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I. Main Parameters 1. Rated Airflow: The volume of air that passes through the filter per unit of time. In China, this is generally expressed in units of m3/h, while abroad it is indicated using the imperial unit cfm (cubic feet per minute), or ft3/min. The conversion between the two units is as follows: 1 cfm ≈ 1.7 m3/h. Of course, we hope that, for the same windward cross-section, the filter will be able to handle as much air flow as possible, which indicates a higher filtering capacity; however, this is mainly determined by its filtering material and structure. 2. Surface velocity and filter velocity: Velocity refers to the speed of the airflow across the windward cross-section, and is generally expressed in m/s ； Surface velocity = Air volume ÷ Frontal area. Filtration velocity refers to the speed of the air flow passing over the filtering material, and is generally expressed in cm/s ； Filtering speed = Air volume ÷ Filtering area. The filtering area refers to the area of the filtering material used; only when the filtering material is flat and perpendicular to the direction of the airflow does the cross-sectional area facing the airflow in the filter equal the filtering area. However, the filtering materials used in vacuum cleaners are often shaped by folding them. Abroad, imperial units for wind speed are also used: fpm (feet per minute), i.e., feet per minute; 1000 fpm ≈ 5.08 m/s. 3. Filtration efficiency: 1) Definition: Filtration efficiency = amount of dust captured by the filter ÷ dust content in the air upstream from the filter = (dust content in air upstream – dust content in air downstream) ÷ dust content in air upstream. Although the definition of efficiency seems simple, its meaning and value are closely related to the testing method used; efficiency values obtained through different testing methods cannot be directly compared. Therefore, when discussing efficiency, it is necessary to specify which testing method was used. Abroad, the filtration performance is sometimes expressed using terms such as Penetration or breakthrough rate; Penetration = 1 – filtration efficiency. 2) Specifications: a) Chinese specifications: Coarse, Medium, High Medium, Sub-HEPA, HEPA. b) European specifications: Use four grades – G, F, H, U; American specifications: Use five grades – C, L, M, H, U. a) The Arrestance test is conducted by weighing the dust collected; the dust used in this test is of large particle size and high concentration. According to American standards, “road dust” from the desert areas of Arizona is used, while Japanese standards specify the use of “sub-clay” from the Kanto region in Japan. Chinese standards call for the use of dust from the Loess Plateau. By adding a specified amount of fine carbon black and short fibers to the specified dust, a dust source for testing is obtained. This method measures the weight of dust passing through the filter, or the ratio of dust collected on the filter to the total amount of dust, thereby allowing the filtration efficiency to be calculated using a weight-based approach. The weight-based method is suitable for measuring low-efficiency filters, which are generally used as primary filters in central air conditioning systems. b. Colorimetric method for Dust spot: a traditional testing method originating in the United States, not used in China. Efficiency is calculated by measuring the amount of light flux on the high-efficiency filter paper at the sampling point. Europe has officially abolished this method. C. Atmospheric dust counting method – Chinese standard: The dust source is “atmospheric dust” in the natural atmosphere. The “amount” of dust refers to the total number of particulate matter particles with a diameter greater than or equal to a certain value. The instrument used to measure dust is a conventional optical or laser dust particle counter. The efficiency value is the initial efficiency of the new filter. The atmospheric dust counting method is used to measure filters for general ventilation. Its efficiency value only represents the performance of the new filter. China’s efficiency classification is based on the atmospheric dust counting method. China’s counting standards predate those in Europe and the United States, but the methods used are relatively rudimentary; they are based on domestic counters and the corresponding measurement technologies available in the 1980s. Standard: GB12218-89. d. American counting method for Particle Efficiency, ASHRAE 52.2 – used in the United States. The test bench is similar to those used in the gravimetric and colorimetric methods, and the high-concentration test dust is also of the same type as that used in those methods. 3) The DOP method originated in the United States and was once widely used internationally. The test dust source consists of 0.3 μm monodisperse DOP (dioctyl phthalate, a plastic industry common plasticizer) droplets. ““The amount” refers to the degree of turbidity of the air containing DOP. The instrument used to measure dust is a photometer. The filtration efficiency of the filter for DOP particles is determined by the difference in turbidity of the gas samples. The DOP liquid is heated to produce steam, which condenses under specific conditions into tiny droplets of about 0.3μm in size; this mist-like DOP then enters the air ducts. Measure the turbidity of the air samples before and after the filter, and use this to determine the filter’s efficiency in filtering 0.3μm dust particles. The DOP method has a history of over 50 years, and it was once the most commonly used method internationally for measuring high-efficiency filters. In the early days, it was believed that filters had the greatest difficulty filtering dust particles of 0.3 μm; therefore, it was specified to use 0.3 μm dust for testing high-efficiency filters. DOP contains a benzene ring, and it is suspected to be carcinogenic; as a result, many laboratories have switched to alternatives with similar properties but without a benzene ring, such as DOS, yet the testing method is still referred to as the “DOP method”. The DOP method for measuring high-efficiency filters is also known as the “thermal DOP method”. The corresponding “cold DOP” refers to the use of compressed air in the nozzle to create bubbles in the DOP liquid, resulting in mist-like artificial dust through splashing. Relevant standard: U.S. military standard MIL-STD-282. 4) MPPS method (Most Penetratiable Particulate Size): Used in Europe, and also in the United States except in the defense industry; it is adopted by some companies in China as well. It can measure almost all HEPA and ULPA filters, showing a strong trend toward replacing traditional methods. It was once believed that filters had the lowest filtration efficiency for 0.3μm dust; therefore, when evaluating high-efficiency filters, their filtration efficiency for 0.3μm dust is used as a representative value. There are many methods for generating and measuring 0.3μm dust, which gives rise to methods such as DOP, oil mist, and sodium flame. Through research and experimentation, it was found that by using a laser particle counter capable of detecting dust particles with a size of 0.1 μm, it is possible to not only measure the dust concentration but also the size of each individual dust particle. This makes it easy to determine the value corresponding to the lowest efficiency; the dust particle size associated with this value is known as MPPS (Most Penetratiable Particulate Size). The efficiency value corresponding to MPPS is referred to as the MPPS efficiency. Relevant standards: American IEST-RP-CC-007.1-1992, European EN 1822-5:2000 5) Resistance 1. Definition: When air flows through a filter, the filter exerts resistance on this airflow, resulting in a reduction in the volume of air flowing. 2. Components of resistance. The resistance of a filter consists of the resistance inherent in the filtering material itself and the structural resistance of the filter. It is desirable for the filter to have as low resistance as possible, because the higher the resistance, the greater the power required from the motor to achieve the desired air flow rate; if the motor’s power is insufficient, humming noises may occur. All these issues need to be addressed properly by selecting appropriate filtering materials. 3. Initial resistance: The resistance of a new filter is referred to as its initial resistance. 4. Final resistance: The resistance of a filter at the point when it becomes unusable is called its final resistance. The value of this final resistance has a direct impact on the filter’s service life, the range of possible air flow rates, and the energy consumption of the system. 5. Unit of resistance: Pascal, abbreviated as Pa. 1 Pa = 1 N/m2. In the imperial system, it is expressed in inches of water column. In the metric system, it is expressed in millimeters of water column; 1 mmH2O = 9.8 Pa, while 1 inch of water column = 250 Pa