With the continuous development of science and technology, the production of modern technological products and modern scientific experimental activities require higher and higher cleanliness of indoor air. Especially, microelectronics, biotechnology, pharmaceutical products, medical treatment, daily chemicals and so on, all require high quality, high purity, precision and high reliability of indoor environment, that is, indoor environment. It's done in the clean room.
Clean room has a certain level of air cleanliness, and many parameters should be controlled (air cleanliness, air volume, wind speed, pressure, temperature, humidity, bacterial concentration, etc.). Clean air conditioning system is the main means to achieve clean room control.
Clean air-conditioning system has the following characteristics compared with general building air-conditioning system:
1) Guarantee the temperature and humidity required by the production process;
2) Ensure air purification and sterilization;
3) Controlling the airflow and wind speed in each region;
4) To ensure the reasonable flow direction and pressure distribution between different functions.
5) Eliminate harmful gases to ensure indoor air quality.
Reasonable airflow direction and pressure distribution are very important for clean air conditioning system.
(Instrument applications: anemometer, wind cap, differential pressure meter, thermohygrometer, plankton sampler, dust particle counter/laser dust particle counter/remote particle counter, etc.)
1. The influence of air return system in clean room on air distribution
Clean rooms are organized according to air flow, including unidirectional flow, non-unidirectional flow, radial flow and mixed flow. One-way flow, non-one-way flow and mixed flow (local one-way flow) are the most frequently used. One-way flow is the use of "piston" like extrusion, rapid discharge of indoor pollutants, the streamline in the working area is one-way parallel, no eddy current, high cleanliness. This paper mainly analyses the non-unidirectional flow and points out the problems in the design of air conditioning system in clean room.
At present, the main form of air distribution in non-unidirectional flow clean room is: top-down air supply (or side-up air supply) with high efficiency. The layout of high efficiency air supply outlet is uniform ceiling, or above the protected production line, and the return air outlet is under the side wall.
From the general air conditioning point of view, the main factor affecting the air distribution and air conditioning parameters of the room is the layout of the air outlet, and the return air outlet has little influence on the working conditions of the room. Therefore, in the design of clean air conditioning, many designers are also out of this concept, the clean room return air outlet layout is extremely arbitrary, find a spare corner or wall layout on the line, completely without considering the impact of indoor air flow on the process operation surface, making the clean room meaningless.
In some designs, the return air outlet is located next to the one-way flow clean workbench, and the polluted air in other areas of the room flows to the clean workbench, which seriously affects the protected area; in others, the return air outlet is far away from the polluting process equipment.
(e.g. crushers) Yes, the polluted air generated by these process equipment flows to relatively clean areas or even clean rooms.
The Code for Design of Clean Workshop in Pharmaceutical Industry GB50457-2008 clearly points out that the position of return air outlet should be far away from clean workbench.
The air outlet should be set near the process equipment which is liable to pollute, and the local air exhaust device or the process equipment which needs air exhaust should be arranged on the lower air side of the clean room. Therefore, in the design of clean room air conditioning, there should be both the floor plan and the process layout plan. According to the process requirements, the design should be carried out: when arranging high-efficiency air outlets on the top, as close as possible to the uniform distribution of process equipment.
When arranging the return air outlet, according to the characteristics of the production process, the cleanliness requirement is high, and the return air outlet is located far away from the equipment side; if dust or other polluted gases are produced, the return air outlet is located nearest to the equipment, so that the cleanliness of the clean room can be guaranteed and the safe and effective use of the process equipment of the clean room can be guaranteed to the greatest extent.
Another problem in the design of return air outlet of clean room is that the return air outlet is too little and the return air speed is too high. Some clean room air conditioning design, a clean area, a large area, arranged four efficient air outlets, but only one return air, arranged in a corner. In this way, the whole room air flow to one place, there must be dead angle, clean air is difficult to reach, so that the area of dust particles increase, affecting cleanliness. The high return air speed will form strong eddy current near the return air outlet and even at the height of the working face, which will make the dust diffuse and also affect the air quality and cleanliness of the clean room.
Therefore, the return air outlet of non-unidirectional flow clean room should be arranged according to the air volume, and the wind speed should not be too high, so it is better not to exceed 2m/s. The return air outlet should be arranged on two (or more) sides of the wall. In order not to increase the sanitary dead angle, it is usually arranged diagonally. When the width of the clean room is not more than 3m, it can be arranged unilaterally.
