or three weeks (as per your determined schedule) to become dry enough to remain stable and reach the qualities that are necessary for perfect furniture. Check with a moisture meter that the moisture ratio at least has come down to 8%, or desired MC for your projects and environment.
  5. When you have achieved the desired MC, you are done, switch the kiln off and allow to cool down before unloading the timber.

Resting

Regardless of the moisture content of the timber at the completion of the drying process, once taken out of the kiln it will be influenced by the MC of the environment and air humidity. It needs to reach a environment balanced moisture ratio.

In humid weather the timber make take up some additional environmental moisture. In dry hot conditions, the MC may drop a little further.

Let the timber rest for a week or two until you think it has become stable in your environment. Confirm this with your moisture meter with some checks over this period. It is good practice to rest the timber indoors in your workshop if you plan on commencing a project with it soon. Or indoors/sheltered area if storing it for later use.

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