Q: We had a salesman stop over and claim that our dry kilns were venting too much and causing all sorts of problems, including high energy use. We sent him on his way quickly, but what is the truth about venting?
A: A kiln vent opens only when the interior RH is too high. It stays open until it has exhausted enough air to drop the RH in the kiln a few points. Then it closes. More modern kilns made during the last 25 years or so with proportional controls actually do not open and close periodically, but open only a small amount and then maintain this small opening for many hours. If working properly, the proportional control will select the correct opening and only make small corrections in the opening from time to time to keep the interior kiln humidity at the proper level. This results in rather even or constant venting. Both off-on venting and proportional venting are excellent techniques to maintain the correct RH efficiently in a kiln.
But, here is a problem. In addition to exhausting moisture, the vent also exhausts warm air, which is an energy loss. Vent losses are about 25 percent of the total energy used in drying. (Again, off-on controls or proportional controls result in the same energy loss.)
Several companies have an option to put a heat recovery system on the vents. With most hardwood drying, the amount of heat per hour lost through the vents and therefore recovered is small. In softwoods, which typically dry many times faster than hardwoods, the heat recovery rate is much higher. The heat recovered is low temperature heat; many mills have limited uses for low temperature heat. Perhaps the main use would be for reheating fresh air that comes into the kilns. Although the net energy savings might only be 5 percent of the total energy used in the kiln, for a boiler that is already operating at its capacity or for a system that is short on fuel, this savings might make the difference between using the old boiler for years to come or having to purchase a new boiler.
I have seen several furniture and cabinet plants consider vent heat recovery. In most cases, the economics are not favorable . . . that is, energy is still too cheap. But increases in energy costs or perhaps government incentives to save energy could easily make vent recovery more attractive.
Technical info: Consider that the incoming outside air is 40 F and the kiln is 130 F. The typical venting would be 21 cubic feet of air maximum per minute per MBF of lumber. Using a heat capacity for air of 0.075 Btu per (F x cu ft), the energy required is 140 Btu per minute or 200,000 Btu per day. This sounds like a lot of energy, but a million Btu costs perhaps $5 when using wood waste, so this is about $1 worth of energy per day per MBF lost as heat through the vents. With a 30 MBF kiln running 350 days a year, this is about $10,000. It should be clear that saving half of this energy with an exchanger will be difficult to justify unless the equipment is not expensive, your fuel costs are really high or the price of energy rises.
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