Water Conditioning & Purification Magazine

For Broader Water Treatment Options, It’s All in the Bag—Bag Filters

By J. Neal DeLettre

Summary: If you’re considering easy ways to filter particulate from liquids, consider using bag filters for your application. That’s because bag filters are easy to install, service, use and perfect for a wide range of applications.


Bag filters are ideally suited when easy bag removal is advantageous because the liquid being filtered is messy or difficult to work with, as well as for process applications when quick and easy bag change-out is essential. Typical applications for bag filters include:

Bag filter housings are generally constructed of carbon steel—304 stainless, 316 stainless and, on occasion, specialty metals such as Monel and Titanium are necessary. ASME code housings are frequently specified in industrial applications, but non-code housings are typically used in commercial installations.

Types
Single, multi- and duplex bag housings are available, and housing requirements are based on flow rate and whether or not automation is necessary. The four most common bag housings and bag sizes are shown below (see Table 1) using industry standards.

Because bag filters are typically used in industrial applications, it’s common for them to be made using thick gauge steel with sturdy mounding legs, swing-bolt lids, eye nuts, and stainless steel strainer baskets for easy bag removal (see Figure 1).

Filter bag selection
Filter bags come in a number of sizes, and they’re made in a variety of materials. Bag size selection is generally based on flow rate and capacity. The liquid to be filtered generally determines the materials of construction. Bag type is based on the application and the filtration efficiency required.

NOTE: Filtration efficiency is typically based on a particular micron (µm) rating to be removed, and efficiency may range from “zero” percent to 100 percent.

Factors to consider before bag size, type and efficiency are selected as follows:

  1. Flow rate,
  2. TSS (total suspended solids),
  3. Temperature,
  4. Maximum pressure,
  5. Pipe size for existing equipment,
  6. Micron rating desired, and
  7. Efficiency requirements.

Materials of construction
Filter bags are typically made using polypropylene felt (PP), polyester felt (PE), nylon felt (NY), polypropylene monofilament mesh (PMO), nylon mono-filament mesh (NMO) and PTFE (PT). Media selection is based on the factors listed above and the chemical and thermal resistance data shown in Table 2.

In the past, bag filters were considered coarse filtration when highly efficient filtration wasn’t required. In recent years, however, manufacturers of filter bags have developed more efficient products to meet increased demands of the marketplace. Now, filter bags are available for precision filtration, offering 100 percent retention efficiencies down to one micron or less.

Surface area
One last consideration regarding bag filters is related to their limited surface area compared with cartridge filtration. For example, the most commonly used filter bag is the #2 bag, which is 7 inches (“) wide and 32” long. It has two sides, so the surface area of an entire #2 bag filter housing is about 3 square feet (sq. ft. or ft2), or more if the bags are pleated or gusseted—a usually diamond-shaped or triangular insert in a seam to provide expansion or reinforcement.

NOTE: To calculate the square-foot surface area of a filter bag, multiply length × width × two (since bags have two sides) and divide by 144 (square inches in a foot).

The above comparison (see Table 3) is for bag and cartridge filtration, based on 100 gallons per minute (gpm) using five-micron filtration.

Conclusion
As one can see from the data presented above, bag filters are convenient since they use a single filter bag. The filter area they provide is limited, however, compared to cartridge filtration. Bag filters are an ideal alternative for general filtration requirements because they’re easy to install, service and use.

About the author
J. Neal DeLettre is vice president of sales and marketing for Dunnellon, Fla.-based Flowmatic Systems Inc., a leading supplier in the water quality improvement and filtration industries. DeLettre can be reached at email: neal@flowmatic.com

 

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