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Dessicant Driven Dehumidifying Kiln?12/14
Reading up on wood-drying kilns and particularly on what appear to be increasingly popular dehumidifying kilns, I'm surprised not to find any information on use of chemical dessicants, eg calcium chloride, silica gel, etc. These dessicants are effective, can "suck up" large amounts of water and are (by the sack) cheap. The main downside I can see is perhaps a difficulty in controlling relative humidity.
1000 BF of oak has around 3000 pounds of water to be evaporated. Check and see how much desiccant is needed for that much water. Also, to achieve 7% MC, we need 30% RH, so you need a desiccant that runs at the low RH...many do not. If you wanted to reuse the desiccant by recharging it, then the cost goes really high. If you want to discard it after one use, then there are environmental issues and the pile of desiccant would be large.
Bottom line...so long as energy is cheap, desiccant a are not viable for wood.
Good points, particularly for larger commercial kilns. For smaller lots, I feel dessicants have a place.
Silica gel can adsorb over 35% of its weight; calcium chloride over 200% And both can be regenerated. Calcium chloride reaches equilibrium at 22% RH at 30 deg C and it's cheap. An entry level "kiln" could probably be put together with little more than a sack of dessicant and an inflated plastic bag. Would this be conducive to a quality product though?
Just a thought.
Doesn't it require energy for recharging? It takes the same amount of energy in a desiccant dryer as without...water still requires the same energy to evaporate. Isn't the energy cost a major cost in drying of lumber? So, what is the advantage of a desiccant compared to just using the energy directly to dry the wood?
You still need to control the humidity during drying throughout the process...for example, with oak, we start at 87% RH and gradually drop the RH and raise the temperature from about 110 F to 160 F (45 to 75 C) over 30 days. Wouldn't this require a more sophisticated control system than we presently use with a hot air kiln and with external air vents? Yet, for a small kiln, wouldn't the cost be prohibited perhaps?
Of course, the advantage of a dehumidifier using a heat pump is the high energy efficiency, even with a small kiln. That is, the latent heat of evaporation is recycled giving a reduction in overall energy exceeding 50%. Heat pumps are indeed attractive, even though they use an expensive form of energy...electricity.
Try it... Go get a bag of desiccant, put it in a plastic garbage bag, put some fresh cut wood in with it... tell us how it works....