CORE ANALYSIS OF CLEAN ROOM

Introduction

Clean room is the basis of pollution control. Without clean room, pollution-sensitive parts cannot be mass-produced. In FED-STD-2, clean room is defined as a room with air filtration, distribution, optimization, construction materials and equipment, in which specific regular operating procedures are used to control the concentration of airborne particles to achieve the appropriate particle cleanliness level.

In order to achieve good cleanliness effect in clean room, it is necessary not only to focus on taking reasonable air conditioning purification measures, but also to require process, construction and other specialties to take corresponding measures: not only reasonable design, but also careful construction and installation in accordance with the specifications, as well as correct use of clean room and scientific maintenance and management. In order to achieve good effect in clean room, many domestic and foreign literatures have been expounded from different perspectives. In fact, it is difficult to achieve ideal coordination between different specialties, and it is difficult for designers to grasp the quality of construction and installation as well as the use and management, especially the latter. As far as clean room purification measures are concerned, many designers, or even construction parties, often do not pay enough attention to their necessary conditions, resulting in unsatisfactory cleanliness effect. This article only briefly discusses the four necessary conditions for achieving cleanliness requirements in clean room purification measures.

1. Air supply cleanliness

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

Filter selection

The final filter of the purification system generally adopts a hepa filter or a sub-hepa filter. According to my country's 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%, Class D is (for particles ≥0.1μm) ≥99.999% (also known as ultra-hepa filters); sub-hepa filters are (for particles ≥0.5μm) 95~99.9%. The higher the efficiency, the more expensive the filter. Therefore, when choosing a filter, we should not only meet the air supply cleanliness requirements, but also consider economic rationality.

From the perspective of cleanliness requirements, the principle is to use low-performance filters for low-level clean rooms and high-performance filters for high-level clean rooms. Generally speaking: high-and medium-efficiency filters can be used for the 1 million level; sub-hepa or Class A hepa filters can be used for levels below class 10,000; Class B filters can be used for class 10,000 to 100; and Class C filters can be used for levels 100 to 1. It seems that there are two types of filters to choose from for each cleanliness level. Whether to choose high-performance or low-performance filters depends on the specific situation: when the environmental pollution is serious, or the indoor exhaust ratio is large, or the clean room is particularly important and requires a larger safety factor, in these or one of these cases, a high-class filter should be selected; otherwise, a lower-performance filter can be selected. For clean rooms that require control of 0.1μm particles, Class D filters should be selected regardless of the controlled particle concentration. The above is only from the perspective of the filter. In fact, to choose a good filter, you must also fully consider the characteristics of the clean room, the filter, and the purification system.

Filter installation

To ensure the cleanliness of the air supply, it is not enough to have only qualified filters, but also to ensure: a. The filter is not damaged during transportation and installation; b. The installation is tight. To achieve the first point, the construction and installation personnel must be well-trained, with both knowledge of installing purification systems and skilled installation skills. Otherwise, it will be difficult to ensure that the filter is not damaged. There are profound lessons in this regard. Secondly, the problem of installation tightness mainly depends on the quality of the installation structure. The design manual generally recommends: for a single filter, an open-type installation is used, so that even if leakage occurs, it will not leak into the room; using a finished hepa air outlet, tightness is also easier to ensure. For the air of multiple filters, gel seal and negative pressure sealing are often used in recent years.

Gel seal must ensure that the liquid tank joint is tight and the overall frame is on the same horizontal plane. Negative pressure sealing is to make the outer periphery of the joint between the filter and the static pressure box and the frame in a negative pressure state. Like the open-type installation, even if there is leakage, it will not leak into the room. In fact, as long as the installation frame is flat and the filter end face is in uniform contact with the installation frame, it should be easy to make the filter meet the installation tightness requirements in any installation type.

2. Airflow organization

The airflow organization of a clean room is different from that of a general air-conditioned room. It requires that the cleanest air be delivered to the operating area first. Its function is to limit and reduce the pollution to the processed objects. To this end, the following principles should be considered when designing the airflow organization: minimize eddy currents to avoid bringing pollution from outside the work area into the work area; try to prevent secondary dust flying to reduce the chance of dust contaminating the workpiece; the airflow in the work area should be as uniform as possible, and its wind speed should meet the process and hygiene requirements. When the airflow flows to the return air outlet, the dust in the air should be effectively taken away. Choose different air delivery and return modes according to different cleanliness requirements.

