Vacuum Drying: Back to Physics 101
Catherine Sidman is the Product Manager at Cascade TEK, an A2LA accredited environmental product testing laboratory. They also manufacture high quality vacuum ovens used for drying, curing and vacuum bake-out applications. You can read more on the Cascade TEK blog.
Imagine a 1 liter glass beaker full of water sitting at sea level. This water has a column of air—gaseous atmosphere comprised of Nitrogen, Oxygen, Carbon Dioxide and particulates—that is as large as the mouth of the beaker and sixty-two miles high. Those gaseous molecules and particulates have mass and are affected by gravity which is why air is denser at sea level and becomes increasingly less dense as you reach the Kármán line, which is the limit between the Earth’s atmosphere and space. If you wanted to boil the water in the beaker at sea level, you would have to add enough thermal energy to increase the temperature to 100°C. Then the water could overcome the 14.7 pounds per square inch of atmospheric pressure and jump from the liquid phase to the gas phase. As you increase in altitude and the pressure on the surface of the water decreases, the amount of energy required to excite those water molecules into the gas phase decreases. Vacuum drying takes advantage of this natural phenomenon by adjusting the pressure in order to remove liquids at low processing temperatures.
Turn Down the Heat!
A classic application for vacuum drying is in the final processing of struck circuit boards and other electronic components. The parts are wet from a sonic bath and need to be dried quickly and efficiently. The heat required to drive off the moisture in a convection oven would melt the delicate components and allowing the parts to “air dry” would result in catastrophic residue. By processing the components at a low temperature like 60°C and a roughing vacuum level of one hundred microns—roughly the equivalent of 160,000 feet in elevation—the liquid water will jump to the gas phase and be drawn out by the vacuum pump. This results in dry and clean parts with no residue from processing.
Better Out Than In
Vacuum drying has some secondary benefits that are essential for applications as diverse as processing wire harnesses for the Mars Rover and Hernia Patches for Aunt Mabel. In both of these applications it is essential that trace chemicals do not off-gas from components into the surrounding area. Space flight hardware will be exposed to ultimate vacuum and components will outgas—leach—chemicals. If components outgassed multiple substances that reacted, the results could range from optical contamination to explosion. In the case of implantable medical devices, chemicals could leach into the recipient’s system. Processing under vacuum does more than simply dry: it also off-gasses chemicals that could foul electronics in space or poison a patient.
Drying Clothes in the Microwave
Budgets are tight and floor space is at a premium so the desire to want one oven that can tackle every application is understandable. Consider how many home appliances it takes to heat, bake and dry. It would be nice to reduce the number we need but it is unlikely that we will be processing our Thanksgiving turkey in the same unit that dries our clothes or heats left overs. Vacuum drying is not a nice upgrade from convection drying; it is a unique process that meets the needs of manufacturers and researchers across the spectrum. When vacuum drying is needed, nothing else will do. That is, unless you don’t mind melted boards or fouled optics in space.