Unidirectional flow clean room testing mechanism (clean room testing)

In the clean room, from the air supply port to the return air port, the cross section of the air flow along the way has almost no change. With the pressure and flow equalization effect of the inlet static pressure box and the filter, the flow velocity on the whole room cross section is relatively uniform, and at least in the working area, the streamlines are unidirectionally parallel and there is no eddy current. These are the three major characteristics of the unidirectional flow clean room. The unidirectional parallelism of the streamlines here means that the time-averaged streamlines are parallel to each other and have a single direction.

In a unidirectional flow cleanroom, airflow is turbulent, differing from the laminar flow concept in fluid mechanics. The term "laminar flow cleanroom" is misleading, as noted by various standards, including the British standard BS-5295 and the German standard VDI-2083.

Clean airflow fills the entire room, pushing indoor dirty air outside rather than relying on mixing. This process is likened to "piston flow" or "push flow," where clean air acts as a piston, ensuring dust particles move forward without returning.

However, reverse airflow can occur along walls and filter overlaps, disrupting the intended flow and introducing external pollutants. Effective design should minimize the filter frame's area and align walls closely with the filter's useful cross-section to maintain air purity.

After the above analysis, it can be considered that there are two important prerequisites to ensure the characteristics of a unidirectional cleanroom (high cleanliness and rapid self-cleaning recovery ability): 

① the cleanliness of the incoming flow; 

② the piston flow state of the incoming flow.

As for the cleanliness of the incoming flow, it is not a problem for a unidirectional cleanroom that supplies air through a filter with high efficiency, but the "piston flow" state of the incoming flow needs further analysis.

According to the principles of fluid mechanics, the incoming flow conditions will have a direct and important impact on the future flow conditions. The airflow at the outlet of the air supply surface is the incoming flow of the working area.

If the turbulence of the incoming flow is large, it will affect the subsequent unidirectional flow characteristics; if the incoming flow does not fill the active cross-section, it will affect whether the "piston flow" can be formed later and how fast it is formed; and the fact that the incoming flow does not fill the active cross-section is also a factor in the flow movement. 

Therefore, under the condition of selecting a filter, the necessary condition to ensure the characteristics of a unidirectional cleanroom is "piston flow", and the necessary condition for "piston flow" is that the incoming flow fills the active cross-section. However, it is unrealistic to let the airflow completely fill the active cross section from beginning to end for a room rather than a section of pipe.

Initially, unidirectional parallel airflow was thought to fill entire clean rooms, but this approach proved impractical. Now, it’s understood that unidirectional flow can occupy specific areas, allowing for controlled airflow in the working space, enhancing cleanliness.