Selecting the right filter to go with your silencer application requires understanding all of the options. There are trade-offs between cost, efficiency, durability, and size that really need to be thought through before selecting a filter-silencer or filter housing. In this week’s blog, I’ll go through the thought process that we use when we are helping a customer make a silencer filter selection.
Contaminate Size
Filter selection should start with knowing the size of the contaminates that need to be removed. Most filters will be rated based on the size and efficiency of the particles that can be filtered out of the air. The most efficient filters can remove particle as small as 0.3 microns with a 99.97% efficiency.
A good example is selecting a filter for a regenerative blower. Most blower manufacturers recommend filters with a 10 micron rating. However, in a dusty food processing environment that has flour dust in the air, a 10 micron filter might not be sufficient to capture all of the flour dust. In this case, the trade-off that needs to be considered is between the cost of more efficient filters and the cost of more frequent blower maintenance.
Filter Material
There are three types of material used in silencer filters: paper, felt and wire mesh. Here is a brief summary of the characteristics of each material type.
Paper – These filters have the best efficiency for filtering out small particles. Due to their low cost, they are generally treated as disposable when they are dirty. However, some types of paper filters can be cleaned with compressed air. A thorough post cleaning inspection is required to identify holes or cracks that may have been caused by the cleaning process.
Felt – Most felt filters are made with a synthetic felt material. They provide the second best efficiency for filtering out particles. The big advantage of felt over paper filters is advantage is that they can be cleaned with either compress air or water and reused multiple times.
Wire-Mesh – Often made with galvanized wire, mesh filters can be cleaned and reused indefinitely. However, they are the least efficient filter material. To enhance the efficiency, they are treated with and adhesive. The adhesive must be reapplied after every cleaning. A big disadvantage of a wire mesh filter is if that if it becomes too dirty, the efficiency will drop dramatically and most particles will pass through the filter.
Two-stage Filters
A two-stage filter uses a wire element or felt element as the primary filter and a paper element as the secondary filter. The advantage of a two-stage filter is that it combines the best features of cleanability and efficiency into one. The primary filter captures most of the particles and when it is dirty, it can be removed for cleaning. The secondary filter doesn’t need to be replaced as frequently because it doesn’t get as dirty as it would as a standard alone filter.
The disadvantage of a two-stage filter is the size of the housing required to hold both filters. That is why the decision to use a two-stage filter needs to be made at the time of silencer selection. Standard filter-silencers or filter housings can’t be modified to accommodate a two-stage filter.
Conclusion
The one lesson that we have learned when evaluating the trade-offs between cost, efficiency and durability is to overestimate the time period between filter cleanings and replacements. This is particularly important when there is not a dedicated maintenance staff to perform the service. Often times, filter cleaning and replacement doesn’t occur until the dirt load on the filter cause a decline in the amount of air moving through the system.