You're familiar with that old adage, "You can lead a horse to water, but you can't make him drink." It's a folksy way of saying that the recipient of your intentions has to be willing to go along with them, or there's no deal, no matter how attractive your offer.
This applies in process heating, too. Over the years, I've filled a lot of column space telling you how to do a better job of bringing heat to a product, but that's only half the story. The other half is the product's willingness to accept the heat. Some materials are easier to heat than others, and if you factor in the need to remove moisture or solvents from some of them, you've got a complex heat transfer situation. Success in process heating is getting high productivity without sacrificing quality. In short, it means heating the product as quickly as it can take the thermal energy, but not hammering so much into it that you cause temperature damage or uneven drying or curing. If you fail to take this into account, you may be disappointed with the results.
How do you determine where that ideal combination of speed and quality occurs? It depends on the product's thermal properties, so I'll begin by looking at some of those that affect how quickly you can heat, dry or cure a product.
The first is specific heat, which is the amount of thermal energy required to elevate a certain weight of a material through a given temperature rise. In U.S. units, specific heat is expressed in BTU/lb-oF. If you're working in metric units, it is usually calories/g-oC or Joules/g-oC. A material with a high specific heat value must absorb more energy to be heated a certain number of degrees. Wood, for example, has an average specific heat of 0.33 BTU/lb-oF, aluminum, about 0.25. Heating a pound of wood through a certain temperature rise will require 1.32 times as much heat input as the same weight of aluminum. Water requires 1 BTU/lb-oF, a relatively high figure, which helps explain why people sizing ovens and dryers put so much emphasis on getting an accurate estimate of the water content of products to be dried.
The second property isthermal conductivity, which is a measure of how readily heat can flow through a material. It's expressed in BTU-ft/ft2-hr-oF (calories or Joules-m/m2-hr-oC). It's usually represented by the letter k. In plain English, conductivity is the amount of heat energy (BTU) that can pass through a given thickness (ft) of that material in an hour, given a temperature difference (oF) from the hotter side to the colder side. The square feet part of the term takes into account that more heat will pass through a piece with a larger cross-sectional area, just as a four-lane highway can carry a higher traffic flow than a two-lane road.
Most ovens and dryers use convection, radiation or a combination of the two to bring the heat from the source to the work, but at that point, the heat is simply deposited on the product's doorstep. Thermal conductivity is the property that opens the door, welcomes the heat inside, and takes it from room to room.
Its value can sometimes be tough to pin down. For one thing, the thermal conductivity of most materials changes with the material's temperature. In addition, if the material contains air cavities or impurities, its apparent conductivity will be affected by their presence. In effect, you have a conductivity which is an average of the base material and the cavities or impurities it contains. To complicate life further, if the material has a directional structure, like the fibers in a piece of wood, the effective conductivity will depend on the direction the heat is traveling through it.
If a material has high thermal conductivity, you instinctively believe it will be easy to heat. In general, that's true, but only if you can guarantee an adequate supply of heat. Next time[links at bottom of page], I'll look at how conductivity and specific heat sometimes can compete with each other. In doing so, I'll introduce a thermal property you may not have heard of before --thermal diffusivity.