Heat or Air: Which is the best way to get stuff dry?

We often get requests from customers for assistance building a heat solution for drying or adding heat to an existing drying system that is under-performing. This is understandable, using heat to dry is a common occurrence in our everyday lives from clothes dryers to hair dryers. In this article we explore why hot air is not always the best drying solution and talk about using fast moving air as an alternative to heat. We also give guidelines to help you decide when to use heat and when to only use air.

What does “drying” mean?

Firstly let’s clarify what drying can mean. The term is used in many ways that can be open to interpretation. The term “drying” is commonly used to describe all the following applications:

• A sheet of metal passes through a cooling bath and sheet must be dried before rolling
• An automotive part is painted and the paint needs to be dried before assembly
• Ink is applied to a package which needs to be dried before filling
• Bottles are rinsed but need to be dried before labelling
• A metal part is stamped and the forming liquid needs to be removed before welding
• An epoxy is applied that must be dried before packaging

​In some cases the term “drying” is misused and it would be more correct to say curing. Regardless of the terminology, the important point is that not all of these applications are good candidates for using heat to accomplish the end goal.

When to use heat and when to use air

Drying applications can be divided into three categories: blow-off, evaporation, and curing. Here we are going to lump evaporation and curing into a single category because, although the underlying chemistry is different, the possible solutions for the two are the same.

Blow-Off

Many manufacturing processes leave a significant amount of excess liquid on a surface that needs to be removed. It is much more efficient to mechanically remove the liquid rather than attempt to evaporate it from the surface. In order to evaporate the liquid, enough energy needs to be applied to raise the volume of liquid from its starting temperature to the boiling point temperature and to change the state from liquid to gas. The number of Watts (units of energy per unit time) required for the heater to evaporate the liquid in a practical timeframe quickly escalates. In these scenarios, blow-off has the advantage of being fast and energy efficient. The outlet of a blower is connected to an air knife to focus the air into a fast moving sheet. The impingement of the air on the surface then literally strips off the liquid.
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See our article on The Basics of Heat Calculations for more details.

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Evaporation/Curing

Applications where evaporation and/or curing need to take place require the introduction of heat. Examples include drying paint, drying ink, or curing epoxy. In the cases of drying paint or ink, the final surface finish is important; fast flowing air would not only damage the surface but would likely strip the paint right off the surface.  In the case of epoxy, and some paints, the heat also acts as a catalyst to the curing process.

Hybrid Solutions

There are times when the optimum solution to a problem is a combination of heat and air. Every application must be evaluated separately but below are some examples of ones that lend themselves to a hybrid solution.

• Blow-off works best on smooth surface where the mechanical stripping action can result in complete drying. There are times when the excess liquid is on a porous material such as wood. Good results can be had by using a blow-off system to remove the bulk of the liquid and then quickly following with heat to remove any residual traces trapped by the porosity.
• There are some plastics that are hydrophilic—meaning they have a propensity to hang onto surface water. In these cases there can be an advantage to either heating the surface water to free it from the surface and quickly following with an air knife or heating the air going into the air knife thereby hitting the surface with a curtain of fast moving hot air.
• Solvent based liquids such as forming oils generally benefit from using a heated air knife. Again, the mechanical action of the high velocity air removes the bulk of the liquid but the heat is efficient in flashing off any residual liquid.

Drying problems may not be as straight forward as they first appear. At STANMECH Technologies we have a great deal of experience in the design of blow-off, heat, and hybrid systems. Contact us to discuss the best solution to your application.

Originally published: 03/20/2015