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Vacuum Problems - Condensable Vapors


Problems achieving low vacuum pressures in the sub 500 micron range can often be traced to contaminants (assuming no air leaks). These contaminants can be present in the product the vacuum is being pulled on, in the system, evacuation lines, filters, or in the vacuum pump lubricant.

The problem is that different products have different vapor pressures. Solvents and water are two common roadblocks to achieving low pressures (higher vacuum). The vacuum pump generically can achieve, for talking sake, 10 micron pressure. The vacuum oil out of the bottle is rated for .001 micron. If a contaminant is present in the system that "out-gasses" at 200 micron then the best pressure attainable on the system will be 200 micron and will be maintained at 200 micron until the contaminant is evacuated from the system and exhausted from the pump and it's oil sump. If this contaminant is present in the system and is removed under vacuum you can be assured it is now in the vacuum pump oil. Once contaminants are in the pump, use of the gas ballast valve may help purge them from the pump oil. Only then will the system be able to perform down to 10 micron pressure the pump is rated at. If a real low pressure is not needed to accomplish a given task consider running the vacuum pump with gas ballast cracked open to throughput condensable vapors through the exhaust port during evacuation process. Make sure adequate ventilation is present as throughput of condensable vapors will end up in the exhaust stream of the vacuum pump and may pollute your work area and could even pose fire hazards depending on the makeup of this exhaust.  If you are evacuating gasoline vapors these vapors will end up in your workspace barring adequate ventilation.

Other things that can outgas include sealants used in sealing the systems, types of elastomers including hoses, and seals. Special attention paid to materials on the front end of designing a system saves time trouble shooting the system. Some out gassing can go on for very long periods of time until the contaminant is removed from the system. System solutions to help include, liberal use of the vacuum pumps gas ballast valve, baking the system using an oven or heat gun to help "burn off" the contaminants and identification of offending products and elimination from the system.  Adding heat to a system adds kinetic energy to molecules that can outgas exciting them from their state of inactivity and allowing them to be "pumped".

Cleanliness of the system, piping and traps are the most important things you can control. Final cleaning with an Acetone rinse and letting dry is a good procedure. Petroleum solvents such as mineral spirits, xylene etc should NOT be used as the they have much greater residual ability. Base cleaning using 111 Dichloromethane, or other chlorinated solvent is okay again using a final acetone rinse. In high vacuum applications less than .001 micron, chlorine contamination may need to be addressed also. Full drying between all steps.  These are subjects that are better left to the cleaning pros.  Consideration of cleaning materials needs to be seriously addressed when attacking pressures less than .001 micron.


11/27/2005 ęCopyright HyVac Products, Inc. All Rights Reserved
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