1 The problem that the return air outlet of the clean room becomes the supply air outlet
According to the process requirements, there must be a certain static pressure difference between adjacent clean rooms. On the one hand, when the doors and windows are closed, the air in the clean room with low cleanliness will be prevented from penetrating into the clean room with high cleanliness. On the other hand, when the door is opened, sufficient air flow shall be ensured to move in the positive direction, In order to reduce the reverse gas flow caused by the opening action and the entrance of people as far as possible, and reduce the pollution. However, in practice, due to design or other reasons, in order to ensure a large static pressure difference in the "relatively important" room, the phenomenon of "less important" clean room return air outlet into air supply outlet will appear which is quite common in the process of purification and commissioning. The analysis is as follows:
1.1 It is difficult to determine the design return air volume to maintain the room pressure difference
In the design of clean air conditioning, design engineers pay more attention to the design of clean room supply air volume, and the design of return air volume is usually estimated, that is, if the return air volume is less than the supply air volume, a certain pressure difference can be ensured. However, the pressure difference between adjacent rooms is greatly affected by the site premise, which is mainly the size of the room door gap. If the sealing function of the door is good, the smaller difference between the supply and return air volume can ensure the pressure difference required by the room. On the contrary, if the sealing function of the door is poor, in order to ensure the positive pressure difference of the clean room during the design, it is necessary to have a larger difference between the air supply volume and the return air volume. Therefore, in the field debugging, even if the design air supply and return air volume of the clean room are ensured, the pressure difference of adjacent rooms will also flow backward. Based on this situation, in actual debugging, the air volume is distributed to the clean room according to the designed air supply volume, and the return air volume is adjusted appropriately according to the requirements of ensuring the pressure difference on site. It is found that the deviation between the return air volume and the designed return air volume can sometimes be reached when the air supply volume into the room is within 10%. Of course, this is not to say that there is no need to calculate the return air volume in the design, but it shows that the design is carried out according to the ideal state, and for the actual clean room, the influencing factors can not even be controlled and speculated.
1.2 The design of return air pipeline is not reasonable
Although there is a big deviation between the return air volume and the design value of the clean room, if the return air pipeline is well designed, it is still possible to debug the pressure difference of the clean room and avoid the occurrence of problems. On the contrary, if the design of return air pipeline is not reasonable, the resistance deviation of parallel branch pipe is too large, and the residual pressure of selected air conditioning unit is significantly insufficient, then in order to ensure the relative positive pressure difference of all rooms on a certain return air duct branch to the outdoor, so as to reduce the main return air valve on this branch road, it will often cause the return air outlet of other rooms on the same branch pipe to have reverse activities, That is, the return air outlet becomes the air supply outlet.
2 Classification of disinfection and exhaust air in clean room
There are two types of disinfection and exhaust air in clean room: one is the regular exhaust of clean room. After a certain period of operation, the production line of clean room needs to be disinfected completely, and the disinfected gas is discharged to the outdoor through disinfection exhaust fan. The other is the non scheduled exhaust during the operation of the special clean room of the Department. When the concentration of indoor pollutants in the clean production workshop reaches a certain degree, the exhaust fan will automatically (or manually) exhaust, and if the upper limit value is not reached, the exhaust fan will stop running.
2.1 The exhaust outlet becomes the air supply outlet
Separate exhaust air is set to exhaust the department room. Because of the design of the pipeline, under the effect of different static pressure difference in the room, the Department exhaust outlet is inverted and becomes the air supply outlet. The reason is the same as that of the return air outlet. It is suggested that in addition to the reasonable selection of exhaust fans, it is recommended to set up their exhaust systems for clean rooms with large pressure difference. If the site does not allow, also try to ensure that the air outlet of clean room with large pressure difference is not arranged on the same branch pipe. It is worth mentioning that the centrifugal fan with higher pressure is used in some exhaust system of purification room, which can be used for reference by other systems.
2.2 Installation of regular disinfection and exhaust air in purification engineering system
If the clean room is in actual operation for a period of time, the whole system needs to be disinfected on schedule. At this time, it is more reasonable to set up the system on the air conditioning unit.
Through the electric closed valve, the return air duct of the system can be reasonably used for system exhaust. The exhaust pipe, fresh air pipe and return air pipe are respectively provided with electric control valve. When the system operates normally without exhaust, the fresh air electric valve 1 and return air electric valve 2 are opened, and the exhaust electric valve 3 is closed. When the system exhausts air on schedule, the fresh air electric valve 1 and exhaust electric valve 3 are opened, and the return air electric valve 2 is closed. When the whole system stops running, all electric control valves are closed.
It is worth mentioning that although the above exhaust system has solved the problem of regular exhaust, there is still another problem in the field commissioning of such a system: when the system is in normal operation, if the electric valve is not tightly sealed, and the residual pressure of the air conditioning unit is large, the fresh air is often sucked into the air conditioning unit through the exhaust pipe. The way to prevent this problem is not only to arrange the pipeline reasonably in design, but also to ensure the selection of valves with high quality.
2.3 Matters needing attention for ventilation setting in dust-free workshop
Most of the design of the non scheduled exhaust system during room operation has not been further considered. In addition to the one-way valve to prevent backflow, it is recommended to set up an electric control valve to automatically open and close with the start and stop of the exhaust fan. In this way, on the one hand, when the fan is not running, there is still a large amount of treated air gushing through the exhaust pipe under the effect of indoor pressure difference, resulting in energy spreading. On the other hand, it can reduce the non operation time, because a large amount of outdoor air through the exhaust outlet generated noise. Because the exhaust pipe is short, and there is no electric closed valve on the exhaust pipe, when the exhaust fan is not running, the indoor air gushes out to the outdoor through the exhaust pipe under the effect of high pressure difference (relative to the outdoor pressure difference of 64pa), and the noise generated by the exhaust outlet is about 68.5db (a). No matter from which point of view, the exhaust air of clean room is not satisfied.
3 Setting of buffer room
On the one hand, the setting of the flushing room is to prevent pollutants from entering the clean room. on the other hand, it also has the function of compensating pressure difference. The buffer room is best to keep negative pressure on the clean room and positive pressure on the outside. Two or more buffer rooms are often set up in purification rooms with strict requirements. However, there are some problems related to buffer rooms as follows.
3.1 The buffer room is not provided with air supply outlet, but only with return air outlet
The buffer room entering the clean area through the non clean area is only provided with return air outlet, but not with air supply outlet. This will inevitably lead to two deficiencies: first, although the buffer for indoor negative pressure, but for outdoor positive pressure is difficult to ensure. Secondly, the buffer room belongs to the quasi clean area, so it is difficult to ensure the cleanliness of the quasi clean area only relying on the compensation air volume of the gap leakage of the dressing room. Therefore, it is suggested that appropriate air supply should be carried out in the buffer room.
3.2 There is no buffer room at the emergency exit of clean corridor leading to outdoor
For emergency exit buffer room setting problem，different design engineers have different opinions, but from the perspective of debugging, it is suggested to add buffer room. From the perspective of pressure difference, the pressure difference of clean corridor relative to outdoor corridor is as high as 50pA. Under such high pressure difference, the noise of the door gap at the emergency exit is very loud, and when the door is opened, the pressure of the whole clean corridor will be released, and the room of clean room will be flooded with pressure backflow. If a buffer room is set up and air supply is carried out, this situation will be completely avoided. It is worth noting that the opening direction of the buffer room door should not be towards the side with high pressure (i.e. clean profile), but should be the same as that of the emergency exit door.
