Unidirectional flow cleanrooms used to be called laminar flow cleanrooms or parallel flow cleanrooms, and have been officially called unidirectional flow cleanrooms since US Federal Standard 209C. The unidirectional flow cleanroom is defined as a cleanroom in which the air flow passes through the entire room section in a single direction along parallel flow lines at a uniform cross-sectional wind speed.

Unidirectional flow cleanroom relies on the "piston" squeezing action of the air supply to quickly discharge indoor pollutants. In the cleanroom, from the air supply port to the return air port, there is basically no change in the cross section of the air flow. The air supply, the static pressure box and the filter play the role of equalizing the pressure, the flow velocity on the cross-section of the whole chamber is relatively uniform, and the streamlines in the working area are parallel in one direction without eddy current.

In the Unidirectional flow cleanroom, the clean air flow is not one or several, but fills the whole section of the room, so this kind of cleanroom does not rely on mixing and dilution, but pushes out the polluted air in the room along the entire section to outdoor, so as to achieve the purpose of purifying indoor air. Therefore, some people call the airflow in the unidirectional flow cleanroom "plug flow", "push flow" and "squeezed weak air jet". Clean air is like an air piston, pushing forward (down) along the "cylinder" of the room, squeezing the original air with high dust concentration out of the room.

In order to ensure the realization of the "piston" effect, the important measure taken is to cover the ceiling or wall with filters. Since both the filter and the ceiling are framed, it is impossible to fill the filter with all over the place.

The fullness of the filter is measured by the fullness ratio. For the unidirectional flow cleanroom, assuming that the filter filling ratio reaches 100% (even without the frame), the entire height and section of the room are parallel unidirectional airflow without vortex areas. Ideally, the dust concentration in the room after the filter depends only on the filter supply air concentration. If the filter is not 100% full, but has a ratio (that is, the fullness ratio), there will be a vortex area at this time, and the dust concentration of the unidirectional flow cleanroom with different fullness ratio is different. Similarly, the dust concentration of the unidirectional flow cleanroom with different personnel density is also different, so the number of personnel in the unidirectional flow cleanroom should be properly controlled.