While we understand there are many nuances to airflow design and unique considerations based on the type of facility, layout of the facility and what is being produced in it, some of the common considerations that apply to most facilities include:
- Air Distribution
Filters are designed to remove contaminants from the air stream. These contaminates can come from the outside air (pollen, dust, products of combustion) brought in for ventilation purposes or it can come from inside the space generated by the occupants or through the production process (powders, textile fibers, cooking oil). There are many types of filters depending on what is being removed. For particulates, filters are typically made of some kind of fibrous material that the air has to pass through and the particles are bonded to the filter media. Filtration of gases can be much harder and in some cases cannot be practically filtered and are merely diluted. Carbon (like charcoal) is commonly used to remove gases, and there are other technologies such as ionic filters that can handle gases as well.
Particulate control is the most common need for filters. In many facilities, there are product quality assurance (QA) staff that will determine the amount of filtration required in each area. Each range of particulates are associated with different potential contaminates (e.g., pollen, textile fibers, mold) and knowing what contaminants are most likely and which will cause product issues, the QA staff can determine the specifications for the filtration system.
Sometimes the user’s QA team will specify a minimum efficiency of the filters and air exchange rate to be used. The most common filter rating system in the U.S. is MERV (minimum efficiency reporting value). MERV ratings represent an efficiency of removing particles of various sizes (e.g., from 1.0 – 3.0 microns, or 0.3 to 1.0 microns). Air exchange rates, sometimes referred to as air changes per hour, determine how often the air in the space passes the filters.
In other cases, the concentration of particles of certain size (e.g., less than 100 parts per million of particles greater than 3 microns) may be specified. In this case, the filter efficiency and air exchange rate of each space has to be determined from calculations and experience in similar facilities.
Evenly distributed air, as mentioned in Part 1 of this series, is necessary to assure that all of the air in the space is being swept and all the particulates have an opportunity to be picked up by the return air systems. It is the return air system that brings all the indoor air and particulates to the filter banks. If you can’t get the particles to the filters, they will just keep circulating within the facility and concentrating.
Filtration for processing areas typically has a pre- and post-filter located in series within the air flow. The pre-filter has a MERV rating much lower than the final filters. This acts as a sacrificial filter to increase the life expectancy of the final filter and reduce the maintenance cost of frequent final filter changes.
In some applications UV lights are used to further clean the air and provide the cleanliness required for the application.
Did you miss Part 1? Read it now. Stay tuned for Part 3 as we explore pressurization.
Would you like to discuss all of the airflow design considerations now? Contact our expert, Frank Mangin at email@example.com.