Points for attention in purification engineering

The airflow speed/number of air changes in the clean room has always been a concern in clean room design. With the increase in the control effect of clean room pollution sources and the improvement of the efficiency of the final filter, the Whether the recommended or reference value is conservative has been discussed a lot; the noise, damage and maintenance that people worry about in the application of FFU have been solved in practice. A hot spot: the control of suspended molecular contamination (AMC) has been increasingly put on the agenda in the microelectronics and IC industries, and has received attention. The following is a summary and analysis of these issues. The determination of the airflow velocity under a certain cleanliness in a dust-free clean room varies with specific conditions such as the use of the cleanroom. It is not only affected by the amount of indoor dust and filter efficiency, but also by other factors. For industrial cleanrooms, it affects the cleanliness. The main factors for selecting the airflow speed are: (1) Indoor pollution sources: building components, number of personnel and operating activities, process equipment, process materials and process processing itself are all sources of dust particle release, which vary according to specific circumstances and vary greatly. (2) Indoor airflow pattern and distribution: Unidirectional flow requires uniform and equal streamlines, but will be disturbed by process equipment layout and position changes and personnel activities to form local eddy currents; non-unidirectional flow requires charging Mixing to avoid dead angle and temperature stratification; (3) Control requirements for self-cleaning time (recovery time): accidental release or introduction of pollutants in the clean room or interruption of air flow or intermittent convective air flow or people and equipment during normal operation The movement of the air conditioner will cause the deterioration of the cleanliness, and the self-cleaning time to restore the original cleanliness depends on the airflow speed; the control requirements for the self-cleaning time depend on the quality of product production and the finished product within this time frame (under the deteriorated cleanliness). (4) Efficiency of the final filter: under a certain amount of indoor dust, a filter can be used to reduce the air flow rate; for energy saving, a filter should be considered and the air flow rate should be reduced, or a higher air flow rate should be used. Low-efficiency filters and high airflow velocity are used to ensure that the product of flow and resistance is small; (5) Economic considerations: Excessive airflow velocity increases investment and operating costs, and the appropriate airflow velocity is the above factors. Reasonable synthesis, too large is often unnecessary and not necessarily effective; (6) For clean rooms with low cleanliness requirements, sometimes the number of air changes is determined by the requirements of indoor heat removal. Application of FFU system FFU has proven to be worry-free in terms of service life and maintenance. The main improvements at present are: (1) Take measures to equalize current and reduce noise, and the noise can be within 50db; (2) The motor adopts DC/EC (electronic rectifier motor) to save nearly 50% of power consumption compared with the original AC motor, because AC motors with small capacity (power < 1/2HP) used in small fans are generally capacitor split-phase or hidden-pole type, and their efficiency is only about 40%, while the efficiency of DC/EC motors can reach 75-80%; In the speed regulation control, each unit can be individually controlled by the filter to reduce the pressure to save energy consumption, but the current investment recovery period is still long and not widely used. Generally, group control or all group control is commonly used. (3) However, the static pressure at the FFU outlet should not be too large. Generally, the outlet wind speed is 0.38m/s. At this time, the static pressure is generally within 250Pa. Advantages of FFU return air system compared with other methods: (1) Great flexibility, easy to transform; (2) Occupies less building space; (3) The air pressure in the clean room is greater than that in the return air static pressure chamber, eliminating the static pressure the potential for cleanroom contamination. 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 in total, and The maximum resistance during operation is within 250Pa. Therefore, the heat transfer area of the dry surface cooler should be larger, the size of the return air duct should be larger, and the resistance of the porous floor and grille should be smaller. The resistance is within 15Pa, otherwise it is necessary to add a pressurized fan system, which reduces the comprehensive advantages of the FFU system. (2) After using DC/EC motor, the energy consumption per unit air volume may be lower than that of the current centralized system of general large-scale centrifugal fans, but some studies have pointed out that the energy consumption of the return air system using the improved large-scale axial flow fan is lower than that of the return air system using the improved large-scale axial fan. Still higher. Therefore, it is necessary to pay attention to the factors that increase the efficiency of large axial fans and reduce the resistance of the system. (3) Generally, the FFU system has a large energy consumption per unit of air volume, so the cooling load of the clean room also increases accordingly. Evaluation under specific circumstances (1) When FFU is used to transform old buildings into clean rooms, its comprehensive economy is generally desirable. (2) In a clean room with strict cleanliness requirements, when the full coverage rate of the final filter is 100%, it is still uneconomical to use FFU for large systems; it is meaningful to make specific comparisons for small systems.