Convection: A Series on the Principles of Industrial Heat Transfer
Convection is the transfer of energy via a moving fluid, and by fluid, I mean a liquid, vapor or gas.
It seems incredible that in seven-and-a-half years of writing this column, I've never gotten into a discussion of convection heat transfer. After all, it is the way most ovens and dryers heat the products they contain. Well, better late than never ...
First, the basics. If you take any course in heat transfer, the first concept to get hammered into your noggin is that there are three modes of heat transfer: conduction, radiation and convection. Conduction deals with heat transfer through solid objects or between objects in contact with one another. Radiation is energy transfer via electromagnetic waves. When the waves come into contact with a material, their energy is converted into heat.
Convection is the transfer of energy via a moving fluid, and by fluid, I mean a liquid, vapor or gas. The fluid can heat or cool the surface it comes into contact with, depending on its temperature relative to the surface. I'll limit this discussion to the heating of solid objects by moving air or gases, which is how most ovens work.
The basic relationship describing convection heat transfer is:
Q = hc x A x ΔT
Q represents the amount of heat transferred per unit time (BTU/hr or equivalent units)
A is the area of the surface in contact with the fluid
ΔT is the temperature difference between the fluid and the surface
hc is the convection transfer coefficient
This coefficient takes into account all the other variables in the process, and there are a lot of them. Some of the key ones are the velocity of the fluid and its viscosity, specific heat, specific weight and thermal conductivity. They affect the turbulence generated at the solid surface and the amount of heat the fluid can carry to it. Geometry -- the shape and size of the object to be heated, its orientation to the moving fluid, and the distance from adjacent objects and surfaces -- has a powerful effect on the value of the coefficient and, consequently, the heat transfer rate.
Convection heat transfer comes in two styles -- natural and forced. When fluids are heated, they become buoyant and tend to rise, just like hot air balloons. If they cool, gravity gets the upper hand and they tend to sink. Natural convection takes advantage of this buoyancy effect to create flow circulation patterns. It's energy-conscious because it doesn't need any powered devices to do the job. The tradeoff is that it's limited in the amount of fluid it can move, so if you want to transfer heat at high rates, mechanical assistance may be required. Early automobile cooling systems relied on natural convection to circulate coolant through the engines and radiators. As engine performance levels increased, however, heat had to be removed faster. Higher coolant flows were needed, and it became necessary to add water pumps to the engines.
The same is true in drying -- you can set something out in the fresh air and eventually, it will dry, but if the process is too slow or if it fails to dry all the way down in the deep recesses of the piece, forced convection might be needed.
Forced convection uses fans to increase the heat transfer rate. By increasing the velocity of the gases, you can make them scrub the product surface with greater turbulence, and that raises the convection coefficient. In addition, it provides more energy to sweep away the blanket of spent gases next to the surface, replacing them with a fresh batch having a higher fluid-to-surface temperature differential.
Forced convection also is beneficial for mass transfer reasons. If moisture or a solvent is evaporating from the surface into the airstream, the air will quickly become saturated if it doesn't move on. Once it is saturated, evaporation ceases, no matter what the temperature differential.
Heat transfer textbooks usually carry examples of convection coefficients for simple geometric arrangements such as flow through a pipe, flow across a bank of tubes or flow parallel to a flat surface. Unfortunately, these simple configurations can't begin to describe the geometry of a curing or drying oven and its contents. The convection coefficient varies from place to place within the oven, and it can't be calculated with one or two simple equations. The textbooks usually cop out by saying complex configurations must be determined by field experimentation -- not much help if you're designing something from scratch.
Through experience, oven and dryer manufacturers have determined the overall, or average, convection coefficients for a variety of their designs. In addition, some of them use computers to model flows and heat transfer rates. This allows them to predict drying rates and heat inputs with a high degree of confidence. Nevertheless, be realistic when considering a new oven or dryer. Even the best computer models can't take all of the design variables into account, and unanticipated field conditions or product properties throw another wildcard onto the table. Some on-site tweaking of the oven's air delivery system may be necessary before it performs as specified.