Can the cleanliness level and HEPA level correspond one to one?

This question often arises in project communication: Which filter level does ISO 7 correspond to, and does ISO 5 need to be equipped with H14?

The assumption for this question is that there is a conversion table between the two sets of levels. Actually no. The cleanliness level describes the state of the space, and the HEPA level describes the product performance. The two evaluation objects are different, separated by a whole set of system factors such as air supply volume, airflow organization, sealing and operation management.

Treating them as a one-to-one correspondence can easily lead to problems in two directions: when selecting, infer the filter element grade according to the room grade, and when accepting, use the filter element report to explain the room results.

What are the two sets of levels talking about?

Cleanliness levels (such as ISO 5, ISO 7, and ISO 8 in ISO 14644) evaluate the number of particles in a unit volume of air, and the test must be conducted under specified states, measuring points, and sampling conditions.

HEPA grade (H13, H14, U15, etc.) evaluates the filtration efficiency of the filter for particles of a specific particle size. The test has specified air volume, dust generation method and detection means.

One is the space acceptance index, and the other is the product factory index. The same HEPA grade product installed in different systems may produce completely different room results; the same standard clean room may use different filter grades.

What variables are in between?

Assuming that the target of a room is a certain cleanliness level, at least these layers must be passed from the filter to the room result:

Air supply volume. Particles are controlled by dilution and removal. If the air volume is insufficient, no matter how good the filter element is, it will not reach the target. The number of air changes and unidirectional flow coverage must be calculated during the design process.

Airflow organization. Whether the air supply surface coverage is complete, whether the return air path is smooth, and whether there are eddies and short circuits directly determines whether the particles can be effectively taken away.

Sealed condition. Bypass air leakage at the end, box, and pipeline will allow unfiltered air to bypass the filter element. This kind of problem is common at the end interface. For the verification method of the liquid tank sealing structure, please refer to the relevant instructions on the knife edge size verification.

Pollution load under operating conditions. If the particles brought in and out by personnel, equipment, and materials exceed the dilution capacity of the system, static compliance does not mean dynamic compliance.

Maintenance status. After the filter element accumulates dust, the resistance increases and the air volume decreases. The performance after running for a period of time may be different from the initial state.

A problem with any one of these variables can cause room results to deviate from expectations without changing the filter grade itself.

Correct order when selecting

A safer approach is to work in reverse: first clarify the control requirements of the target area, then determine the air supply volume and air flow organization, then select the fan based on the total resistance of the air path, and finally determine the grade and structure of the filter element.

In this sequence, the filter element grade is the result, not the starting point. If you set the grade first and then configure the system, it is easy to have a mismatch where the grade is sufficient but the air volume is insufficient, or the air volume is sufficient but the sealing is not in place.

Renovation projects are a bit more complicated. The fans, air ducts, and ends of the original system are all fixed. When replacing the filter element, if the grade change brings about a change in resistance, you need to go back and recheck whether the fan operating point is sufficient. You cannot just see whether the filter element can be installed.

Acceptance data should be filed separately

It is recommended in the project document that the two types of requirements should be written separately and accepted separately:

Filter side: grade, applicable standards, rated air volume, test report and unit-by-unit number.

Room side: air volume, air flow organization, measuring point distribution, test status (static or dynamic) and cleanliness results.

In this way, when a problem occurs, it can be determined whether the product fails to meet the agreed indicators or the system operating status does not meet the design conditions. Mixed into one document, the boundaries of responsibilities are unclear and the investigation has no direction.

For such local areas inside semiconductor equipment, the evaluation object is the small environment within the equipment, which is different from the entire clean room. The test method and acceptance conditions need to be determined separately according to the equipment structure.

About Aili Industrial’s solution packages

Ai Li industrial production HEPA/ULPA air filter and matching FFU fan filter unit、EFU equipment-side fan filter unit and high-efficiency air supply outlet. Filters and air supply equipment are matched in the same system, and grade selection, air volume matching and interface sealing can be confirmed together during the planning stage.

For clean room projects, it is recommended to provide the target area, test status, air supply volume requirements and terminal interfaces together to facilitate the judgment of how to combine the filter and system more appropriately. For equipment-side applications, equipment drawings, available heights and maintenance directions are important inputs for solution confirmation. Specific performance is subject to product technical data and project acceptance results.