How to Configure Pre, Medium & HEPA Filters for Cleanrooms

3-Stage Air Filter Matching Guide

How should filters be configured for Class 10,000 and Class 100,000 cleanrooms? Is it safer to simply stack as many filter stages as possible? A common misconception regarding this issue is treating filter configuration as “piling on components”—assuming that more stages are better and that higher-grade filters are always superior. In actual engineering practice, both extremes are seen: in Class 100,000 cleanrooms, H14 filters are installed at the end of the system while the pre-filter is omitted, resulting in the high-efficiency filters becoming clogged within six months; conversely, there are cases where three layers of medium-efficiency filters are stacked at the front end, but the operator is reluctant to install high-efficiency filters at the end, making it impossible to maintain the required cleanliness level no matter how adjustments are made. The logic behind a three-stage filtration system is simple: relay filtration. Each stage has two primary tasks: to capture the particles it is designed to capture, and to protect the next stage.

What does each of the three levels do?

The pre-filter (coarse filter) serves as the first line of defense. It traps large particles—dust, hair, and fibers—that are visible to the naked eye. Its value lies not in its filtration efficiency, but in its low cost, high dust-holding capacity, and the fact that it can be replaced or washed. If the pre-filter is missing or not replaced for a long time, large particles will bypass it and hit the medium-efficiency filter directly, rapidly shortening its lifespan—saving a little money but costing a lot in the long run.

The medium-efficiency filter is the primary interception stage. It is primarily responsible for capturing particles 0.5 μm and larger. Its real purpose is to protect the high-efficiency filter at the end of the system—since high-efficiency filters are expensive, have high pressure drop, and are difficult to replace, if the medium-efficiency filter is properly configured, the high-efficiency filter won’t need to be replaced for years; skimping on the medium-efficiency filter is equivalent to using the high-efficiency filter as a primary filter.

High efficiency depends on the terminal filter. It ultimately determines the cleanliness level: whether the air supplied to the room is clean or not comes down to this final stage. The Chinese national standard GB/T 13554-2020 classifies HEPA filters into three efficiency levels: 35, 40, and 45 (ultra-high-efficiency filters are further divided into six levels ranging from 50 to 75). Structurally, they are categorized as either pleated or non-pleated. The letter-based classifications commonly used in the industry, such as H13 and H14, actually originate from the European standard EN 1822 and do not follow the notation used in the Chinese national standard.

Several classification systems often appear mixed together on quotes and drawings, but if you refer to this table, you won’t get confused:

Please note that the last two classification systems are based on different test methods. In the industry, statements such as “H13 roughly corresponds to which grade in the national standard” are based on empirical correlations and should not be treated as strict equivalencies—when writing technical documents, clearly specify which standard you are referencing.

A quick tip to remember the division of labor: The efficiency of the primary filter determines how long the high-efficiency filter will last, and the high-efficiency filter determines how high the cleanliness level can be.

The signal for replacing the filter element is based on industry best practices, not a mandatory standard—the final resistance varies by manufacturer, so always refer to your filter’s nameplate and the manufacturer’s documentation.

When it comes to filters, more layers do not necessarily mean better performance; there are three key factors to consider here.

• The first consideration is system resistance. As filters are connected in series, system resistance accumulates accordingly. In a three-stage filtration system consisting of pre-filters, medium-efficiency filters, and high-efficiency filters, the filtration section often consumes the vast majority of the fan’s total pressure. Blindly adding an extra stage of medium-efficiency filtration will lead to a long-term increase in the fan’s energy consumption, yet will yield virtually no improvement in filtration performance—since the vast majority of large-particle contaminants have already been intercepted by the pre-filters.

• The second consideration is filter lifespan. If filters of an excessively high grade are indiscriminately selected for installation upstream of the HEPA filter or at the system’s outlet, it is equivalent to using expensive filters to handle coarse particulate contaminants. The purchase cost of a HEPA filter is more than ten times that of a medium-efficiency filter, and its dust-holding capacity is much lower; if used to trap large quantities of coarse impurities, the pressure drop will reach the replacement threshold within just a few months, resulting in premature filter failure.

• The third consideration is the actual operational requirements. The cleanliness class of a cleanroom is determined by the filtration efficiency of the terminal filter and the operating airflow rate, not by the number of filter stages installed. Once the filtration level meets process requirements, adding additional filters will not result in a detectable improvement in particle concentration, and the final cleanroom acceptance data will remain unchanged.

How is it configured in actual engineering projects?

The type of terminal unit is determined by the cleanliness class: It is standard practice to install HEPA filters in terminal units for high-class cleanrooms (ISO 7 and above); For hospital cleanrooms, refer to Design Manual 22K505—for Class II, III, and IV cleanrooms, terminal supply air outlets should be equipped with sub-HEPA (YG-class) or HEPA filters; exhaust air from positive-pressure operating rooms should use medium-to-high-efficiency filters, while exhaust air from negative-pressure operating rooms should use HEPA filters. Understanding this hierarchy makes it clear: HEPA filters are not used everywhere; instead, filter grades are assigned based on the actual needs of each room.

For the fresh air side, it depends on the local atmospheric dust concentration. According to 22K505, the number of filters—whether one, two, or three in series—is selected based on the ambient air particulate matter concentration: in areas with good air quality, a single medium-efficiency filter is sufficient, while heavily polluted areas require a three-stage configuration of “pre-filter + medium-efficiency filter + higher-efficiency filter.” There are several other common configuration points in air purification and conditioning systems: install a medium-efficiency filter (Class Z2, with a particle counting efficiency of at least 40% for particles ≥0.5 μm) in the positive pressure section of the recirculation unit for pre-filtration, and install a medium-efficiency filter (Class Z3, with a particle counting efficiency of at least 60%) at the return air grille.

The 22K505 standard provides the complete specifications for hospital cleanrooms; those working on medical projects can refer to it directly:

Non-medical industrial cleanrooms do not have such detailed specifications. The standard practice is to use “pre-filters plus medium-efficiency filters” at the air intake to extend service life, and to install high-efficiency filters at the outlet according to the cleanliness class—the logic is the same; don’t treat this as a set of standard provisions to be followed rigidly. As a final note regarding the table: When high-efficiency filters are operated at their rated airflow, both pressure drop and efficiency decline occur much more rapidly. Leaving a margin in airflow capacity during selection is far more effective than switching to a higher-grade filter media.