The Special Situations
In my last column (see link at end of article), I examined how the process heating requirements of a load are determined. Briefly, it consists of calculating how much thermal energy is required by the load and dividing that by the time allowed to heat it. That heat requirement is figured by calculating the energy absorbed to make the load’s temperature rise (weight x specific heat x temperature rise) and adding any energy absorbed if the material goes through a change of state (melting, vaporization or internal structural changes). These changes occur with little or no rise in temperature, so the energy absorbed per unit of material weight is known as the latent (hidden) heat of the change.
As examples of phase changes, I used the melting and boiling of water because they’re well understood and follow the rules, so to speak. The temperature of the ice or water rises until the melting or boiling point is reached. Then, it holds steady as the phase change preferentially absorbs all the incoming energy. Once the phase change is complete, and all the water is in its new state, its temperature will resume increasing if more heat is added.
Not all materials behave exactly like this, however. Changes of state with no temperature increase occur with pure elements and compounds, and many metal alloys, for example, show slight temperature increases as they melt. Alloys are mixtures of two or more metals, and because of that, they melt over a range of temperatures (figure 1) rather than at one fixed temperature. The mechanics of why this happens would take a whole column (or two) to explain, so just trust me on this.
Occasionally, a transformation, once started, will do a turnabout and give off heat. This is most common where a mixture of two or more materials are heated to get them to react chemically. Many of these reactions are exothermic; that is, they liberate heat. The process is akin to pushing a ball up a hill (supplying heat from an outside source) until the ball crests the hill and begins rolling down the other side (exothermic reaction). At that point, many processes shut off the heat source -- the reaction provides enough energy to keep things going.
So, now do you have all you need to know about figuring the heat ovens and furnaces have to provide? Not quite. There are a couple of other points to consider.
First, does the load come completely up to temperature? The obvious answer would be, “Of course! Why do it any other way?” But the truth is, in many processes, it’s not necessary or even desirable for everything to be fully saturated with heat. Take curing a powder coating on a casting, for example. As long as the powder and the surface of the casting it bonds to are up to the proper temperature, you’ll get a good finish. There’s nothing to be gained by heating the interior of the casting where the powder isn’t applied -- all it does is consume extra energy for no real purpose.
Second, in addition to exothermic reactions, there’s another example of the load giving back energy -- processes where the heating cycle consists of raising the product temperature, followed by cooling it while it’s still in the oven or furnace. The process of heat absorption is reversible -- cool steam to 212oF (100oC), and it condenses back to liquid water, returning all that heat it absorbed when it became steam. Continue cooling the water, and each pound of it will release 1 BTU for every 1oF it loses. Freeze the water, and the latent heat of fusion will be returned. In a process heating application, this returned heat may be able to be put to use in the oven, offsetting some of the heat lost out through the insulation or carried out by the conveyor, or best of all, recycled to incoming fresh material, preheating it and reducing the amount of energy that has to be consumed by the oven.
And when you’ve taken all of this into account, it’s time to figure the thermal input to the oven or furnace.