Even if the clean room pressure difference is qualified, external air may still enter, because the fresh air itself can carry molecular contamination, and door opening and dynamic air flow may also cause input. Differential pressure manages air flow and cannot replace gas processing and detection. The differential pressure gauge only controls 'where the wind blows', but not 'what the wind carries'.

The pressure difference controls the flow direction, not the composition.

The function of positive pressure in the clean room is to allow air to flow outward through door cracks and gaps to reduce the infiltration of outside air. But positive pressure cannot prevent the fresh air system from actively bringing in outside air. When fresh air enters the clean room through the air-conditioning box, if the gas-phase filter section is insufficiently configured or the media does not correspond, external acidic gases, organic emissions, or gases from nearby pollution sources will enter the room with the fresh air. A qualified pressure difference indicates that the flow direction is correct, but does not indicate that the composition of the incoming air has met the process requirements. These two things are taken care of by airflow organization and filtration configuration respectively.

Fresh air is the main input channel for external air

Clean rooms need to continuously replenish fresh air, which comes directly from the outdoors or near the air intake. If the air intake is close to traffic arteries, industrial emission sources or building exhaust outlets, the molecular pollution load in the fresh air may be higher. Particle filters can intercept aerosol particles, but have limited effect on gas molecules and require chemical filtration section treatment. Whether the fresh air chemical filtration configuration covers the main local gas objects determines whether the external gas can be effectively reduced. For configuration ideas of the fresh air filter section, please refer toDoes the chemical filter section need to be reselected when the fresh air volume changes seasonally?。

Door opening and dynamic conditions will cause short-term input

Qualified pressure difference usually refers to the average pressure difference under steady state. At the moment of opening the door and when the exhaust air fluctuates, the local pressure difference may drop or even reverse. Personnel entry and exit, material transfer, and exhaust equipment starting and stopping may allow outside air to enter in a short period of time. Such inputs are not necessarily evident in differential pressure records but may create short-term concentration peaks in critical areas. If the critical area is close to the door area or transfer port, the impact of dynamic input should be determined based on the door opening record and concentration period. For inspection ideas on the dynamic working conditions of the door area, please refer toWill more people coming in and out affect gaseous pollution control?。

Adjacent areas may also be input sources

If there are cleaning, coating or other release sources around the clean room, air from adjacent areas may still enter through gaps, ducts or shared return air, even if the overall room maintains positive pressure. This type of input does not necessarily go through the fresh air system, and the differential pressure meter may not reflect it. During the investigation, the concentration and time period changes in the room and adjacent areas should be compared to confirm whether there is cross-area transmission. If cross-zone paths are obvious, improving partitioning, pressure gradients, or local exhaust may be more straightforward than increasing room filtration.

Aili industrial supporting equipment

For projects with qualified pressure difference but external air entering, we usually recommend that customers check the three paths of fresh air, door area and adjacent area separately, and then determine which section should be covered by the filter section. Aili Industry can provide selection packages for box-type, flat-plate and V-type chemical filtration products. For box-type MAU packages, please refer toMAU box type chemical filter , product series can be viewedChemical filter product range . The V-shaped structure provides a larger filtration area in the same box space, which helps control surface speed and resistance when the fresh air volume changes. Device-side applications can useChemical filter FFU type, integrated chemical adsorption layer withHEPA/ULPA filter particle filtration, circulation section or local protection position can be configured according to actual air flow organization. The manufacturing end covers independent R&D and production of motors and fans, control systems, sheet metal and complete machine assembly. The fresh air path, door area pressure difference and filter section position can be checked in the same package, and non-standard air paths can be adjusted according to on-site conditions. The specific gas effect, applicable environment and replacement cycle still need to be verified based on the corresponding configuration data and on-site working conditions. The actual results are subject to on-site leak detection and gas detection results.

Related questions and answers

If the pressure difference is qualified, do I still need to measure the external gas?
Recommended to test. Differential pressure reflects flow direction, not composition. Fresh air may carry molecular pollution, and dynamic working conditions in the door area may also cause short-term input, which should be judged based on target gas detection.

Fresh air has a particle filter, why does it need chemical filtration?
Particle filters intercept aerosol particles and have limited effect on gas molecules. Acidic gases, organic vapors, etc. need to be processed by chemical filtration sections, and the tasks of the two are different.

Can Aili Industrial help check the external gas input to the clean room?
It can help determine the input path and filtering configuration based on the fresh air path, door area conditions and adjacent area conditions. The specific plan needs to be confirmed based on product data and on-site working conditions.

Related articles

• How to distinguish whether workshop pollution comes from outside or inside?

• The indoor odor is different from the fresh air entrance odor. How should I investigate?

• Will more people coming in and out affect gaseous pollution control?

• How should gaseous contamination be prevented from crossing into the wafer transfer area?

Key points for selection and acceptance inspection