Corrosive Environments require specialty air filters to properly offer corrosion control; however most air filter suppliers are too eager to sell a filter without doing the proper site evaluation or invest in the needed capabilities to create the optimal filtration for a variety of applications. This has lead to misapplication in many facilities, causing a need for premature filter replacements or flat-out ineffectiveness. To combat this the following steps should be taken:
First: Determine if a multi-pass system is possible, or if a single-pass system is the only option. Because of how molecular filtration and carbon filters work, multi-pass systems offer a much higher removal rate than single pass systems. This is partly due to leakage of cheaper systems and also because the lower a concentration of gas gets, the more difficult it is to capture, so the more times we can pass the contaminated air through a remediation system the better.
Second: Understand Carbon Bed Depth and Residence Time. These two items are linked, as a larger the Carbon Bed offers longer residence time of the polluted corrosive air in the filter media. A higher residence time will be more effective at removing gasses and a larger Carbon Bed produces longer filter life due to the increased space of the Mass Transfer Zone. The Mass Transfer Zone is critical, because this is the area in the filter where the gasses are removed from the air and as the mass transfer zone is loaded it moves deeper into the filter. Once it starts to break-trough the filter needs to be either reactivated or replaced. Here is the residence time of some common molecular filter options:
Third: Incorperate a testing procedure. Your filter supplier should be able to have in-house testing that conforms to current ANSI/IEST/ASHRAE standards - if they don't then they do not have the hands on experiance and knowledge of how molecular filtration works. If a supplier is truly interested in providing you with the optimal corrosion control soultion (which we define as longest life and highest gas removal) the hands on experiance of testing carbon sorbent media after use is needed. We typically recommend a minimumum of semi-annual testing for corrosive environments, and sometimes quarterly or bi-monthly testing may be needed.
Fourth: Don't take filter literature/brochures as gospel. Efficacy charts on a filter's ability to remove gasses/molecular compounds from the air is not a standardized procedure and there are old IEST/ANSI methods that most manufacturers use because it is easier to show favorable results with the old standards (which are still "current" or unrevised) becaue they start with very high concentrations - many times higher than what is likely in your facility. As stated earlier the lower the conectration of gas gets the harder it is to remove - this is because the molecules speperate as far from eachother as possible to "balance" in the space. You should demand to see efficay charts that show the effectiveness of the filter at something close to the concentration levels in your facility - typically 1-5 ppm/2,000cfm - and following the procedure outlined by ASHRAE 145.2
Fifth: Determine if the filtration needs to be incorperated into current ventilation systems, or if a stand-alone system is needed. There are a couple variables in play in making this determination, like the size of space being filtered, where the source of the corrosive gases are, and the layout/design and limitations within the existing ventilation. Typically stand alone systems are more effective because they are deisgned for the sole purpose of reducing or eliminating corrosive gases. Acting as supplemental air cleaners stand alone systems are ideal for smaller areas like control rooms, gowning/ante rooms, and administrative/office space adjoining or within the corrosive environment. Larger scale systems are available to address large spaces like production areas and wastewater treatment plants.
Want some free advice on corrosion control? Contact one of our Application Specialists to take care of your corrosive environment.