|By not harnessing energy effectively, your kiln
can be costing you money.
Editor's note: This is the first of a three part series discussing how to save energy during kiln drying lumber.
Read Part 2.
Read Part 3.
Is your dry kiln wasting energy?
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, it 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 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 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.
The reason this occurs is because 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. In essence, we bring in outside air, add energy/heat and then exhaust a portion of this heated air before it does its work. Having the kiln call for heat while venting at the same time results in a tremendous waste of energy.
Controlling Wasted Energy
One solution would be to alter the controller operation so that it would always shut the heat off any time the vents open. The kiln controller already is trying to lower the wet bulb temperature (energy), so it is redundant to add energy in order 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.
There are options available, including alternate heat/dump cycle (AHDC) for lumber drying. This technique dries lumber with a period of heating, during which no venting occurs, followed by a period of intense venting, or dump cycle, during which there is no heat or spray. The AHDC can be applied to most dry kilns, with little or no modifications.
Editor’s note: This is the first in a series of three articles that focuses on how to avoid wasting energy in the dry kiln process. The articles can be found online at WoodworkingNetwork.com under Technical Articles. Part 2 will be posted at WWN on Monday, November 22, followed by Part 3 on Monday, November 29.
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 [email protected].
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