Compressed Air Best Practices - September 2008 - (Page 51) Compressed Air Industry FOOD PACKAGING & PROCESSING | 09/08 | vapor) condensation in the pump oil reservoir. Discharge temperature must therefore be balanced between the risk of condensation in the oil reservoir and excessive oil carryover. Another important factor is how well the coalesced oil can be removed from the “dry” side of the separator element. If coalesced oil cannot be removed quickly enough, the separator element will begin to fill up with liquid oil. As it fills with oil, there is less media surface area available for air flow resulting in an increase in velocity and higher oil bypass. The most common method of reducing excess oil carryover is to install an external oil mist filter, also called a demister, at the discharge port of a vacuum pump. These filters utilize specialty elements that coalesce most of the excess oil and provide for much cleaner discharge air. Installing the correct size filter is important so that air velocities remain low enough for effective separation. A good rule of thumb is to size the external exhaust filter for the nominal rating of the vacuum pump. A 40-hp vacuum pump rated for 500 acfm will need an exhaust filter rated for 500 scfm. Downsizing the demister is acceptable in some applications where the system is operating at a steady vacuum and there is little chance of fluctuations. Liquid oil that is collected in an external oil mist filter can be drained to a separate collection system for proper disposal. A lot of research has been done on improving auxiliary oil mist filters over the last several years. The designs in use today are effective and worth the investment. Note that even with an external exhaust filter there will still be some oil carryover from the vacuum system but the amount of carryover from a vacuum system with an exhaust demister is significantly less than a vacuum system without one. One other point to note is that in systems with discharge piping to the outdoors or other areas it is important to install drip legs in the vertical run of pipe so that particulate or dirty oil does not drain back down into the vacuum pump. This arrangement will create a spiraling contamination loop by keeping all non-desirables in the vacuum pump. Water is of particular concern because it is very likely to condense in the relatively cool vertical discharge piping. Keep your work environment green by maintaining a clean air discharge from your vacuum pumps. It is easy to obtain and install oil mist exhaust filters that will provide for years of clean operation. For more information, contact Dan Bott, Dan Bott Consulting LLC, tel: 251-609-1429, email: dan@dbott.com, www.danbottconsulting.com “Keep your work environment green by maintaining a clean air discharge from your vacuum pumps.” www.airbestpractices.com 51 http://www.danbottconsulting.com http://www.airbestpractices.com
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