Drying – Conduction vs. Radiation

It should be clear by now that heat plays an important role in drying.  Delivery of heat to a drying system is critical and can be accomplished in a number of ways.  Today’s blog will explore two options for heat delivery – conduction and radiation.

Conduction seems to be the “go to” when it comes to drying systems.  Heat from a heat source such as an electric heater is used to heat air by conduction.  The heat moves from the heater to the air by physical contact between the two.  The heated air is then delivered to the drying site using a blower or fan.  The heat in the air is then used to facilitate the drying process which involves evaporating any water (or other liquid) that remains on the part after being cleaned and rinsed.  Let’s think through this scenario.

In earlier blogs we learned that air is not only a poor conductor of heat but also has a relatively low capacity to store heat.  Once the heat in the air is delivered to the drying site, its motion over the wet part creates evaporative cooling of the remaining liquid as the top layer of the liquid evaporates.  Since the liquid is a better conductor than air, the cooled liquid pulls heat from the part beneath it thereby cooling it as well.  Quite simply, the heat it the part is much more available and abundant than the heat in the air blowing over the part.  In many cases, the temperature of the part being dried is actually reduced only to increase after all of the water is evaporated.  Those of us who have conducted drying trials with hot air dryers know how difficult it can be to dry parts that hold significant amounts of water.

One option to a hot air dryer is one using radiant heat.  Radiant heat is transmitted by electromagnetic waves rather than by contact between two surfaces.  Radiant heat from the sun easily travels 93 million miles through a total vacuum.   Radiant heat from the sun even passes easily through normal window glass to heat the spot on the rug where your pet likes to nap without significantly heating the window glass itself.  Radiant heat only heats opaque and/or non reflective objects which are in its path.

Now let’s consider how radiant heat might be used to dry parts.  First off, the drying of glass and other transparent or reflective objects is off the table!  That’s not going to work.  However, there are lot of parts that are good absorbers of radiant heat.  Heat from a radiant heater will pass through air and water remaining on a part and heat the part itself directly.  The part, then, will efficiently deliver that heat to the water on the part by conduction (assuming that the part is a good conductor).  The heat delivered to the water from the part will facilitate its evaporation while more heat is added from the hot part below to overcome evaporative cooling much more effectively than air will in the conduction example given above.  A low flow of dry (not necessarily hot) air over the surface being dried will accelerate evaporation (which is its only purpose).

In the next blog, we’ll look at the comparison between hot air and radiant heat as a means of drying parts in somewhat more detail and consider how an efficient and fast radiant heat dryer might be designed.

The more I think about it, the more I wonder why we don’t utilize radiant heat more frequently as an efficient and fast way to dry parts.

–  FJF  –