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Alternative methods of monitoring the kiln load

Started by jimF, December 24, 2014, 10:49:27 AM

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jimF

Even though most of the folks on this board are either interested in solar, dehumidification or vacuum drying, you may be interested in being aware of what is out there to monitor the kiln load.  With the holiday free time I'll see if there is some interest.  Technical papers for more in depth information are on the internet if you become interested in any one of the following methods. This is too long to digest in one post, so it will be two posts.

Since the goal in drying wood is to achieve a specific moisture content, just about every dry kiln process is monitored by keeping track of the moisture content. This can be accomplished by using many different methods.  Traditionally this has been performed by a person taking out of the kiln samples which are weighed and the moisture content is calculated.  This required planning your daily schedule around going into the kiln, weighing, putting the samples back in and performing the calculations.  The various drying schedules that are followed were based on commercial practice in the fifties and sixties which were not necessarily optimum for each species.  These schedules have been modified since then and while the existing schedules will dry lumber with acceptable quality, there has been no systematic study to find optimum settings for each species or group of species. Optimum is defined here as the fastest schedule which will not develop defects in the dried lumber and will improve the final quality compared to existing schedules.

To overcome the time consuming physical work, a patent was filed to use an electronic load cell to weigh and estimate the moisture content of the  samples insitu or the full stack of lumber. But not until the 1990's was this commercially performed after the initial patent ran out.  This eliminated the physical work but not the less than optimum nature of the various schedules. The traditional schedules have been used and modified for this advanced method, such that there are not steps in the schedule but ramp-ups. This increased the speed at which the drying is accomplished.  Further increases in the rate may be possible but, on what basis would this be accomplished. We'll come back to this point later.

Another method that has been developed to estimate the moisture content uses electrical resistance.  The electrical resistance of wood increases as it dries.  This method uses two "nails" that are driven into the lumber with wires connected to a recorder/computer. While this is used, the problem with this is that the method is not sensitive to moisture changes at the high moisture contents.  And it is at the high moisture contents that is most critical because that is when the defects, checks, are initiated.

Moisture content can be estimated using the dielectric properties of wood similar to the dielectric hand held moisture meters by installing large plates in the kiln.  The application of this method has been more limited than either the load cell or electrical resistance methods.

The next two final methods to estimate moisture content have been researched but have not been developed for commercial use.  The first being Infrared radiation detection.  This measures the intensity of various frequencies of infrared radiation emitted from the wood.  The second uses ultra-sonics. The principle is that moisture content has an influence on the speed of sound. 

These methods center on the effort to measure the moisture content.  The next post will continue talking about monitoring the kiln looking at the process from a different perspective.

jimF

Monitoring the kiln by looking at the process from a different perspective than measuring moisture content is what will follow.

Related to the concept of monitoring the moisture content is monitoring the moisture content loss and following schedules the associated schedules.  This can be accomplished by initially calculating the moisture content using any of the above methods and then calculating the moisture content loss per day or hour.  This method keeps the drying rate constant, with the thought that this would keep the drying stresses constant.Again the schedules were developed using data obtained from current commercial schedules which were not necessarily optimum for each species and no studies have been performed to find optimum schedules. 

Along the line of constant drying rate but avoiding the labor of obtaining moisture content, the Temperature Drop Across the Load (TDAL) was also developed.  This centers on the concept that the thermal energy in the air is conducted into the lumber, used to evaporate the moisture and returned to the air carried by moisture vapor; resulting in a lower temperature.  The more moisture evaporated the more the temperature is reduced as it goes through the lumber stack.  So as the moisture content is reduced, the rate moisture is removed is reduced and the air temperature reduction is less.  When the recorder notices the air temperature difference between the two sides of the stack is less, the schedule is advanced to increase the moisture loss again.    This method is usually used for construction grade lumber.

So far the moisture content has been the topic of concern because the goal is to reach a specific moisture content.  However, while drying, one should not be concerned with the final goal but rather with what will cause drying defects.  And all defects (stain, mold etc.) but one can be avoided by drying fast at the lowest practical temperature.  That exception, checks, dictates to slow down the drying rate.  Realizing this, shouldn't a person monitor the limiting factor during drying, not the final goal?.  Now it is true moisture content indirectly influences drying stresses that cause checks, but in such an indirect why that a person cannot use moisture content to achieve this.  Therefore, other methods have been looked at.

Actually, it is not the moisture content level that causes drying stresses but moisture content gradient or the difference between the surface and the interior of the board.  To this goal, a version of electrical resistance to obtain the moisture content at the surface and in the center of the board has been used, two nails on the surface and two nails in the center.  The previous problem still applies; the critical time is still at the high moisture contents when the resistance method is not accurate.  Also an additional problem, that stress development is not only dependent on the moisture gradient and therefore moisture differential still cannot predict what the stress level is.

It is with the thought of measuring something that indicates the actual stress levels that causes checks the following method has been developed.  Measure something that is a direct response to stress levels.  The same as a bathroom scale pointer spins around actually measures the stretching of a spring as it responses to the weight of a person standing on it. The lumber actually shrinks and stretches in response to the drying stresses contained within it.  This method measures the shrinking and stretching of the board.  In commercial trials this method reduced the drying time by 35-50% and as a consequence improves the quality.  Commercial units have not been produced and the base schedules used were the standard schedule which would still need to be optimized.

There has been a lot of study on the final two methods but they have not been brought to a successful stage to approach a commercial unit.  The first of the two is acoustic emission.  This basically a microphone attached to the lumber to monitor the amount of noise the lumber produces.  Noise is produced during drying when one of two reactions; 1) when the stresses become so high that cracks develop and 2) when the water empties out of each cell and an air bubble develops called cavitation.  It has been difficult to distinguish between the two and to determine how much of the first instance is detrimental.  Equipment noise also causes problems.

The last method of monitoring is associated to acoustic emissions, piezoelectric response to the formation of cracks.  As with any formation of a crack, chemical bonds are broken and when chemical bonds are broken an electrical discharge occurs.  This electrical discharge is the piezoelectric response.  The recording of the number of discharges indicates the degree of cracks that has occurred in the board, similar to acoustic emissions.  And similar to acoustic emissions, determination of how many discharges is beyond the acceptable level is difficult to determine, as well as the difficulty of recording the discharges in a commercial setting.

While most of these methods of monitoring the lumber in a kiln is beyond most of the readers needs I hope you enjoyed learning more about the material and processes we are all interested in.

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