Different airflow organizations have their own characteristics and scopes:

(1). Vertical unidirectional flow

In addition to the common advantages of obtaining uniform downward airflow, facilitating the arrangement of process equipment, strong self-purification ability, and simplifying common facilities such as personal purification facilities, the four air supply methods also have their own advantages and disadvantages: full-covered hepa filters have the advantages of low resistance and long filter replacement cycle, but the ceiling structure is complex and the cost is high; the advantages and disadvantages of side-covered hepa filter top delivery and full-hole plate top delivery are opposite to those of full-covered hepa filter top delivery. Among them, the full-hole plate top delivery is easy to accumulate dust on the inner surface of the orifice plate when the system is non-continuously running, and poor maintenance has some impact on the cleanliness; dense diffuser top delivery requires a mixing layer, so it is only suitable for tall clean rooms above 4m, and its characteristics are similar to full-hole plate top delivery; the return air method for the plate with grilles on both sides and the return air outlets evenly arranged at the bottom of the opposite walls is only suitable for clean rooms with a net spacing of less than 6m on both sides; the return air outlets arranged at the bottom of the single-side wall are only suitable for clean rooms with a small distance between the walls (such as ≤<2~3m).

(2). Horizontal unidirectional flow

Only the first working area can reach the cleanliness level of 100. When the air flows to the other side, the dust concentration gradually increases. Therefore, it is only suitable for clean rooms with different cleanliness requirements for the same process in the same room. The local distribution of hepa filters on the air supply wall can reduce the use of hepa filters and save initial investment, but there are eddies in local areas.

(3). Turbulent airflow

The characteristics of top delivery of orifice plates and top delivery of dense diffusers are the same as those mentioned above: the advantages of side delivery are easy to arrange pipelines, no technical interlayer is required, low cost, and conducive to the renovation of old factories. The disadvantages are that the wind speed in the working area is large, and the dust concentration on the downwind side is higher than that on the upwind side; the top delivery of hepa filter outlets has the advantages of simple system, no pipelines behind the hepa filter, and clean airflow directly delivered to the working area, but the clean airflow diffuses slowly and the airflow in the working area is more uniform; however, when multiple air outlets are evenly arranged or hepa filter air outlets with diffusers are used, the airflow in the working area can also be made more uniform; but when the system is not running continuously, the diffuser is prone to dust accumulation.

The above discussion is all in an ideal state and is recommended by relevant national specifications, standards or design manuals. In actual projects, the airflow organization is not well designed due to objective conditions or subjective reasons of the designer. Common ones include: vertical unidirectional flow adopts return air from the lower part of the adjacent two walls, local class 100 adopts upper delivery and upper return (that is, no hanging curtain is added under the local air outlet), and turbulent clean rooms adopt hepa filter air outlet top delivery and upper return or single-side lower return (larger spacing between walls), etc. These airflow organization methods have been measured and most of their cleanliness does not meet the design requirements. Due to the current specifications for empty or static acceptance, some of these clean rooms barely reach the designed cleanliness level in empty or static conditions, but the anti-pollution interference ability is very low, and once the clean room enters the working state, it does not meet the requirements.

The correct airflow organization should be set with curtains hanging down to the height of the working area in local area, and the class 100,000 should not adopt upper delivery and upper return. In addition, most factories currently produce high-efficiency air outlets with diffusers, and their diffusers are only decorative orifice plates and do not play the role of diffusing airflow. Designers and users should pay special attention to this.

3. Air supply volume or air velocity

Sufficient ventilation volume is to dilute and remove indoor polluted air. According to different cleanliness requirements, when the net height of the clean room is high, the ventilation frequency should be appropriately increased. Among them, the ventilation volume of the 1 million-level clean room is considered according to the high-efficiency purification system, and the rest are considered according to the high-efficiency purification system; when the hepa filters of the class 100,000 clean room are concentrated in the machine room or the sub-hepa filters are used at the end of the system, the ventilation frequency can be appropriately increased by 10-20%.

For the above ventilation volume recommended values, the author believes that: the wind speed through the room section of the unidirectional flow clean room is low, and the turbulent clean room has a recommended value with a sufficient safety factor. Vertical unidirectional flow ≥ 0.25m/s, horizontal unidirectional flow ≥ 0.35m/s. Although the cleanliness requirements can be met when tested in empty or static conditions, the anti-pollution ability is poor. Once the room enters the working state, the cleanliness may not meet the requirements. This type of example is not an isolated case. At the same time, there are no fans suitable for purification systems in my country's ventilator series. Generally, designers often do not make accurate calculations of the system's air resistance, or do not notice whether the selected fan is at a more favorable working point on the characteristic curve, resulting in the air volume or wind speed failing to reach the design value shortly after the system is put into operation. The US federal standard (FS209A~B) stipulated that the airflow velocity of a unidirectional clean room through the clean room cross section is usually maintained at 90ft/min (0.45m/s), and the velocity non-uniformity is within ±20% under the condition of no interference in entire room. Any significant decrease in airflow velocity will increase the possibility of self-cleaning time and pollution between working positions (after the promulgation of FS209C in October 1987, no regulations were made for all parameter indicators other than dust concentration).

