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Don't Forget Drying


In my experience, drying deserves a lot more attention than is usually gets in the design of a cleaning system.  If for no other reason, drying deserves extra attention because it is almost always the most time-consuming of all steps in the parts cleaning process. The effectiveness of a dryer is affected by far more variables than one would imagine at a glance.  The purpose of a dryer, of course, is to remove residual liquid that remains after parts are cleaned and rinsed.  Liquid is removed either manually as in the case of a “blow-off” or by evaporation as in the case of a hot air dryer.  There are also some “high tech” drying techniques which include displacement drying, vacuum drying and a few others that I will cover in upcoming blogs. Mechanical removal of liquid can be as simple as orienting the part so that liquids will drain from cavities and other geometries which would otherwise harbor large “puddles” of liquid.  Part re-orientation in the dryer may be required in cases where these geometries would otherwise hamper the cleaning process if the same orientation was maintained for both cleaning and drying.  The use of rotation devices is common.  Mechanical removal is also useful for removing liquids trapped in capillary spaces, screens and, in some cases, blind holes.  Although effective when properly applied, blow-offs may be difficult to execute with sufficient accuracy in some volume parts cleaning applications.  This is especially true when a variety of parts need to be processed in a single cleaning machine. Re-circulating hot air dryers are, by far, the most commonly used drying means.  Air is re-circulated by blowers in a closed chamber.  Heaters, usually electric, are used to increase the air temperature to speed drying.  The benefits of heated air are multi-fold.  The addition of heat increases the vapor pressure of the liquid thereby promoting and speeding its evaporation.  The addition of heat also reduces the relative humidity of the air which enhances its ability to absorb evaporating liquid.  It must be remembered that evaporation of liquid has a cooling effect which must be considered in the design of a dryer.  Parts which have a large surface area to mass ratio (a heat exchanger with fins for example) require more heat input than similar-sized parts that are the same size but have less surface area. One of the common mistakes made in specifying a hot air drying process is to assume that the controlled air temperature in the dryer must not exceed the temperature of concern for the safety of the part being dried.  The maximum dryer temperature should be established by determining the temperature that the part will attain when dried using air at various temperatures for various times rather than by simply limiting the air temperature to the temperature of concern for the part.  Controls which limit the exposure time while allowing a higher temperature set-point may result in significantly faster drying and lower equipment cost. There will be more to come about drying in future blogs.