Where the lumber drying art becomes a science

Our kilns worked very well when they were new. Now, after a period of time, the results are not as good. We have checked everything and followed the manufacturer's recommendations. What else can we do?

Try calibrating your temperature sensors (either thermocouple or RTD). We have found that, with time, the readings are shifting and this is causing a loss of control of the kiln. Don't put your temperature sensor in an electric kettle to compare it with boiling water. The heating element is causing some form of induction and it's damaging the electronic module reading the temperature. We have designed a calibration oven to do just that. If your software does not allow for calibration, you can try ours, free, and see for yourself if this was the problem.

It could be also non-condensable gases (CO2, O2) trapped in your steam coils which are reducing their heat transfer capacity. The CO2 is corrosive and sometimes will produce a red powder in your steam traps if you have a steel piping network. The CO2 is due to the alkalinity of your water and you have to learn to either live with it or buy a reverse osmosis system to remove it. One way of fighting this problem is to install some vents at the highest location of your steam network, where the CO2 will accumulate and you can trap it.


We have two (2) identical kilns side by side and the one further away from the boiler room always takes more time to dry the same lumber. We have looked at every possibility but we can't explain this phenomenon, other than the steam having to travel a longer distance.


The wet bulb of your kiln that is further away could be the problem. If the water for that bath has to travel through the first kiln it could be arriving at the second kiln too hot to give a proper reading. The water rising by capillarity on the wick does not have time to get in equilibrium with the air in the kiln and the sensor is reading the temperature of the water instead of the wet bulb. The solutions are either to run the water supply on the outside of the first kiln if possible or try our stainless steel wet bulb bath. Our design will keep the bath where the wick is with stagnant water in equilibrium with the surrounding air, independently of the incoming water temperature or flow, and the stainless steel will prevent heat conduction that will warm up the water. We recommend the installation of two (2) wet bulbs per kiln (one on each wall) otherwise, you may have a"sawtooth" (wet bulb line goes up or down after each fan reversal) effect on your wet bulb graph which will impair your drying.

The opposite is also true if you have a wet bulb bath close to the control room and the water is very cold. It will have a tendency to lower the value of the wet bulb and needlessly start the humidification system, thereby stopping the drying process.


We have followed your recommendations of calibrating our sensors and modifying our wet bulb baths, with great results, thank you. What else can we do to improve on our drying?



There are three (3) more things you can try:


  1. Protect against radiation your sensors near the booster coils. The radiation will cause your top coils to close if you are controlling on the leaving air side. If you are on the entering air side, it will close your booster coils. Try using a lighter color which will reflect the radiation and will not conduct heat. We recommend something made of stainless steel.

  2. If from time to time because of false readings, you must cancel certain temperature sensors during the drying process, you may need to improve their positioning. Try our thermocouple or RTD handle, it's easy to install and you will get a proper reading each time for TDAL control.

  3. If you are controlling your kilns from beginning to end with entering or leaving air side, try the following:

    • Warm up your lumber on the entering air side to protect it against large depression when it's colder.

    • Dry it at the average temperature to diminish the effects of fan reversal.

    • Shut down the load on leaving air side to cut back on the heat when the load approaches the final humidity required.


If you are calibrating your sensors, you may want to try a schedule that will operate on TDAL from beginning to end (instead of time-based) and adapt to the conditions of your lumber. That way the schedule will change segments automatically, based on the humidity of the lumber, and improve the drying time and quality of lumber.