In order to obtain good clean effect, the clean room should not only focus on taking reasonable air conditioning purification measures, but also require the process, construction and other professional to take corresponding measures. Not only should there be reasonable design, but also careful construction and installation in line with the specifications, as well as the correct use of clean rooms and scientific maintenance and management. In FACT, IT IS DIFFICULT TO achieve ideal cooperation between different majors, and the designer is difficult to grasp the quality of construction and installation and use and management, especially the latter. In terms of clean room purification measures, many designers, or construction parties, often do not pay enough attention to the necessary conditions, resulting in unsatisfactory cleaning effect. This article only briefly discusses the four necessary conditions to achieve the cleanliness requirement in the clean room purification measures.

First, air supply cleanliness:

To ensure the air supply cleanliness meets the requirements, the key is the performance and installation of the final filter of the purification system.

The final filter of the purification system usually adopts HEPA filter or sub-HEPA filter. According to Chinese standards, the efficiency of HEPA filters is divided into four grades: class A is ≥99.9%, class B is ≥99.9%, class C is ≥99.999%, and class D is ≥99.999%(for ≥0.1μm particles) (also known as ultra-high efficiency filter); The sub-high efficiency filter is 95~99.9% (for ≥0.5μm particles). The more efficient the filter, the more expensive it is. So the choice of filter should be in line with both to meet the air supply cleanliness requirements, but also to consider economic and reasonable. Considering the cleanliness requirements, low-performance filters should be selected for low-grade clean rooms and high-performance filters should be selected for high-level clean rooms. Generally speaking :1 million grade can choose high efficiency filter; Less than 10,000 grade can choose sub-high efficiency or class A high efficiency filter; Class B filter is selected for 10,000 ~100 grade; Class C filters are selected for grades 100~1. Here each cleanliness level seems to have two types of filters to choose from, choose high performance and low performance, depends on the specific situation: when the environment pollution is serious, or indoor exhaust air ratio is large, or the cleanroom is particularly important, need bigger safety coefficient, in one of those or circumstances, to choose high class of filter; Otherwise, you can select a lower performance filter. For clean rooms requiring control of 0.1μm particles, class D filters should be selected regardless of the controlled particle concentration. The above mentioned only from the perspective of filter, in fact, it is necessary to choose a good filter and comprehensively consider the characteristics of clean room, filter and purification system.

To ensure air supply cleanliness, only qualified filter is not enough, but also to ensure that :a. Transport, installation process does not damage the filter; B. Install tightly. To do so, the construction and installation personnel are required to be well trained, both with the knowledge of the installation and purification system, and with more skilled installation skills. Otherwise, it will be difficult to ensure that the filter is not damaged. There are profound lessons in this aspect. Secondly, the problem of installation tightness, mainly depends on the advantages and disadvantages of the installation structure type, the design manual generally recommends: for a single filter using the installation type, so that even if leakage occurs, it will not leak into the room; The use of finished high efficiency air outlet, the tightness is also relatively easy to be guaranteed. For the air of multiple filters, liquid tank seal and negative pressure seal are often used in recent years. The liquid tank sealing must ensure that the liquid tank joint is tight and the overall frame is on the same level. The negative pressure seal is to make the filter and the plenum and the frame joint around the negative pressure state, and the same as the open type even if the leakage will not leak into the room. In fact, as long as the installation frame is flat and the end face of the filter is in even contact with the installation frame, it should be easy for any installation type to make the filter meet the installation tightness requirements.

Two, air distribution:

The air flow organization of the clean room is different from that of the general air conditioning room. It requires the cleanest air to be sent to the operating part first. Its role is to limit and reduce the pollution of the processed products. To this end, these principles should be taken into account in the design of air flow organization: minimize eddy currents and avoid bringing pollution from outside the work area into the work area; Try to prevent the dust of the second float in order to reduce the dust to the workpiece pollution opportunity; The air flow in the working area should be as uniform as possible, and the wind speed should meet the process and health requirements. When the air flow to the return air outlet, the dust in the air can be effectively taken away. According to different cleanliness requirements, choose different air supply and return mode.

Different air distribution has its own characteristics and scope:

Vertical uniflow: 1) a uniform air flow down, can be obtained for process equipment layout, self-purification ability strong, can simplify the personal purification facilities such as common facilities, such as the common advantages, and their respective advantages and disadvantages of four air supply way: with high efficiency filter has the advantages of low resistance, change the filter cycle is long, but the roof structure is complex, high cost; The advantages and disadvantages of side cloth HEPA top feeding and full hole plate top feeding are opposite to those of full cloth HEPA top feeding. When the system is not running continuously, the inner surface of the hole plate is easy to collect dust, and the maintenance is not good, which has some influence on the cleanliness. Dense diffuser top delivery, because a mixing layer is needed, so it is only suitable for tall clean rooms above 4M, its characteristics are similar to the full hole plate top delivery; The return air outlet is only suitable for clean rooms with a net distance of less than 6m on both sides of the grill plate and the lower part of the wall on both sides. The return air outlet at the lower part of the unilateral wall is only suitable for the clean room with small wall spacing (such as ≤>2~3m).

