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each piece of lumber, while some use heat coils or microwaves. All of these systems are extremely expensive to run when compared to dehumidification or conventional kilns. Operating costs are usually three to four times higher than costs for dehumidification kilns. Initial or capital costs are also much higher, and handling costs are much higher because of the smaller loads. Uneven drying is a problem. When compared on the basis of cost per thousand board feet of annual production, the costs for a vacuum system are much higher than dehumidification. Vacuum drying can often be justified when drying thicker hardwoods. What Is A Dehumidification Kiln? A dehumidification kiln uses a heat pump system to remove the water from lumber. One primary advantage of this type of system is that it recycles heat continuously instead of venting away heated air, as a conventional kiln does. So it is more energy efficient and its operating cost is usually lower. This is true even though a dehumidification kiln uses electric energy to run the fans, the blower that draws the air over the dehumidification coil, and the refrigeration compressor; while a conventional system burns less expensive fuel such as gas or wood. The reason a dehumidification system costs less to run even though a conventional system burns cheaper fuel lies in the dehumidification system’s ability to conserve energy by recycling heat. With the heat being constantly recycled, the amount of electricity demanded by the system is small, so it comes out ahead of a conventional system that may use cheaper fuel, but needs a lot more of that fuel to do the same job. In a dehumidification kiln, heated air, (usually starting at a heat of about 85°F or 29°C) is circulated over the lumber with separate circulating fans, evaporating the water contained in the wood. The hot, moist air then passes over a cold refrigeration coil where air is cooled to about 60°F (15°C). At the cooling coil, the evaporated water in the air condenses into liquid form and flows down the drain as a stream of cool water—instead of as a cloud of steam carried by heated air, as in a conventional kiln. When the air is cooled at the cold coil, the heat removed from the air is immediately used by the system to heat the air back up again. The energy efficiency of the heat return is such that each time this process occurs, the air leaves the dehumidifier at an even hotter temperature than when it entered. As the air temperature in the kiln rises, it can ultimately reach temperatures as high as 160°F (72°C), or even as high as 225oF in Nyle’s new VHT system. If the temperature becomes higher than desired, the operator can vent surplus heat to the outside. Dehumidification kilns are very easy to operate and are very popular with beginning lumber dryers. They are also popular with experienced operators who want a system that requires minimum attention to get zero defect drying. Dehumidification is usually the least expensive to run and to install by a wide margin. Drying times with a Nyle XDH kiln are about the same as conventional kilns. 10PDF Image | INTRODUCTION TO KILN DRYING
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