Hot Air Drying – Tips and Pitfalls

Drying is the last and often a critical part of a cleaning process.  In many cases it is also the most time and energy consuming.  It is often the most difficult part of the process to specify and control.  Although there are several drying technologies available, blowing with hot air is simple and by far the most prevalent.  But even this simple technology has variables that are often overlooked.  If you are not familiar with the basics of hot air dryers, check out this blog.

In the end, drying is accomplished by evaporation of liquid from the part surface.  The things the promote evaporation are higher temperature, increased airflow and reduced relative humidity.  The hot air dryer is engineered to provide all of these.

But here are a few more tips.

Drying Starts in the Rinse?? –
Yes – Using a higher rinse temperature provides a higher part temperature as the part enters the dryer and can significantly reduce drying time.  In some cases it is felt that hot rinses consume too much energy and, therefore, are replaced with cold rinses. The fact is that any energy savings in the rinse is probably lost in the dryer as the part still has to be brought up to temperature to dry.  Hot rinses speed drying.

Part Temperature Lags the Hot Air Temperature in a Dryer –
Air is a very poor conductor of heat compared to most part materials.  The rate of heat transfer depends on the differential temperature between the air and the part surface.  Although not a subject of this blog, evaporative cooling also slows part heating as liquid evaporates from the part surface and cools it.  If a part has a temperature limit of 150°F, it is very possible that the dryer temperature could be safely set at 200°F or more without the part ever reaching the critical 150°F temperature before it is dry.  This is a little like baking a potato in an oven at a temperature of 375°F – – the inside of the potato reaches the boiling temperature of water at the end of a one hour baking cycle although the temperature of the air in the oven is much higher.  Done under controlled conditions of exposure time and part contact increasing dryer temperature may be a solution to shorter drying times even for temperature sensitive parts.  It’s an easy test to run using thermocouples attached to the part or a non-contact infrared thermometer.  I think you’ll be surprised at the result.

More Liquid Takes Longer to Evaporate –
Remove as much liquid as possible before the part enters the dryer.  Parts that “pool” liquid should be oriented to allow liquid to drain.  If simple orientation won’t do it, an investment in part rotation prior to drying may justify itself by saving both time and energy in the long run.  Use of a compressed air blow-off or some other means to remove liquid puddles prior to drying is another option.

Humidity Plays a Role –
Dry air absorbs liquid more readily than wet air.  There are a couple of ways to decrease the relative humidity in the dryer to increase drying speed.  The first is to introduce lower humidity air from the surrounding environment.  In a domestic clothes dryer, for example, the air is not re-circulated at all.  Recirculated air would eventually become saturated with liquid and drying would slow considerably.  If venting is not an option, a chilled condenser can be used to collect and remove excess liquid from the air (much like a dehumidifier in your home).  When low temperature drying is necessary, dry nitrogen is often used as a drying agent instead of air.

While we’re on the subject of humidity, simply heating air reduces its relative humidity as hot air can absorb more liquid than colder air – think dew point.  A few degrees can make a big difference in drying time.