Filtration is an activity that we tend to associate with the clarification of liquids as in the purification of water by municipal treatment plants. While this association is certainly a valid one, the need to remove particulate matter from air and gasses is equally widespread. As it happens, filter bags, in one form or another, can be used
both for the cleansing of liquids and gasses.
Though not as common as they once were, they were often used to provide the filtration system for home swimming pools. Before their widespread replacement with sand filters, the turbid water was pumped and passed through a number of woven fabric sections in order to trap sediment within, while allowing the clarified water to pass and to be returned to the pool. Other liquid filtration tasks for which these units are still widely used include the treatment of wastewater, cooling water, and industrial process water.
In these roles, filter bags are seen as a highly cost-effective option, as they offer a combination of good-quality filtration and high flow rates which, in turn, results in a low cost per litre. They are also a versatile option as, depending on the amount of sediment to be removed, they can be used singly or mounted in a multi-bag housing.
Atmospheric pollution has become one of the greatest hazards to the health and safety of workers, as well as a major threat to the environment in general. To deal with these threats, filter bags will often prove to be the most effective countermeasure. Because the conditions under which they are required to operate can vary quite widely, the materials from which they are made must vary in order to resist them.
At temperatures of up to just over 80°C, for example, woven cotton should work well for general applications, such as removing sawdust and cement dust from ambient air. For the filtration of air and gasses at higher temperatures, polyester felt offers a better choice as, in addition to its resistant to chemicals and abrasives, filter bags made from this material can operate continuously at a temperature of around 140°C. Resistant to most organic solvents and oxidising agents, polyester is not recommended for use with strong acids and alkalis at high temperature.
There are, however, occasions on which even polyester is not sufficiently resistant to heat, and a viable alternative is necessary. In such cases, fibreglass and Teflon filter bags can be used in steel foundries and coal-fired power statins at temperatures of up to 260°C. For less demanding conditions, other choices include materials, such as acrylic, nylon, and polypropylene felt, each with its own physical qualities.