Unidirectional flow clean room are generally of two types, namely horizontal flow and vertical flow. In a horizontal flow system, the airflow flows from one wall to the other. In a vertical flow system, the air flows from the ceiling to the ground. For situations requiring lower concentrations of suspended particles or microbial concentrations in clean rooms, unidirectional airflow is used. Previously called this kind of clean room "laminar flow" clean room. The name of unidirectional flow and laminar flow are explained the situation: the air flow in one direction (either vertical or horizontal), and in general is 0.3 M / s to 0.5 m / S (60 ft / min to 100 ft / min) of the average speed through the entire space. The HEPA filter with clean room ceiling to indoor air. The airflow is like an air piston that flows down through the chamber, removing contaminants and then discharging from the ground. Also, after mixing with some fresh air from the outside, recycle to the HEPA filter. Suspended contamination generated by personnel and processes can be immediately removed by this air, while the turbulent ventilation system uses the principle of mixing and dilution. In an empty room without any obstructions, unidirectional flow can quickly remove pollutants, much less than the wind velocity previously mentioned. But in an operating room, the machines and the people moving around the machine form obstacles to the airflow. Obstacles can cause turbulence to flow in one direction and form a mass of air around the obstruction. The activities of the people can also make one-way turbulence into turbulence. In these turbulence, because of the low wind speed, the air dilution degree is smaller, thus causes the pollution concentration to be higher. Therefore, the wind speed must be maintained at 0.3 M / s to 0.5 m / S (60 ft / min to 100 ft / min) range, so that the one-way flow interruption can be quickly recovered, fully diluted turbulent zone around the obstacles of pollution. The wind speed can correctly represent a unidirectional flow, because the higher the wind speed, the cleaner the room. The hourly ventilation rate is related to the volume of the room, such as the height of the ceiling, so it is not suitable for one-way flow. The airflow rate in a one-way flow chamber is many times (10 to 100 times) of the turbulence chamber.