William W. Moschler, Jr. Research Scientist

There is a large secondary wood processing industry in Tennessee and the surrounding states. This industry provides markets for the extensive timber industry in Tennessee and provides a large local value added opportunity for Tennessee timber. Tennessee has a blend of small and large manufacturing firms. Four individual plants in east and middle Tennessee each process and use over 50 million board feet of lumber per year. Memphis has long been known in the forest products industry as "the hardwood capital of the world". There are hundreds of small and medium sized lumber companies and secondary manufacturing firms in Tennessee.

Drying is the most time consuming and expensive part of the manufacture of products from green lumber (secondary processing). In 1987 the National Hardwood Lumber Association (NHLA) rated drying systems, automated kiln control, and improved techniques for drying as the three highest hardwood utilization research priorities. The primary wood used in manufacturing in east and middle Tennessee is oak. Oak is the most difficult and most expensive wood to dry in commercial use. It takes between 30 and 45 days to dry nominal 1" thick white or red oak in a kiln; it takes longer to dry thicker oak lumber. Consequently, drying is of utmost concern to the Tennessee wood utilization industry.

In response to the need for better drying systems by Tennessee lumber producers, a program focused on drying was initiated at UT. A large factor in the development and success of this program was the keen interest of Dr. Robert Little in all aspects of wood drying. Forest products research has received strong support over the last decade from both the Tennessee Agricultural Experiment Station and the College of Agriculture Sciences and Natural Resources. Additional contributions for forest products research at UT have come from interested private donors, from local and national commercial firms, and from TVA. This outside support financed the building of the Mengel Forest Products Laboratory, the acquisition of 2 dry kilns, and the purchase of related equipment and instrumentation. The research undertaken was mostly applied in nature. The intent was that the results be followed by our program as far as possible towards their eventual application to industry. Three projects from this drying research program are discussed in this article.

In current commercial lumber drying practice, the control of a dry kiln is done from a schedule of temperature and humidity for different moisture content stages during the drying cycle. Samples about 2 feet long are taken from the lumber to be dried. These samples are weighed and their moisture content (MC) is determined from sub-samples. The samples are placed in the kiln, in pockets in the lumber stack, and are removed, weighed, and replaced each day. The sample moisture contents are calculated, compared with the "schedule", and the kiln adjusted to new set points if needed.

There are automated, computer based kiln control systems presently on the market. These systems use electrical resistance probes to determine the sample moisture content and are not very good for drying oak. The control probes are inaccurate above 25% MC. Oak takes most of the time in the drying cycle, and is most subject to damage, at moisture contents above 25% MC. The focus of our kiln control project was to develop a system that is accurate over the entire MC range. Some of the significant steps in the development of this system were:

Principle idea behind system: The real breakthrough for the development of our system was the discovery that samples carefully located on the kiln walls give almost identical moisture content loss results as samples located in pockets in the lumber stack. This discovery allowed us to develop a system in which samples taken according to current commercial practice are monitored continuously by load cells permanently attached to the computer control system.

Proof of principle and patent: Tests were conducted in 2 test kilns at UT and in several different kilns at 3 commercial drying facilities. These tests showed that with proper orientation and location, samples mounted in holders on the kiln wall will have the same air flow rates around them, and the same drying rate, as matched samples located in pockets in the lumber stack. These tests were used as the "proof of principle" basis for a successful patent application by The University of Tennessee. A research sponsor was found to pay the equipment and field testing costs for further development of a kiln control system based on this patent.

Prototype and Software Development: A major task in prototype development was the selection of data acquisition hardware that would perform reliably in the corrosive atmosphere inside a kiln and in the industrial environment of the lumber yard. The most time consuming part of the development process was the writing and testing of the extensive software package. The software developed provides data acquisition, kiln control, and real time display of the drying process. Each kiln has a independent control system that is monitored and set by the computer. Up to 15 kilns may be monitored by a single computer.

Initial Testing: The prototype control system was installed on a test kiln in the Mengel Forest Products Laboratory at UT. During the next 2 years this system received extensive testing and modification. Starting at the component manufacturers level, improvements were made in various parts of the system to increase its reliability for commercial use. The results from this project were presented at the 1995 NHLA meeting. The UT kiln control system received the NHLA outstanding research award for 1995.

