Compressed Air Best Practices - October 2008 - (Page 52) | 10/08 | AUTOMOTIVE TRANSIT BY DOCTOR VACUUM Inlet Filtration Nearly all vacuum pumping technologies have some degree of sensitivity to inlet particulate contamination. Since everything from a vacuum-assisted production process ends up at the inlet of the vacuum pump, it is important to figure out how to best protect the pump in that particular environment. In many cases, the expected service life of a vacuum pump comes down to how well it is protected from incoming contamination. Vacuum pump inlet filtration comes in many forms. From simple screen-type filters to elaborate bag houses, there are an enormous number of configurations to choose from. The key is to match the requirements of the vacuum pump to the process or application so that the vacuum pump is adequately protected without spending excessive capital. In other words, do the job, but don’t overdo it. There are three key factors to consider when designing an inlet filtration system for a vacuum pump: particulate retention, pressure drop and serviceability. Factor #1: Particulate Retention Particulate retention is simply how efficient a filter media is at trapping a particle. In the filtration business, particles are usually measured in microns. A micron is 1/1000th of a millimeter or 1/25,400th of an inch. To give you an idea of the scale we are talking about, the average human hair is about 50 to 100 microns in diameter. The smallest object a human eye can see is about 40 microns. Filtration media is specified by providing an attenuation efficiency for a given size particle. Each type of filter media has an efficiency curve that illustrates how well that media does at trapping a range of particulates. It is not descriptive enough to use terms like 10-micron media to describe a particular media. A media that filters at an efficiency of 80% at 10 microns is clearly not as effective as a media that filters at an efficiency of 99.7% at 10 microns. Typical filter media’s are paper, polyester and metal/mesh screen. Also note that higher velocity through a media will reduce its efficiency. It is therefore important to keep airflows below the recommended maximum velocity so that rated efficiencies are maintained. Particulate retention is an important consideration in vacuum pump installations and it is important to know exactly what is getting to the vacuum pump. A good example of why this is important is an installation where a significant portion of the particulate load is less than one micron in diameter. A standard 10- or 5-micron filter will trap a portion of the smaller particulates but over time there will be a buildup of small diameter contamination in the vacuum pump. Some vacuum pump technologies are able to handle this situation without incident, but many other technologies will have severe problems ingesting a heavy load of small particulates. The result will be vacuum pumps that need to be taken off line to be cleaned or in more serious situations, vacuum pumps that will have to be repaired or replaced. In either case, this is a problem that has high maintenance or production downtime costs associated with it. With a little bit of advance research, this problem can be avoided. 52 www.airbestpractices.com http://www.airbestpractices.com
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