In my last two columns, I discussed how a product's characteristics affect how quickly it can be heated in an oven or dryer. Now I'll look at what happens when the load contains a liquid that has to be removed.
I'll begin by looking at the effect of heat on liquids. As the temperature of a liquid increases, so does its vapor pressure; it tends to evaporate faster. An open pan of water heated to 150oF (66oC) will evaporate faster than one left at room temperature. At 212oF (100oC), the water begins to boil, greatly speeding up the process. For good drying results, evaporation has to take place at the surface of the product, not inside it. Consequently, you have to be careful not to push the heating process too hard until the liquid reaches the surface.
Moisture migrates out of the interior of a product sort of like a blotter working in reverse, passing from fiber to fiber or grain to grain in the base material. Decreasing its viscosity and surface tension hastens this migration. When moisture reaches the surface of the product, it begins to evaporate. This process lays first claim on incoming heat, so until the product's surface is nearly dry, its temperature won't increase much above the liquid's evaporation temperature. This way, the liquid protects the solid material from overheating.
How, then, can products be overheated and damaged when they still contain a significant amount of moisture?
The answer is poor moisture distribution. With heat coming to the product from the outside, the skin dries first. Moisture migrates from the interior, where it's highly concentrated, to the surface, where it's less concentrated, until drying is complete. However, if heat enters the product faster than the moisture can return from the interior, the surface temperature will begin to rise, leading to overheating. Several unwanted consequences can occur:
Incomplete Drying.If a product is overheated, an insufficiently dried product may result because it was removed from the dryer based on its surface moisture content. In time, the interior moisture will work its way out to the surface, and if the product is checked then, you may think it has re-absorbed moisture after leaving the dryer. If you wait long enough, it may even air-dry to the required moisture level, but the uniformity of dryness is apt be poor. If some improperly dried products are placed into bulk storage, they may decay or even spontaneously combust.
Trapped Moisture.Due to overheating, some products may form sealed-over surfaces that won't allow the trapped moisture to escape, no matter what you do to the product. This can happen with coatings exposed to heat too quickly. The surface skins over, trapping solvent underneath. With continued heating, some of the solvent vaporizes, forming bubbles which break through the skin, leaving pits and dimples behind. Often, the coating remains soft, gummy and easily marred because it's still in a semi-liquid state underneath.
Damage.Overheating also can result in swollen, cracked or exploded product. The trapped moisture expands and builds up enough pressure to swell or fracture the product.
Destruction.If overheated, the product can discolor or even ignite in the dryer while part of it is still wet.
What Should You Do?If moisture content varies from place to place in the load, investigate ways to expose the load to the incoming heat more uniformly. Perhaps tumbling it, rather than letting it sit on a conveyor belt, will do the trick. If the product is too fragile to take agitation, cannot be exposed to heat more effectively, or if it's uniformly underdried, you've done about all you can, and you have to accept that the product is the limit on the speed of the process -- bringing in heat faster will only make it worse.
It might even be necessary to reduce the rate of heat transfer to the product until the internal moisture flow can catch up. In some cases, a drastic slowdown or complete stop in the heating process may be necessary.