Two core features of a clean room（2）

2023-09-27

original

Marya

Adequate ventilation is necessary to dilute and eliminate indoor air pollution. According to different cleanliness requirements, when the net height of the clean room is high, the number of air changes should be appropriately increased.

1. Supply air volume or wind speed

Adequate ventilation is necessary to dilute and eliminate indoor air pollution. According to different cleanliness requirements, when the net height of the clean room is high, the number of air changes should be appropriately increased. Among them, the ventilation capacity of a 1 million level clean room is considered based on a high and medium efficiency purification system, while the rest are considered based on an efficient purification system; When the 100000 level clean room high-efficiency filter is centrally arranged in the machine room or at the end of the system using sub high-efficiency filters, the air change frequency can be appropriately increased by 10-20%.
A unidirectional flow clean room has a relatively low cross-sectional wind speed through the room, and a turbulent flow clean room is a recommended value with a sufficient safety factor. Vertical unidirectional flow ≥ 0.25m/s, horizontal unidirectional flow ≥ 0.35m/s, and cleanliness can be tested in an empty or static state. Although it can meet the requirements, its anti pollution ability is poor. Once the room enters the working state, the cleanliness may not meet the requirements, and such examples are no longer individual; At the same time, there is no fan series in China that is more suitable for purification systems. Generally, designers often fail to accurately calculate the air resistance of the system or pay attention to whether the selected fan is at a more favorable working point on the characteristic curve, resulting in the system's air volume or wind speed not reaching the design value shortly after being put into operation. The US Federal Standard (FS209A~B) had always stipulated before October 27, 1987 that the airflow velocity of a unidirectional flow cleanroom passing through the cleanroom section is usually maintained at 9Oft/min (0.45m/s), and under the condition of no interference throughout the room, its velocity non-uniformity is within ± 20%, Any significant decrease in airflow velocity will increase the likelihood of pollution effects between self-cleaning time and working position (after the issuance of FS209C in October 1987, all parameter indicators other than dust concentration were not specified).
A turbulent flow clean room is a recommended value with a sufficient safety factor, but many designers are still not confident. When making specific designs, increasing the ventilation rate of a 100000 level clean room to 20-25 times/h, increasing the ventilation rate of a 10000 level clean room to 30-40 times/h, and increasing the ventilation rate of a 1000 level clean room to 60-70 times/h. This not only increases equipment capacity and initial investment, but also increases future maintenance and management costs. In fact, it is not necessary to do so. During the preparation of China's air cleaning technology measures, more than 100 clean rooms in China were surveyed and measured, and many clean rooms were still dynamically tested. The results showed that the ventilation volume of 100000 level ≥ 10 times/h, 10000 level ≥ 20 times/h, and 1000 level ≥ 50 times/h can meet the requirements. According to the US Federal Standard (FS2O9A-B), non unidirectional flow clean rooms (Class 100000, Class 10000) have a height of 8-2ft (2.44-3.66m), and it is generally considered that the entire room should be ventilated at least once every 3 minutes (i.e. 20 times/h). Therefore, the provisions of the design specifications have taken into account a larger margin coefficient, and designers can confidently choose according to the recommended values for ventilation in Table 1.

2. Static pressure difference

Maintaining a certain positive pressure in a clean room is one of the essential conditions to ensure that the clean room is not or less contaminated and to maintain the designed cleanliness level. Even in a negative pressure clean room, it must maintain a certain positive pressure in adjacent rooms or suites that are not lower than its cleanliness level in order to maintain the cleanliness of the negative pressure clean room.
The positive pressure value of the clean room refers to the value at which the indoor static pressure is greater than the outdoor static pressure when all doors and windows are closed. It is achieved by purifying the system's supply air volume by exceeding the return air volume and exhaust air volume. In order to ensure the positive pressure value of the clean room, it is best to interlock the supply, return, and exhaust fans. When the system is started, the supply fan should be started first, and then the return fan and exhaust fan should be started; When the system is shut down, first turn off the exhaust fan, then turn off the return fan and supply fan to prevent contamination of the clean room during system startup and shutdown.
The air volume required to maintain positive pressure in the clean room is mainly determined based on the airtightness of the maintenance structure. In the early stage of clean room construction in China, due to the poor airtightness of the enclosure structure, maintaining a positive pressure of ≥ 5Pa requires 2-6 times/h of air supply; At present, the airtightness of the maintenance structure has been greatly improved, and maintaining the same positive pressure only requires 1-2 times/h of air supply; Maintaining ≥ 1 OPa also requires 2-3 times/h of air supply.
According to China's design code [6], the static pressure difference between clean rooms of different levels and between clean and non clean areas should not be less than 0.5mmH2O (~5Pa), and the static pressure difference between clean areas and outdoor areas should not be less than 1. OmmH2O (~10Pa). The value seems to be low for three reasons:
1) Positive pressure refers to the ability of a clean room to suppress indoor air pollution through gaps in doors and windows, or to minimize the infiltration of pollutants into the room when doors and windows are opened for a short period of time. The size of positive pressure indicates the strength of the ability to suppress pollution, but of course, the greater the positive pressure, the better (as will be discussed later).
2) The required air volume for positive pressure is limited, and the difference in air volume between 5Pa positive pressure and 10Pa positive pressure is only about 1 time/h. Obviously, it is better to set the lower limit of positive pressure value as 10Pa.
3) The US Federal Standard (FS209A-B) stipulates that the minimum positive pressure difference between a clean room and any adjacent low cleanliness area is 0.05 inches of water column (12.5Pa) when all entrances and exits are closed, which has been adopted by many countries.

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Two core features of a clean room（1）