Filtration Terminology
Filter Specifications
Pressure drop: The resistance to air flow created by an air filter is an important consideration. The higher the resistance, the greater the energy required to overcome it. Excessive pressure drop will lead to excessive energy consumption. Consequently, all other considerations being equal, the filter with the lowest pressure drop (the lowest resistance to air flow) is preferred.
Efficiency: The most important consideration is the ability of a filter to remove from an air stream the greatest number of dust particles whose size is of most concern. This ability is described as efficacy .
Atmospheric dust holding capacity: The term "dust
holding capacity" is used in a variety of ways. As defined here,
it means the amount of atmospheric dust that a filter will capture
during its service life. If the amount of dust in the air that is being
filtered is constant, the filter with the highest atmospheric dust
holding capacity will have the longest life and must be serviced less
frequently. There are long-standing disagreements as to whether "dust
holding capacity" as defined in the ASHRAE test method using a
synthetic dust can be correlated with atmospheric dust holding capacity.
Capacity: This is the amount of air that a filter can handle. Usually capacity is defined as the volume of air per unit time that a clean filter can handle at a specified pressure drop. This volume is expressed in cfm (cubic feet of air per minute) or in SI metric as L/s (liters of air per second). Changing the amount of air being handled by a filter will affect other performance values such as pressure drop and dust holding capacity. It may also affect efficiency.
The DOP [dioctyl phthalate] test does not measure efficiency, rather it measures penetration, i.e., and the fraction of DOP that passes through the filter. Efficiency is determined by subtracting the penetration (percent) from 100%. The original maximum [HEPA] penetration was 0.5% so that the efficiency was 99.95% (100-0.05). Later improvements in media manufacture and materials allowed efficiency to be extended to 99.97% without any increase in pressure drop.
Source: NAFA Guide to Air Filtration