Drying – Compressed Air Blow-Off

High velocity compressed air blow-offs are a viable means of drying or de-watering parts in many applications.  They are especially useful if parts don’t need to be “super dry” (as completely free of water) or if there is a requirement to remove “excess” water from a part prior to final drying using another means such as hot air or radiant drying.  Compressed air blow-offs are also often used in instances where there is a benefit of leaving some residual liquid on a part as in the case of a rust inhibitor.  Although the above might convey the idea that achieving a completely dry part using only compressed air is not possible, that is not the case.  There are parts that, under certain circumstances, can be completely dried using only compressed air.

Compressed air for blow offs is usually delivered using specialized nozzles designed for use with air or by devices called air knives.  In most cases, air delivery nozzles utilize technology that shapes the flow of air for a specific application.  Often, these devices “amplify” the flow of air using techniques similar to those used in eductor nozzles designed for use with liquids.  But just with submerged liquid sprays, turbulation encountered after the air leaves the nozzle, causes rapid decay of the stream of flow.

Compressed air, as useful as it is in some applications, has its associated list of limitations just like any other process.

• In many facilities, compressed air is expensive and is often in limited supply.  This is especially pertinent as effective compressed air blow-offs consume a relatively large volume of air.
• An effective air blow-off requires that the surface to be de-watered be accessible and relatively close to the blow-off nozzles.  The velocity of air diminishes rapidly as the distance from the nozzle is increased.  A distance of not more than 3″ from nozzle to part is a good target for drying parts completely using compressed air.  Access to blind holes and enclosed spaces is difficult.  Such features may preclude drying by compressed air in some applications.
• Our old nemasis, evaporative cooling, runs rampant when attempting to dry parts using compressed air.  This is especially true after the physical displacement is complete and all that is left is a film of water on the part which must be evaporated.
• Most of us are familiar with the fact that air as it is compressed increases in temperature.  This is that high school physics demonstration using a bicycle pump with no outlet with a little wad of cotton affixed to the end of the plunger.  The cotton is actually ignited by the increase in temperature as the plunger is pushed down to rapidly compress the air inside the pump. The opposite effect occurs as air expands due to a reduction in pressure as it exits the nozzle or other delivery device.  This is called adiabatic cooling (OK, I googled it!).  Expansion of the air (or any gas) requires energy which in this case results in a decrease of temperature.

For all its limitations, compressed air blow-offs remain (if nothing else) a valuable tool in reducing the load on other drying techniques.  It makes sense that the any reduction in the amount of water that needs to be evaporated or removed from a part my other means results in a faster, more reliable, and frequently less costly process overall.

The next blog will discuss additional ways to reduce final drying requirements using means to partially de-water parts prior to drying.

–  FJF  –