|Multi-bag filter housings accommodate high volume processes demanding continuous operations and applications where limiting exposure to workers is an important factor. Housings are installed in the process piping, which hold filter media during the filtration process. (Photo courtesy of Eaton's Filtration Division)|
There are many filter types to choose from when specifying an industrial application liquid processing filter. The optimal choice ultimately comes down to the technology that best meets the performance and operating cost needs of the application. Common filter types include self-cleaning filters and removable media types, such as bag and cartridge filters. Each type has advantages and disadvantages when compared with the others, and the different types are appropriate for different applications.
1. Self-Cleaning Filters
Self-cleaning filters are most appropriate for applications where the process requires maximum uptime or little interruption, filter media replacement and disposal costs are high, the process fluids are valuable, and/or exposure of the process liquid to workers or the environment is undesirable.
There are several types of self-cleaning filters, including backwashing systems that regenerate by reversing flow across a permanent filter media and mechanically cleaned filters that use a scraper to physically wipe particles from the media.
Automatic Backwashing Filter
Automatic backwashing filter systems can be used to remove suspended solids of approximately one micron or larger and work best with water-like liquid process streams at flow rates typically of several hundred to several thousand gallons per minute.
Mechanically Cleaned Filter
Mechanically cleaned filter systems can work with many industrial applications where viscosities or solids concentrations are higher, but they generally have lower flow rate capacities than backwashing units and work best at retentions of 25 microns and greater.
All self-cleaning filter technologies are pressure filters (they have certain minimum operating pressure requirements) and they generate a waste stream that must be accommodated either by reprocessing or disposal. However, mechanically cleaned systems generally produce lower volumes of waste as a percentage of total process flow rate.
|A single bag filter housing has one-piece body design that can house different filter media, determined by a variety of factors including the size of particulate being filtered, flow rates and viscosity. (Photo courtesy of Eaton's Filtration Division)|
2. Disposable Media Filters
Disposable media systems such as bag- and cartridge-type filters are available in many media compositions and retention rates, allowing for very specific solutions that are not available with more general self-cleaning units. In general, bag and cartridge solutions are limited by the amount of surface area that can be applied to the filter application. Both types require a permanent filter housing to be installed to the process piping, which hold the media during the filtration operation.
Bags and cartridges must be selected from materials that are compatible with the chemistry and temperature of the process, and they can remove suspended solids for applications with lower flow rates, where operator exposure to the process liquid is not a problem, and where lower volumes of solids must be removed.
Bag and cartridge filters are roughly equivalent in price, while self-cleaning systems have higher initial costs. However, users should consider the total operating costs of the filtration system, not merely the initial purchase price. Media replacement and disposal costs, labor costs, process liquid losses and downtime should all be included when evaluating filtration systems.
Bag filters may be the most appropriate choice in many situations. These filters operate by capturing solids inside the filter media “bag,” and tend to be low-cost and generally have lower surface areas than comparable cartridge systems. Higher surface area and higher filtration efficiency bags are available as well.
Bag filters tend to have the lowest equipment investment costs, and are generally tolerant of a wide range of process conditions, making them a good choice for many different applications. For example, for small batch applications, the cost of a self-cleaning system may not be justified. In this type of application, process pressure conditions may not meet the requirements of self-cleaning systems, or bag replacement may be so infrequent that disposal costs are not a significant factor in deciding which type of filter system to select. Bag filters generally produce less solid waste than cartridge filters so they may be appropriate in situations where users want to minimize the waste volume (especially when it is hazardous waste).
Cartridge filters operate by capturing solids on the exterior of the filter media “sticks.” Cartridges range from simple low-cost string-wound and resin-bonded units to high-capacity pleated units available in many filter media types. There are multiple standards for cartridge sizes and end connection configurations.
Cartridge filters can be the logical choice for a wide variety of applications with varied flow rates that require sub-micron retention rating, including food and beverage; chemicals; paint; and the filtration of acids and bases. Cartridge filtration can also be an effective choice for pharmaceutical and ultra-pure water applications. Although cartridge filters generate more solid waste than bags, and require more labor for media change out, there are advantages to the variety of materials that can be used in cartridge filters, often a consideration in high-temperature applications where metallic materials are required.
In situations where a cartridge filter user wants to convert to a self-cleaning system, but cannot for whatever reason, bag filters may provide a compromise solution. Bag filters generate 10 to 15 times less solid waste than cartridges, and they require less labor for media changeout. Plus, some bag filters can be cleaned and reused, lowering waste and disposal costs even further.
Bag filters can also be used in conjunction with self-cleaning filter systems when processors want to recycle the waste liquid containing the solid contaminants. Installing a bag filter in the waste stream line removes contaminant from this fluid and allows it to be recycled back into the process.
Multibag and multicartridge filter units can accommodate higher flow applications by holding more filter media within a single pressure vessel housing. Multibag units provide longer run times between bag changeouts, while some multifilter units provide continuous operation by sequentially taking individual filters off-line for backwash cleaning while the other filters continue operating.
Chris Rau has 15 years of filtration industry experience, including product development, application engineering and laboratory roles. He now manages the new product development teams for filtration hardware, replaceable filter media, and measurement systems within Eaton’s Filtration division.