Question: What is the difference between steam and dry heating for pallet heat treatment.
Short answer:
Steam heating was the initial favorite method by the kiln expert and it certainly the fastest heating method (which does not means the best) for penetrating a piece of wood. However, the free market and the industry has found the dry heat to be both the most economical and technical solution for several reasons. Today we have seen no problem using temperatures as high as 250-300F (120-150C). This seems unbelieveable on the surface, but the pallet is only dried by about 3-10%, which is not sufficient to create any damage. The cycle time is very short which also help reduce any potential damage.
Learn more about the difference between direct heating and indirect heating.
Detailed answer:
Steam was initial seen as the solution by many kiln experts and it has many characteristic that makes it the obvious choise. It was speculated that dry heat would probably damage the pallets due to drying, which would not be an issue with direct steam heated.
The key to efficient and fast heat to 56C or 133F is to have a wet bulb temperature significantly higher than this. You can achieve this high wet bulb temperature either by higher humidity or high dry bulb temperature. We found that raising the dry bulb was the most practical. During many actual test runs on pallets we have found that it is no problem using temperatures as high as 250-300F (120-150C) in the kiln without any significant signs of damage to the pallets. This seems unbelieveable on the surface, but the pallet is only dried by about 3-10%, which is not sufficient to create any damage.
Another significant advantage in raising the dry bulb temperature is that a little more drying occours, which help reduce the chance of mold after the heat treating process, which is a big problem with steam heating pallets.
Learn more about the problem on mold on heat treated pallets.
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