by Stevan Brown, CAFS
What is the right filter for your job?
Today, more than ever, there are a large variety of filter choices for every application. Filters come in a wide range of efficiencies, pressure drops, depths, media types and methods of filtration. There are filters that will withstand very wet environments as well as filters designed for high velocities. Our goal in filtration is to get the best efficiency or achieve a required level of filtration with the least pressure drop within given constraints.
Fortunately, today, there are tools that will help us in product selection. First we need to determine the purpose of the filter? Is it to protect the machine, improve indoor air quality, address a specific contaminant or to protect a product or process in the conditioned space? Is the target contaminant particulate or gaseous? Each of these may require a particular type of filter and with the right tools you can make the proper selection.
Many of us are familiar with filter ratings under ASHRAE 52.1 which describe efficiency and pressure drop. The ASHRAE 52.1 test told us how much particulate matter, by weight, the filter removed. This may have been adequate for some situations but it really did not tell us anything about the size of the particles being removed. ASHRAE , recognizing the need for a better rating system, developed Standard 52.2 which gives each filter a Minimum Efficiency Reporting Value (MERV). The MERV value gives us specific information about the minimum performance of the filter in relation to a specific particle size and airstream velocity. If we know the contaminant that needs to be removed we can determine particle size. The MERV charts can then be used to determine exactly which filter will remove this particle size.
Filters come in a range of depths so it is important to determine the depth that is best suited for the job. An increase in the depth of the filter usually provides less pressure drop and a longer service life. Filter efficiency may be increased without increasing pressure drop by changing to a filter with a greater depth. You may choose a filter depth based on the service cycle of the equipment. You may not want to replace filters monthly if you service the facility on a quarterly basis.
You must consider the capability or capacity of the machine. Be sure that the machine is rated to handle your filter selection, especially when changing to a different filter system or increasing the level of filtration. Be certain that your filter selection will install into the existing system or review the modifications that may be necessary. Also, review any special conditions that may effect filter selection such as operating in a wet environment.
Carefully consider the price of the filter. While, initially, some filter choices may seem more expensive they may actually be more economical over the life of the filter. Consider all the operating costs of the filter system such as installation labor, disposal, and operating and maintenance costs of the HVAC unit.
Also, be sure that you know any requirements or regulations that may exist for your application. While ASHRAE 62.2 recommends a minimum MERV 6 rated filter for commercial spaces most environments do not have specific requirements. However, some environments such as hospitals, food and drug processes, paint booths and clean rooms do. Be sure you know what, if any, requirements exist for your application.
You may find situations which require the removal of gaseous contaminants. Often these are situations where odorous gases are causing complaints within the conditioned space. Generally this requires a three step approach. First attempt to remove the source of the odorous gases, next ventilate and finally use filtration designed to remove these contaminants. There are also environments such as museums, art galleries and libraries where the removal of gaseous elements is required to protect the building contents or other areas where specific elements must be removed to protect the occupants. When you are dealing with gaseous contaminants it is important to understand the requirements and consult with an expert in their removal.
This should give you a guide to filter selection but cannot address every situation. When you have a question do not hesitate to call your Certified Air Filtration Specialist. The CAFS designation is your assurance that a high level of proficiency and knowledge has been obtained and demonstrated.