The air flow rate/ventilation times of clean room has always been a concern in clean room design. With the increase of the control effect of clean room pollution sources and the improvement of the efficiency of the final filter, there has been a lot of discussion on whether the recommended or reference values of relevant specifications and guidelines are conservative; FFU in the application of people's concerns about noise, damage maintenance and other problems have been solved in practice, with the continuous improvement of FFU, whether to use FFU return air system is also a hot spot: suspended molecular pollution (AMC) control in the microelectronics and IC industry has been increasingly mentioned on the agenda, attention. The following are the cases of these problems respectively summarized and analyzed.

The determination of air velocity under a certain degree of cleanliness in a clean room varies with the specific circumstances such as the use of a clean room. It is not only affected by the amount of indoor dust and the efficiency of the filter, but also by other factors. As far as industrial clean rooms are concerned, the factors affecting the cleanliness and selection of air velocity are mainly: (1) Indoor pollution sources: Building components, the number of personnel and operational activities, process equipment, process materials and process itself are all dust particle release sources, which vary greatly according to the specific situation; (2) Indoor air flow pattern and distribution: unidirectional flow requires a uniform and equal flow line, but it will be affected by the disturbance of process equipment layout and position changes and personnel activities to form local eddy currents; Non-unidirectional flow requires mixing to avoid dead corners and temperature stratification; (3) The control requirements of self-cleaning time (recovery time) : the interruption of pollutants or air flow released or brought into the clean room by the accident or the intermittent convection air flow or the movement of people and equipment during normal operation will cause the deterioration of cleanliness, and the self-cleaning time of restoring to the original cleanliness is determined by the air speed; The control requirement for the self-cleaning time depends on the ability to withstand the impact on the quality and yield of the product produced within this time frame (under the deterioration of cleanliness); (4) The efficiency of the final filter: under a certain amount of indoor dust, the filter can be used to reduce the airflow speed; In order to save energy, the filter should be considered and the airflow speed should be reduced, or the filter with lower efficiency and the airflow speed should be higher, so that the product of flow and resistance is small; (5) Economic considerations: too much air speed causes an increase in investment and operating costs, the appropriate air speed is a reasonable synthesis of the above factors, too much is often unnecessary, and may not be effective; (6) For clean rooms with low cleanliness requirements, sometimes the number of air exchanges is determined by the requirements of indoor heat exhaust.

Application of FFU system FFU has been proven to be no concern in terms of service life and maintenance. At present, its main improvements are: (1) to take measures to equalize flow and reduce noise, noise can be within 50db; (2) The motor uses DC/EC(electronic rectifier motor) to save nearly 50% compared with the original AC motor, because the small capacity (power <1/2HP) of the AC motor used by the small fan is generally capacitive phase separation or hidden pole type, and its efficiency is only about 40%, while the efficiency of the DC/EC motor can reach 75 ~ 80%; In the speed control, each individual can be controlled by the filter pressure reduction to save energy consumption, but the current investment recovery period is still long and not widely used, generally commonly used group group control or all group control. (3) However, the static pressure at the FFU outlet cannot be too large, and the outlet wind speed is generally 0.38m/s, at which time the static pressure is generally within 250Pa. Advantages of FFU return air system compared to other methods

Advantages: (1) flexibility, easy to transform; (2) Occupy less building space; (3) The air pressure in the clean room is greater than that in the return air plenum room, and the possibility of pollution of the plenum room to the clean room is excluded.

Disadvantages: (1) The total resistance of the return air duct (including the porous floor, grille and air duct), the resistance of the dry surface cooler and the resistance of the final filter (at the initial resistance) should be controlled at about 165Pa to meet the maximum resistance within 250Pa during operation. Therefore, the heat transfer area of the dry surface cooler should be larger, the size of the return air duct should be larger, the resistance of the porous floor and grille should be small, the general practice is: control the resistance of the dry surface cooler at about 50Pa, and the resistance of the return air duct is within 15Pa, otherwise it is necessary to add a pressurized fan system, which reduces the comprehensive advantages of the FFU system. (2) After the use of DC/EC motors, the energy consumption per unit of air volume may be lower than the current centralized system of the general large centrifugal fan, but it has been pointed out that the energy consumption is still higher than that of the return air system of the improved large axial fan. Therefore, it is necessary to pay attention to the factors that improve the efficiency of large axial flow fans and reduce the resistance of the system. (3) The general FFU system has a large energy consumption per unit air volume, so the cooling load of the clean room also increases accordingly.

Evaluation in specific cases (1) When FFU is used to transform old buildings into clean rooms, its comprehensive economy is generally preferable. (2) For clean rooms with strict cleanliness requirements, when the final filter is 100% full, FFU is still uneconomical for large systems; Make a specific comparison of the significance of small systems.