The selection of air filters is generally based on the following criteria.
I. Filter Efficiency of Air Filters

The "filter efficiency" of an air filter is the ratio of the amount of dust captured to the original dust content of the air: Filter efficiency = Filtered dust amount / Upstream dust content = 1 - Downstream dust content / Upstream dust content. The meaning of the efficiency seems simple, but its value and significance vary depending on the testing method.
In determining the filter efficiency, the meaning of the dust amount is diverse, and the resulting filter efficiency values calculated and measured are also varied.
When the same filter is tested using different methods, the efficiency values obtained will be different. Different countries and manufacturers do not use the same testing methods, and the interpretation and expression of filter efficiency vary greatly.

II. Types of Current Filter (High Efficiency) Efficiency Marking

1. Sodium Flame Method, mainly used by some domestic manufacturers. Some small individual businesses may also mark this efficiency value without testing equipment. Generally, the marked efficiency value of an efficient filter is greater than 99.99%. The prospect of this method: The testing accuracy of the sodium flame method determines that this method can only complete the testing of generally efficient filters. The tested efficiency is the comprehensive efficiency of the filter and cannot reflect the efficiency for specific particles. Although the country is revising this standard, personally, I believe that if a filter manufacturer only has this one testing method, it will definitely be difficult to meet the increasingly diverse and high-standard requirements of customers.
2. DOP Method, with a history of over 50 years, was once the most commonly used marking method in the world.
3. MPPS Method, also known as the most easily penetrated efficiency method. It is considered the most strict method for testing high-efficiency air filters.

III. Airflow Rate and Filtration Rate of High-Efficiency Filters

Airflow rate and filtration rate can reflect the ability of the filter to pass air volume. Airflow rate refers to the speed of the airflow passing through the filter surface, generally expressed in m/s, V = Q/F * 3600. Airflow rate is an important parameter reflecting the structural characteristics of the filter. Filtration rate refers to the speed of the airflow passing through the filter material area, generally expressed in L/cm2.min or cm/s. Filtration rate reflects the through capacity of the filter material and the filtering performance. A lower filtration rate can achieve higher efficiency. The lower the allowable filtration rate, the greater the resistance of the filter material. 

IV. Penetration Rate of Filters

That is, initial resistance. Penetration rate is used to indicate the degree to which dust particles can still pass through after passing through the filter. K = (1 - η) × 100%. Resistance. The resistance of the filter is mainly composed of two parts, the resistance of the filter material and the resistance of the filter structure. According to experience, when the resistance of the filter material is determined, the resistance of the filter is closely related to the structure of the filter. When the filter accumulates dust and the resistance increases to a certain specified value, the filter is scrapped. The resistance of a new filter is called "initial resistance"; the resistance value corresponding to the scrap of the filter is called "final resistance". When designing, a representative resistance value is often needed to calculate the design air volume of the system. This resistance value is called "design resistance", and the commonly used method is to take the average of the initial resistance and the final resistance.

V. Dust Capacity

Dust capacity: In simple terms: the ability of the filter to hold dust. The dust capacity of the filter is directly related to the service life of the filter. Generally, it refers to the value when the final resistance of the filter reaches one times its initial resistance, or when the efficiency drops below 85% of the initial efficiency. The accumulated dust on the filter is taken as the standard dust capacity of the filter, abbreviated as dust capacity. If the filter operates with a wind volume exceeding the rated wind volume, its resistance will increase faster as the accumulated dust increases. During the dust collection process where the filter reaches its dust holding capacity, the less efficient filters tend to show a characteristic where their efficiency first increases and then decreases. The less efficient filters collect a large amount of dust and have a sparse filter material. Due to the increase in resistance, the dust particles are prone to penetrate the filter material and fall off, causing secondary pollution. For the high-efficiency filters in use, as the dust accumulation increases, their efficiency generally rises. When the air volume is 1000 m³/h, the dust holding capacity of the generally folded non-woven fabric filter is around 100g, the glass fiber filter is between 250-300g, and the high-efficiency filter is around 400-500g. For similar filters with different sizes, the dust holding capacity will also vary.

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