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Element Life (Pressure Drop) and Compressed Air Filter Efficiency

Efficiency and pressure drop (element life) are two key criteria which tend to work against each other in filter designs. The higher the efficiency of a filter, the greater the resistance to airflow. A less efficient, less restrictive design will result in lower pressure drop, but will pass more contamination downstream. The trick is to design filters which balance these two criteria for specific applications.

Filtration Efficiency

The performance of a filter in removing contaminants is expressed as filtration efficiency. Filtration efficiency is a representation of the amount of contamination remaining in the air-stream after filtering, compared to the amount of contamination entering the filter. Efficiency ratings can take several forms, depending on the type of filter and what it is designed to accomplish.

Coalescing filters are designed primarily to filter liquid oil from the compressed air-stream. Efficiency ratings for coalescing filters are usually stated in one of two forms:

Percent (%) of aerosol particles of a given size and larger (typically measured in microns) removed from the air-stream or

Maximum liquid oil carryover (typically in parts per million by weight (ppmw)) based on a maximum inlet contamination.

Examples:

99.99% efficient at 0.3 micron
Maximum carryover 0.5 ppmw with inlet concentration of 20 ppmw

Particulate filters are designed primarily to remove solid particles. Efficiency ratings for particulate filters are usually stated as a percent (%) at a given particle size, similar to the first example above, or in "absolute" terms. An absolute rating at a given particle size means that no particles at the specified size or larger will pass downstream of the filter.

Vapor filters are designed primarily to remove oil in the vapor state. Efficiency ratings for vapor filters are usually stated as maximum oil carryover (ppmw), based on a maximum inlet concentration.

Experienced filter manufacturers and distributors are well qualified to assess individual needs and recommend the appropriate filter in a particular application.

Pressure Drop

As the filter element begins to plug with solid material removed from the air-stream, the airflow is restricted, increasing the pressure drop through the filter.

Pressure drop costs money. A 10 psi pressure drop requires approximately 1.5 HP or an 1100 watt increase in electrical power consumption for a compressor generating 100 scfm at 100 psig. Additional power translates directly into higher operating costs. In contrast, if the pressure drop could be held to 2 psi, increase in power consumption would be reduced to 220 watts.

Depending on filter design, as more and more contaminants are trapped in the filter, the pressure drop can increase significantly. In compressed air filters, element life is determined primarily by pressure drop. As solid particles cling to the fibers of these filters, they clog the paths between the fibers, and airflow is restricted, increasing pressure drop until the element must be replaced. If a dirt-loaded element is not replaced as needed, excessive pressure drop may cause filter seals to rupture, and gross contaminants will be passed into the air system.

Element Life

Probably the most frequently asked question about compressed air filters is "How long do the elements last before having to be replaced?"

For coalescing and particulate filters, the element life is determined primarily by pressure drop. For this reason, many users specify a differential pressure gauge or indicator on all designs where pressure drop is the primary determinant of element life. As mentioned above, the cost of excessive pressure drop is high, and economics dictate that filter elements be changed regularly to minimize the cost of pressure drop. Most high quality filters are designed to last 3 to 12 months in typical industrial compressed air systems, while operating at or below 10 psid pressure drop. Some designs will last this long and never exceed S — 7 psid. Of course, the pressure drop is directly related to the inlet contamination to the filter, which cannot easily be predicted without detailed system measurements. Therefore, a precise prediction of the filter element life on any given system is almost impossible prior to actual installation and operating experience with that filter.

Element life for adsorption filters is governed by saturation. In general, the element life is directly related to the volume of adsorption material contained in the element. In most adsorption filters, the only external indication of saturation is when the effects of contamination become evident downstream.

Maintenance

Regardless of the type of filter and regardless of pressure drop or other external indication of element life, it is good operating procedure to replace any element at least once a year. Compressed air filters operate in harsh environments, usually very wet and dirty, and often subject to inadvertent but severe pressure surges. Microorganism growth within the element, as well as physical damage to the element, can go unnoticed unless the element is routinely inspected and changed.

The ease of element replacement is often overlooked when the initial purchase is made, and only becomes an issue when the maintenance department makes the first element change. Some designers require tie rods to hold the element in place. Element change-out with tie rods usually takes longer, and the mounting hardware is subject to corrosion, deterioration and loss.

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