2. The influence of return air system in clean room on pressure difference in clean room

Because of the high cost of clean room, the small area of general clean room and the small space left for air conditioning designers after the technicians have arranged the process equipment, it is common for two adjacent clean rooms to share a return air chute in the air conditioning design of clean room. The consequence of this design is that the clean room with low relative pressure can not return air and the room with high pressure can not. The positive pressure can not be adjusted accurately when the air between rooms is compressed into relatively low-pressure rooms. There is no orderly gradient pressure distribution in the clean rooms. Polluted gases may pollute high-cleanliness rooms and may cause cross-contamination. Take a project I met as an example.

(Instrument applications: anemometer, wind cap, differential pressure meter, thermohygrometer, plankton sampler, dust particle counter/laser dust particle counter/remote particle counter, etc.)
The original designer designed the return air system as follows: the clean corridor shared a return air chute with crushing and mixing of drinks, and the return air chute was opened on both sides of the return air chute; the return air chute was shared with buffering and washing hands separately; and the clean room shared the same return air chute with the clean corridor and temporary storage of medicinal materials. From Figure 2 (pressure difference distribution of process requirements), we can see that the pressure of clean corridor is different from that of mixing powder, clean corridor and crushing of drinks, clean room and buffer toilet. What are the consequences of this?
Since the debugging stage, the system has been in trouble. The pressure difference is not adjusted correctly. It can't return to the air by mixing powder and crushing drinks, or the positive pressure of + 15Pa can't be guaranteed in the clean corridor. Second, it is under the same pressure as the buffer wash. The construction unit finally succeeded in debugging the pressure by blocking the return air outlet and closing the supply air outlet of the low-pressure room such as mixing powder.
In the use of the problem comes, low-pressure level room cleanliness does not meet the standard, can only open large air outlet valve. Later, either the polluted air leaks out to the clean corridor and pollutes the whole area, or the air from the clean corridor blows into these low-pressure rooms, causing them unable to return air. The fresh air volume in the room is seriously insufficient, which seriously affects the use, and the users can only turn off the clean air conditioning for production. In this way, the renovation design will be carried out in less than one year.
The author makes the following modification design for its return air outlet:
According to the production process and positive pressure requirements, different positive pressure values require that the room be divided into return air column, each return air outlet or return air pipe valve to adjust, and the return air outlet is equipped with filter screen. In addition, in the clean rooms polluted by crushing and mixing of drinks, an air outlet is set up near the polluting process equipment to discharge the polluted air out of the clean room in time.
After debugging, the pressure difference between clean area and common area is + 10Pa, the pressure difference between clean corridor and polluted room is + 5Pa, the positive pressure value and cleanliness of the test pass once, and there is no problem in operation for nearly two years.
3 conclusion
It is useless to maintain sterility and cleanliness of a clean room by means of space sealing. Only by keeping the pressure in the sterile area higher than that in the outside can pollutants be prevented from intruding into the clean room. In order to ensure the directional flow of clean air, different pressure control is needed between clean function rooms of different levels in the clean area. Air flow can only flow from clean area to non-clean area, from high-level clean room to low-level clean room, and from high-level clean room to low-level clean room. Only on-the-job is guaranteed. In any case, the orderly gradient pressure distribution in the clean room can be maintained unchanged, so that the cleanliness can be effectively controlled.
The air conditioner designer should pay attention to the influence of return air system on the clean room and arrange the return air outlet.
1) The air velocity of the tuyere should not be too high. The tuyere should be arranged diagonally or opposite to the cleanroom as far as possible. At the same time, it should be avoided to be located near the area or equipment with high cleanliness requirement, and the exhaust tuyere should be as close as possible to the pollution source.
2) Cleaning rooms with different positive pressure values should not share return air ducts.
Shandong Aide Purification Engineering Co., Ltd. devotes itself to the fields of medical purification engineering, laboratory system engineering, clean workshop system and so on. After years of development, it has accumulated rich industry experience and gained "China's 3.15 Consumer Trusted Products", "Excellent Engineers in HVAC Purification Industry in 2017" and "Enterprises in 2017". The prize of listing and listing in the stock market reform is a powerful enterprise in the clean Engineering industry. To learn more about the medical purification industry, please pay attention to the Wechat Public No. Aide-188 (Ed Purification and Oxygen Supply)