Drying – Vacuum Drying

Using a vacuum to assist or effect total drying is not a new concept.  A reduction of air pressure lowers the boiling point of water and other liquids and thereby speeds their rate of evaporation.  Although not practical in many applications for reasons I will discuss below, vacuum drying remains a viable alternative in some specialized applications.  Vacuum drying is frequently applied in the case of relatively small parts with small surface features or other attributes that make them difficult to dry using other means.  In most cases, the amount of moisture remaining retained by the part is relatively small – surely, parts with standing “puddles” of water are not good candidates for vacuum drying.  Successful application of vacuum drying usually involves presenting the vacuum dryer with a part that has already been dried to the limit of other, more conventional and usually less costly technology.

The first thing to know is that there is more to vacuum drying than just placing parts in a vacuum chamber.  Remember evaporative cooling, a subject we discussed in earlier blogs?  In vacuum drying, the effect of evaporative cooling is magnified as, not only is the rate of evaporation greatly increased, but, in a vacuum, there is no air to conduct and deliver heat to the surface being dried.  Almost invariably, a source of heat must be used to overcome the increased effect of evaporative cooling in vacuum drying.  In most cases, the required heat is supplied in the in the form of radiant heat from heaters within the vacuum chamber or by heating the chamber using external means.  Some vacuum drying equipment even has heated shelves on which parts can be placed although this is more applicable in the case of powders.

As stated above, vacuum drying has application in situations where water must be extracted from small surface features which might include deep capillary spaces.  But, even with vacuum and added heat, thorough drying requires time.  This is true especially in cases where, as in a capillary, there is little exposed surface available for interaction.

I already hinted that vacuum drying is a relatively expensive technology.

• The vacuum drying chamber must be strong enough to withstand the pressure of the atmosphere when a vacuum is drawn inside.  In many cases, the required strength is achieved by using rounded shapes and domes.  The alternative to achieve the required strength is using box-like chambers with very thick walls.
• High capacity vacuum pumps are required to achieve the required vacuum in a minimum time to make vacuum drying practical from a throughput standpoint.  Vacuum pumps are relatively expensive and require some degree of maintenance.  In the case of vacuum drying, the pumps used must be tolerant of moisture which is inevitable in this application.
• Vacuum drying is difficult and often expensive to automate due to the construction requirements of the drying chambers.

One answer to high volume requirements is using a series of chambers with progressively higher vacuum with automated doors separating the chambers.  The vacuum is increased in stages as the object being dried moves from one stage to the next.

One very positive note about vacuum drying is that it does allow the recovery of evaporated liquids by condensation.  This can be a big plus in the case of expensive solvents.

One final note – vacuum drying is, like many things in cleaning technology, more of a science driven “art” than just a science.  Consultation with a source experienced in vacuum drying is highly recommended prior to specifying any process  involving vacuum drying.

–  FJF  –