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.
|"In many cases, the expected service life of a vacuum pump comes down to how well it is protected from incoming contamination."|
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 air flows 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.
Factor #2: Pressure Drop
The next consideration is pressure drop across inlet filters. Pressure drop is simply the difference in vacuum between the inlet of the filter and the discharge of the filter. Pressure differential across inlet filtration can be important in that if it is excessive, there are higher costs associated with operation of the vacuum pump. High vacuum differentials result in a higher capacity requirement. In simple terms, the vacuum pump has to create a deeper vacuum to account for the loss across the inlet filter. If vacuum pump capacity is fixed, the result will be a loss of vacuum at production equipment.
Factor #3: Serviceability
The final key component is serviceability. It is recommended that whatever design being used has elements that are easy to clean or change. Most vacuum pump applications have single inlet filter configurations. In this scenario, the vacuum pump must be shut down to change or clean inlet filter elements. A better arrangement is to install a dual inlet filter system so that one side can be cleaned while the other side is in operation. In this manner, low differentials can be maintained throughout the production cycle and demand applications can realize maximum vacuum at the point of use. Also note that when servicing cartridge style filter elements, it is important to keep debris out of the pump inlet when the loaded element is removed from the filter housing.
Inlet filter design should not be taken lightly. The best course of action is to obtain a particulate sample and send it to a capable lab for analysis. They will provide a full report on particulate size distribution so that decisions can be made regarding media and filter configurations. Also, contact your vacuum pump manufacturer for information on how much and what size particulate their vacuum pump can tolerate. For very small particulates, specialty media such as HEPA (high efficiency particulate air) or ULPA (ultra low penetration air) may be required. Once this information is gathered, design the inlet filter for low pressure drop and easy serviceability. The benefits to this process will be longer service life of vacuum pumps and extended production uptime.
For more information contact Dan Bott, Dan Bott Consulting LLC, tel: (251) 609-1429