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Is Your Dry Kiln Wasting ENERGY?

Dennis Clay, a well known lumber drying consultant, has written a three part series concerning the waste of energy in kiln drying. Parts 1 and 2 are printed here with part 3 scheduled for the May issue. He has an interesting way of presenting the alternate heat/dump cycle drying concept, sometimes called “dump cycle drying”,that he is promoting.

By Dennis Clay
Date Posted: 4/1/2012

        Let’s get straight to the point. Energy is expensive and likely only to increase in cost. Even if you use wood waste to fuel your boiler, there is a real cost for the energy produced when you consider the open market value of the wood waste, initial cost and maintenance costs of boilers and associated equipment, water, chemicals, electricity, labor to operate and many other costs. And if the energy source uses fossil fuels (gas, oil, etc.), the energy cost can increase the total cost of drying lumber to the point that it may not be profitable. Do you know what your energy cost is in drying a typical load of lumber and how to decrease this cost? If you answered “no”, then you are missing an opportunity to decrease your lumber drying costs substantially.

        Consider what happens in conventional lumber drying. As the moisture comes out of the lumber, it tends to increase the wet bulb temperature (EMC, RH) inside the kiln, causing the vents to open. When the vents open, both wet bulb and dry bulb temperatures begin to fall, causing the heat valve to open and stay open the entire time the vents are open. Even after the vents close, the heat valve stays open long enough to replace the energy (notice that I did not say “heat”) lost during the vent cycle. Depending on the lumber moisture content and many other factors, this vent cycle will occur several times per hour, each time wasting precious and costly energy. If the steam spray is set to automatic (instead of manually shut) to maintain a specific wet bulb temperature, this vent cycle will occur more often because the controller gets into a vicious cycle of vent/spray, vent/spray. Don’t believe it? Watch your kiln and see this phenomenon for yourself. You will begin to see why it requires so much energy to dry lumber.

        Standard vents on most dry kilns are static, meaning that they are just openings through which the hot (moist?) air from the kiln can escape due to the differential pressure created by the circulating fans. Some kilns have powered vents with their own motors/fans to evacuate moist air from the kiln. However, unless these powered vents are properly designed to reverse direction in conjunction with the circulating fans, they are still grossly inefficient. Why?

        Typically, when the vents open, outside air enters through one set of vents, travels through the heating coils and circulating fans and exits the other sets of vents before going through the lumber packs! What?! We bring in outside air, add energy (heat) and then exhaust a portion of this heated air before it does its work? This is ridiculous. It is also a tremendous waste. Why should the kiln call for heat and venting at the same time?

        One solution would be to alter the controller operation so that it would always shut the heat off any time the vents open. After all, the kiln controller is trying to lower the wet bulb temperature (energy), so why add additional energy when you are trying to remove energy from the kiln? You can save energy, and the kiln can attain a lower wet bulb temperature quicker, if heat and venting do not occur at the same time. Depending on the controller, this is usually an inexpensive, quick fix.

        Obviously, the kiln needs to vent to maintain the required wet bulb temperature – or does it? In conventional drying, the kiln controller is set to a specific dry bulb/wet bulb temperature and held there for a number of hours or days, depending on the specific schedule used, requiring venting and/or steam spray in addition to heat. Are there other options?

        We (the lumber industry) have been drying lumber using the same technique since dry kilns were first designed and installed. It has done the job to a degree, yet kiln operators continue to have drying problems even when they adhere to the best drying practices. Additionally, kiln operation managers are challenged to reduce costs, including energy costs and still produce high quality dried lumber in less time.

        Consider another drying technique, one that does not require the standard frequent venting associated with conventional drying. You may have heard people talk about “Dump Cycle Drying” and may have even read comments dismissing “Dump Cycle Drying” as ineffective, nothing new, etc. Before you miss out on a revolutionary drying technique, talk to one of the many companies that are using this technique very effectively and listen to their enthusiastic endorsements – that is, if they are willing to share a technique that they admit gives them a tremendous competitive advantage. These are the people who make their living by operating dry kilns and drying lumber every day. If it didn’t work, you know that they would not use it and they would be the first ones to tell you that this technique did not work. Instead, once they adopt AHDC (Alternate Heat/Dump Cycle) as their default drying technique, they would never return to conventional drying.

        A little background is in order. A graduate mechanical engineer, I thought I would work as an engineer the rest of my life. However, Fate dealt me an opportunity to work in the lumber department at a large furniture manufacturer where I had previously worked for 8 years in their engineering department, designing machinery and systems to help make furniture manufacturing more efficient. Reluctantly, I accepted the challenge to change my career, eventually becoming the lumber manager with responsibility for 4 predryers, 40 dry kilns and a central lumber yard that received, dried and supplied lumber to all of the manufacturing plants. We routinely dried about 1.5 million board-feet every week, including most species of hardwood lumber plus white pine, 4/4 through 8/4. The production plants needed and expected quality lumber. However, the lumber that they received did not always meet these expectations. Plant managers, rough mill managers, division managers, corporate management – all the way to the CEO – wanted answers, not excuses! And we had to deliver or the plants could not operate profitably.

        We incorporated all the best drying practices outlined in the DRY KILN OPERATOR’S MANUAL in addition to applying wisdom gained from frequent lumber drying seminars and associational meetings. Our drying quality and costs were on par with, or better than, industry standards. Yet there was something missing. We still had problems with quality, our steam usage was too high and we needed to increase our drying production. (An objective and thorough analysis of your lumber drying operation would probably produce the same observations). With the large volume of lumber and dry kilns available for experimenting, we could, and did, try any different kiln schedules and/or techniques that other kiln operators shared with us, plus our own innovative solutions. Some of the experiments produced positive results, but most resulted in either no change or were dismal failures. However, we learned from each trial, especially the failures.

        During these trial runs we observed all aspects of the kilns, including the vents. That’s when we realized that a massive amount of energy was wasted as steam (heat energy) poured into the kiln’s heating coils every time the vents opened. We eventually installed powered vents (Wet Air Venting) on almost all of the kilns, increasing our venting capacity, especially for whitewoods. However, we never developed an effective method to address the energy wasted during conventional venting.



        Editor’s note:  The conclusion of this article will appear in the May 2012 issue of TimberLine, where Dennis will outline the Dump Cycle Drying technique. His conclusion may cause you to reconsider your current conventional drying method.  Please plan to read the May issue.

        Known as “The Kiln Consultant,” Dennis Clay has more than 20 years of hands-on experience drying lumber and managing the drying operations for a large furniture company. As a consultant, he conducts analysis of current lumber handling and drying practices as well as on-site kiln operator training. For information, contact him at or call (828)754-2095.


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