2) Horizontal unidirectional flow: only in the first work area to reach level 100 cleanliness, when the air flow to the other side of the process of dust concentration gradually increased, so it is only suitable for the same room process has different cleanliness requirements of the clean room; The local distribution of HEPA filter on the air supply wall can reduce the consumption of HEPA filter and save the initial investment compared with the full horizontal distribution, but there is eddy current in the local area.

3) Turbulence airflow: the characteristics of orifice top feeding and dense diffuser top feeding are the same as the above: side feeding has the advantages of easy pipeline layout, no technical interlayer, low cost, and is conducive to the transformation of the old workshop. The disadvantage is that the wind speed in the working area is large, and the dust concentration in the downwind side is higher than that in the upwind side; The high efficiency filter has the advantages of simple system, no pipe behind the high efficiency filter, clean air directly to the working area, but the clean air diffusion is slow, the airflow in the working area is uniform; However, when multiple air outlets are uniformly arranged or the air outlets of HEPA filters with diffusion plates are adopted, the air flow in the work area can be more uniform. But in the case of discontinuous operation of the system, the diffusion plate is easy to accumulate dust.

The above discussion belongs to the more ideal state, is also the country related specification, standard or design manual recommended. Or in practical engineering is limited by the objective conditions, or because the designers to design has a better airflow organization subjective reason, common are: vertical uniflow take on both sides of the adjacent wall bottom return air, the local best level by giving last time (that is, the local air diffuser without sagging under the curtain around), turbulence model send you back to the clean room with high efficiency filter tuyers top or unilateral next time (for wall spacing is larger), According to the measurement, most of the air distribution patterns can not meet the design requirements. Due to the existing specifications for air or static acceptance, individual clean rooms of this kind barely reach the designed cleanliness level under air or static conditions, but the anti-pollution interference ability is very low, and once the clean room enters the working state, it cannot meet the requirements. The proper air distribution should be arranged in the local area of the curtain hanging down to the height of the work area, 100,000 level should not be used to send back. At present, most factories produce high efficiency air supply outlet with diffusion plate, its diffusion plate is only decorative orifice plate and does not have the effect of diffusion air flow, please designers and users should pay special attention to.

Three, air supply or wind speed:

Adequate ventilation is to dilute and eliminate indoor polluted air. According to different cleanliness requirements, when the clean room is high, the number of air changes should be increased appropriately. Among them, the ventilation volume of class 1 million clean rooms is considered by the high efficiency purification system, and the rest are considered by the high efficiency purification system. When class 100,000 clean room HEPA filters are centrally arranged in the machine room or sub-HEPA filters are used at the end of the system, the air change times can be appropriately increased by 10-20%.

Four, static pressure difference:

Maintaining a certain positive pressure in the clean room is one of the essential conditions to ensure that the clean room is not polluted or less, so as to maintain the design cleanliness level. Even if it is a negative pressure clean room, it must have adjacent rooms or suites no lower than its cleanliness level to maintain a certain positive pressure, so that the cleanliness of the negative pressure clean room can be maintained.

The positive pressure value of clean room refers to the value of indoor static pressure greater than outdoor static pressure when all doors and Windows are closed. It is achieved by the method that the air supply volume of the purification system is greater than the return air volume and exhaust air volume. In order to ensure the positive pressure value of the clean room, the supply air, return air and exhaust fan are best interlocked. When the system is opened, the supply fan is started first, and then the return fan and exhaust fan are started. When the system is closed, the exhaust fan is turned off first, and then the return fan and the supply fan are turned off to prevent the clean room from being polluted when the system is opened and closed.

The air volume required to maintain the positive pressure of the clean room is mainly determined according to the airtightness of the maintenance structure. In the early stage of the construction of clean room, because of the poor sealing of envelope structure, it is necessary to maintain the positive pressure of ≥5Pa 2~6 times /h air supply; At present, the airtightness of the maintenance structure has been greatly improved. Only 1~2 times /h air supply volume is required to maintain the same positive pressure. Maintain ≥1OPa only 2~3 times /h air supply volume.

For a clean room, the above four necessary conditions must be met at the same time to ensure that the cleanliness meets the requirements and has the due ability to resist pollution interference.