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There are a plethora of air and liquid filters available on the market. For residential, industrial and commercial purposes, bag filters are commonly used for all. Learn more about popular types of filter bags and their common purposes. Let’s learn about bag filters this time.
What Are Bag Filters?
“Sock filter” is a common name for bag filters used for liquid because they tend to have a tubular, sock shape. This flexible bag-shape differs from the traditional solid cylindrical cartridge filter found in many household filter systems. Air bag filters tend to have a tubular or rectangular shape with one or more pocket compartments that expand as air passes through, which is different from the flatter, sturdy rectangular box filter you will find in your home’s HVAC system.
Types of Bag Filters
First, which one you need can be sorted to liquid or air filters. For liquid, the categorization breaks down into different media types and number bags used in the system. For air, there are smaller pocket filters categorized by number of pockets and material, and then larger industrial filters by cleaning method.
Single Bag Filters
Single filters are a category of liquid bag filter systems. They consist of a filter housing holding a single liquid filter bag sock. Generally, they’re composed of stainless steel, carbon steel, polypropylene, or aluminum. Ideal for smaller applications, single bag filters present a more economical and less complex solution. They come in a variety of sizes, pressure and temperature abilities, and materials.
Multi-Bag Filters
There are two different types of multi-bag liquid filters. The first is a vessel that holds multiple bags within different filter baskets, numbering anywhere from 2-24 liquid filter bags. They offer a higher dirt-holding capacity and better flow rates for higher volume applications. The second option is a multi-housing system, composed of multiple single-bag housings that have been piped together. This configuration allows for continuous flow, even during cartridge change-outs.
Liquid Bag Filters
The filter used within single and multi-bag filters are liquid ones. These sock-like filters are different lengths characterized as #1, #2, #3, and #4 sized filter bags, and come in a variety of micron ratings. Different media types allow filtration of various liquids including water, chemicals, fuels, paints, and beverages.
Air Bag Filters
Filter bags used in air filtration may be smaller commercial filters called pocket air filters, or larger industrial filters used within a baghouse system. Pocket filters offer different numbers of pockets that inflate when air flows through the system, for a large surface area. The configuration and materials used as well as the MERV rating depend on the filter’s intended use.
They typically minimize air pollution produced by different industries, and include three main types depending on the cleaning method of shaker, reverse-air, and pulse jet. Shakers manually agitate the filter to remove dust, reverse-air filters reverse the airflow direction to gently collapse the filter to remove the dust, and pulse jet filters have bursts of air along the length of the filter that blast off the collected dust.
Materials Used in Bag Filters
Liquid filter bags come in a few different media options, the most common being needle felts, woven monofilament meshes, and melt-blown, with the most common materials being polypropylene, polyester, and nylon (polyamide). The opening may have a polypropylene plastic ring, steel ring, or a drawstring. Polypropylene and polyester bags can have glazed or singed finishes, which reduces fiber shedding.
Pocket air filters are typically composed of fiberglass or non-woven synthetics like polypropylene or polyester, and are either rigid or soft. The pockets are sewn or bonded to a plastic or metal frame. Industrial filters can be composed of a number of different materials, including polypropylene, polyester, nylon, nomex, teflon, fiberglass, and more.
How Bag Filters Work
Liquid filter bags sit inside of a filter housing. Water enters the housing, the water pass through the sock while contaminants are trapped inside. The higher the micron rating, the thicker the material, which allows for smaller contaminants to be captured.
Pocket and baghouse air filters work similarly to one another, with air passing through and inflating the filter bag. For pocket and reverse-air baghouse filters, the dust and other airborne particles stick to the inside of the filter bag. For pulse jet and shaker baghouse filters, the dust cakes on the outside of the filter bag, and the filtered air passes through and out of the system.
Common Applications of Bag Filters
Liquid filter bags are capable of filtering large amounts of liquid at once, and given the variety of customizations, can be used for a variety of applications. They are commonly used for commercial purposes, such as water treatment, chemicals, food and beverage, paints, fuels, pharmaceuticals, solvents, and more.
Smaller pocket air filters are generally used in applications requiring constant air flow. They make highly efficient filters and pre-filters in HVAC systems for commercial, industrial, medical, and institutional applications, such as schools, hospitals, offices, and auto shops.
Advantages of Using Bag Filters
Liquid filter bags economically filter large volumes of water without impeding the flow rates as much as some other filter solutions. The larger surface area creates a typically larger dirt holding capacity for less frequent replacement and cleaning. Filter bags are also easy to install and replace, and offer a high temperature resistance.
Like their liquid counterparts, larger surface areas on pocket filters allow higher dirt capacity and fewer changeouts. They also offer lower air resistance. Pocket filters have a low initial pressure drop, which requires less energy over time. They provide continuous airflow with minimal to no interruptions. They can also be easily customized for different industries and applications.
How to Select the Right Bag Filter
When selecting a liquid filter bag, consider what you want to filter. Different materials handle certain fluids better than others. When determining micron size, think about what will be filtered and the effect on flow rates. Higher micron ratings filter less but allower higher flow rates. Also consider the size of the filter bag, number of bags needed, and the process conditions.
Selection requires knowledge of size to determine if you need a large baghouse filter or a smaller pocket filter. For baghouse filters, determine if more economical shaker filters are better, or if you need a continuous flow pulse jet filter. For pocket filters, if you need less risk of contamination from a bag collapse, rigid pocket air filters would be a better choice over soft bag filters. Other factors are material types, size, frames, and other customizable options.
Bag Filters vs. Other Filtration Methods
In all cases, bag filters tend to be the more economical solution. For example, cartridge water filters typically have higher purchase costs, and may need more frequent replacement. They might also be harder to install. However, cartridge water filters can be more durable, and are very diverse in their media and filtration capabilities.
Pleated air filters are an alternative too. The pleated cartridge filters tend to trap dust particles within the folds more completely than a bag filter. They are also very compact, sturdy, and durable. However, the pleated filters tend to clog faster with less dirt holding capacity, and the pleated media itself may wear out faster than a bag filter. Bag filters are also very customizable and versatile. Like liquid bag filters, the upfront cost for bag air filters is usually less.