For this reason, the author believes that it is appropriate to appropriately increase the current domestic design value of unidirectional flow velocity. Our unit has done this in actual projects, and the effect is relatively good. Turbulent clean room have a recommended value with a relatively sufficient safety factor, but many designers are still not assured. When making specific designs, they increase the ventilation volume of class 100,000 clean room to 20-25 times/h, class 10,000 clean room to 30-40 times/h, and class 1000 clean room to 60-70 times/h. This not only increases the equipment capacity and initial investment, but also increases future maintenance and management costs. In fact, there is no need to do so. When compiling my country's air cleaning technical measures, more than class 100 clean room in China were investigated and measured. Many clean rooms were tested under dynamic conditions. The results showed that ventilation volumes of class 100,000 clean rooms ≥10 times/h, class 10,000 clean rooms ≥20 times/h, and class 1000 clean rooms ≥50 times/h can meet the requirements. The US Federal Standard (FS2O9A~B) stipulates: non-unidirectional clean rooms (class 100,000, class 10,000), room height 8~12ft (2.44~3.66m), usually consider the whole room to be ventilated at least once every 3 minutes (i.e. 20 times/h). Therefore, the design specification has taken into account a large surplus coefficient, and the designer can safely choose according to the recommended value of ventilation volume.

4. Static pressure difference

Maintaining a certain positive pressure in clean room is one of the essential conditions to ensure that the clean room is not or less polluted to maintain the designed cleanliness level. Even for negative pressure clean rooms, it must have adjacent rooms or suites with a cleanliness level not lower than its 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 the clean room refers to the value when the indoor static pressure is greater than the 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, return and exhaust fans are preferably interlocked. When the system is turned on, the supply fan is started first, and then the return and exhaust fans are started; when the system is turned off, the exhaust fan is turned off first, and then the return and supply fans are turned off to prevent the clean room from being contaminated when the system is turned on and off.

The air volume required to maintain the positive pressure of the clean room is mainly determined by the airtightness of the maintenance structure. In the early days of clean room construction in my country, due to the poor airtightness of the enclosure structure, it took 2 to 6 times/h of air supply to maintain a positive pressure of ≥5Pa; at present, the airtightness of the maintenance structure has been greatly improved, and only 1 to 2 times/h of air supply is required to maintain the same positive pressure; and only 2 to 3 times/h of air supply is required to maintain ≥10Pa.

my country's design specifications [6] stipulate that the static pressure difference between clean rooms of different grades and between clean areas and non-clean areas should be no less than 0.5mm H2O (~5Pa), and the static pressure difference between the clean area and the outdoors should be no less than 1.0mm H2O (~10Pa). The author believes that this value seems to be too low for three reasons:

(1) Positive pressure refers to the ability of a clean room to suppress indoor air pollution through the gaps between doors and windows, or to minimize the pollutants that penetrate into the room when the doors and windows are opened for a short time. The size of the positive pressure indicates the strength of the pollution suppression ability. Of course, the larger the positive pressure, the better (which will be discussed later).

(2) The air volume required for positive pressure is limited. The air volume required for 5Pa positive pressure and 10Pa positive pressure is only about 1 time/h different. Why not do it? Obviously, it is better to take the lower limit of positive pressure as 10Pa.

(3) The US Federal Standard (FS209A~B) stipulates that when all entrances and exits are closed, the minimum positive pressure difference between the clean room and any adjacent low cleanliness area is 0.05 inches of water column (12.5Pa). This value has been adopted by many countries. But the positive pressure value of the clean room is not the higher the better. According to the actual engineering tests of our unit for more than 30 years, when the positive pressure value is ≥ 30Pa, it is difficult to open the door. If you close the door carelessly, it will make a bang! It will scare people. When the positive pressure value is ≥ 50~70Pa, the gaps between doors and windows will make a whistle, and the weak or those with some inappropriate symptoms will feel uncomfortable. However, the relevant specifications or standards of many countries at home and abroad do not specify the upper limit of positive pressure. As a result, many units only seek to meet the requirements of the lower limit, regardless of how much the upper limit is. In the actual clean room encountered by the author, the positive pressure value is as high as 100Pa or more, resulting in very bad effects. In fact, adjusting the positive pressure is not a difficult thing. It is entirely possible to control it within a certain range. There was a document introducing that a certain country in Eastern Europe stipulates the positive pressure value as 1-3mm H20 (about 10~30Pa). The author believes that this range is more appropriate.