Field Trials: After completing the development testing on the prototype on the UT test kiln, 2 complete control systems were built for to be used for commercial field trials. The primary purpose of these trials was to provide feed back from commercial users for further development of the system. Secondary purposes of the tests were to provide an evaluation of the configuration and reliability of the hardware and to evaluate the drying rate and quality. In field tests, the weight-based automated control system proved to be compatible with current drying equipment and methods. In direct comparisons drying times were significantly reduced and drying quality was increased.

The idea of conducting research on kiln corrosion at UT was formed when components in the University's experimental kilns began showing severe corrosion. UT researchers visited local kiln operators and contacted kiln manufacturers for advice on how to deal with the corrosion problem in our kiln. Among these sources, we found that there was little knowledge of effective ways to reduce corrosion. Kiln manufacturers did not want to discuss any aspect of corrosion in dry kilns. At UT we began several small scale tests to identify useful coatings to determine the nature of the atmosphere in a kiln that contributes to accelerated corrosion. The results from these initial tests generated interest among kiln owners, operators, and manufacturers.

UT conducted a nationwide survey to try to estimate extent and nature of kiln corrosion and deterioration. This survey was conducted by distributing a questionnaire through 10 regional dry kiln associations covering most of the USA and parts of Canada. In general, the occurrence of significant damage to kilns caused by corrosion was found to be pervasive and wide spread through out the USA and Canada. Over 80% of the 785 firms included in this survey reported significant damage caused by metal corrosion. The highest occurrence of corrosion damage (87%) was reported by firms belonging to Tennessee Valley Dry Kiln Association. Oak, the predominate wood used in secondary manufacturing Tennessee, was found to be highly associated with severe corrosion in kilns.

Because of the wide spread occurrence and economic importance of the corrosion problem, particularly to firms in Tennessee, we increased our efforts to determine effective ways to control corrosion in kilns. Areas that were investigated were the factors that cause corrosion in kilns, coatings to reduce or prevent corrosion, methods of protecting and sealing the kiln insulation, and elements of kiln design that cause corrosion. Experiments were conducted both in the test kilns at UT and on site at several large commercial installations. Effective coatings and other measures for reducing and correcting corrosion problems were identified during this work.

During the final stages of drying lumber, steam is sprayed into the kiln to raise the relative humidity. This process is known as conditioning. Conditioning serves to relieve internal stresses that are produced during kiln drying. For effective conditioning, it is necessary to quickly raise moisture content of the outside shell of the lumber by 3-5% moisture content. To raise the shell MC, the humidity or wet bulb temperature has to be quickly raised in the kiln without increasing the dry bulb temperature. When steam is sprayed into the kiln, the combination of the latent heat from the steam and the heat given off by the absorption of water by the wood causes the dry bulb temperature to overshoot set point, resulting in slow or poor conditioning of the lumber. Some kiln operators find it necessary to open the kiln, allow the lumber to cool, and then re-heat and humidify the kiln, thereby wasting both time and energy.

Researchers at UT were able to successfully adapt water spray systems previously used in green house humidification to replace steam spray for control and conditioning in a kiln. The kiln spray system consists of a high pressure pump that forces the conditioning water through special small diameter orifices to produce a droplet size in the range of 30 microns in diameter. Among development problems addressed were the sizing and location of spray nozzles and the development of effective filtration to reduce clogging of the nozzles. Comparative testing with steam spray showed the water system to be very efficient for kiln humidification and for lumber conditioning The water spray system is attractive to firms who have marginal boiler capacity, often allowing them to increase their kiln capacity without increasing the size of their existing boiler.

UT's patent rights to the kiln control system were sold to a private company. Commercialization and further development of the system was carried out by that company. Currently about 50 systems have been installed at about 20 firms, and the company feels that growth of the application has just started.

The most important result of the corrosion project was that it convinced kiln designers and manufactures to address dry kiln corrosion openly and in a positive manner. The corrosion work eventually lead a nationwide conference at UT dealing with the occurrence, causes, and prevention of dry kiln corrosion . The proceedings from this conference represent the first comprehensive treatment of the dry kiln corrosion problem. Resistance to corrosion is now treated as an important feature in the design and marketing of dry kilns.

The spray system as developed at UT is in production by a kiln manufacturing company. Further development work to improve filtration, to reduce system maintenance, and to optimize performance has been conducted by this firm. The water spray system marketed by this firm has been installed in over 200 dry kilns in at least 4 countries. The application is being expanded from the initial use in hardwood kilns to include high temperature softwood kilns and de-humidification kilns.