Hunan Purification Engineering: Common Problems and Improvement Suggestions in the Debugging of Cleanroom Purification Air Conditioning Systems
Design, construction, and commissioning are the three important links to ensure the normal operation of a purification air conditioning system. Many books and literature have provided specialized introductions to the design and construction of purification air conditioning systems, but there is relatively limited information on the commissioning of purification air conditioning systems and the practical problems often encountered during commissioning. During the process of debugging multiple purification air conditioners, the author discovered several issues that need attention and exploration.
The problem of the cleanroom return air outlet becoming a supply air outlet
According to the process requirements, there should be an improved static pressure difference between adjacent clean rooms. On the one hand, this is to prevent air from low cleanliness clean rooms from seeping into high cleanliness clean rooms through gaps when doors and windows are tightly closed; On the other hand, when the door is opened, ensure that there is sufficient airflow flowing in the positive direction to minimize the instantaneous reverse airflow caused by the opening action and human entry, and reduce pollution. However, in practice, due to design or other reasons, in order to ensure a large static pressure difference in "relatively important" rooms, there may be a phenomenon where the "less important" clean room return air outlet becomes a supply air outlet. This is quite common during the purification and commissioning process. The analysis is as follows:
1.1 The design return air volume for maintaining room pressure difference is difficult to determine
In the design of purification air conditioning, designers tend to focus more on the design of clean room air supply volume, while for the design of return air volume, a rough estimate is usually used, that is, if the return air volume is less than the supply air volume, the pressure difference can be improved. However, the pressure difference between adjacent rooms is greatly affected by the site conditions, mainly the size of the room door gap. If the sealing performance of the door is good, a smaller difference in air supply and return can ensure the opposite pressure difference required by the room. If the sealing performance of the door is poor, in order to ensure the positive pressure difference of the clean room during design, a larger difference in air supply and return is needed. Therefore, during on-site debugging, there was a phenomenon where the pressure difference between adjacent rooms would backflow even while ensuring the designed supply and return air volume of the clean room. Based on this situation, in actual debugging, the air volume distribution for the clean room is first carried out according to the designed air supply volume, and the return air volume is adjusted appropriately according to the requirements of ensuring pressure difference on site. The author once tested the supply and return air volume of the already debugged clean room and found that the deviation between the return air volume and the designed return air volume can sometimes reach 10% when ensuring that the supply air volume into the room is within the range of 10%. Of course, this does not mean that there is no need to calculate the return air volume in the design, but rather that the design is carried out according to the ideal state, and for actual cleanrooms, the influencing factors can sometimes be uncontrollable and unpredictable.
1.2 Unreasonable design of return air duct
Although there is a significant deviation between the return air volume of the clean room and the design value, if the return air pipeline is well designed, the pressure difference in the clean room can still be well adjusted to avoid problems. On the contrary, if the design of the return air duct is unreasonable, the resistance deviation of the parallel branch pipes is too large, and the residual pressure of the selected air conditioning unit is obviously insufficient, then in order to ensure the relative positive pressure difference between all rooms on a certain return air duct branch and the outside, closing the main return air valve on this branch often causes reverse flow in the return air outlets of other rooms on the same branch, that is, the return air outlet becomes the supply air outlet. We qualitatively analyze and illustrate this issue using Figures 1 and 2.
Figure 1 is the floor plan of the rooms, and for analysis purposes, the actual pressure distribution maps of these rooms are provided. Figure 2 is a schematic diagram of the corresponding room's return air duct system. From the figure, it can be seen that the pressure difference between the cover chamber and the outside is 66Pa, while the pressure difference between the buffer chamber and the outside is 28Pa. If the negative pressure formed by the return air branch pipe on the entire return air pipeline is small enough to overcome the positive pressure formed between the cover chamber and the buffer chamber, the return air from the cover chamber will be pressed into the buffer chamber through the return air pipe. For other quasi clean rooms, the same is true, especially for those that, in order to ensure a positive pressure difference with the outer corridor, turn down the return air regulating valve of the branch pipe, often resulting in the phenomenon of the return air outlet becoming the supply air outlet. Of course, further theoretical analysis can also be based on the pressure difference characteristics of the pipeline. Usually, the four major lines of the pipeline (total pressure line, potential pressure line, potential pressure line, and zero pressure line) are drawn for detailed discussion, which will not be repeated here.
Therefore, the author suggests that for clean rooms with high requirements for excellent pressure and quasi clean areas with lower requirements in the same system, the return pipeline should not be set on the same branch pipe as much as possible under the conditions permitted on site, in order to avoid the problem of the return port becoming a supply air outlet. Of course, the occurrence of this phenomenon is closely related to the residual pressure of the selected air conditioning unit, and this should be taken seriously in the design.
2 Cleanroom * * exhaust problem
The exhaust of clean rooms can generally be divided into two types: one is regular exhaust of clean rooms, where the production line needs to undergo * * * after running for a long time, and the * * exhaust gas is discharged to the outside through the * * exhaust fan; Another type is irregular exhaust during the operation of some clean rooms. When the concentration of pollutants in the clean production workshop reaches a certain level, the exhaust will automatically (or manually) exhaust. If the upper limit is not reached, the exhaust fan will stop running. During the debugging process, the exhaust system often encounters the following issues.
2.1 Change the exhaust outlet to the supply outlet
Set up separate ventilation to exhaust some rooms. Due to the design of the pipeline, under the action of different static pressure differences in the room, some exhaust outlets are reversed and become supply outlets, which is the same reason as the return air outlet becoming a supply outlet. It is recommended to use exhaust fans reasonably and set up separate exhaust systems for clean rooms with large pressure differences. If it is not allowed on site, it is also advisable to ensure that the exhaust outlets of clean rooms with large pressure differences are not arranged on the same branch pipe. It is worth mentioning that some purification room exhaust systems currently use centrifugal fans with higher pressure to achieve better results, and other systems can also learn from it.
2.2 System Regular * * Exhaust Setting
If the cleanroom needs to perform regular * * on the entire system after a period of actual operation, it is more reasonable to install a regular exhaust system on the air conditioning unit, as shown in Figure 3.
The electric sealed valve in the picture can effectively utilize the system return air duct for system exhaust. Electric control valves are installed on the exhaust duct, fresh air duct, and return duct respectively. When the system is running normally without exhaust, the fresh air electric valve 1 and the return air electric valve 2 are opened, and the exhaust air electric valve 3 is sealed; When the system performs regular ventilation, the fresh air electric valve 1 and the exhaust electric valve 3 are opened, and the return air electric valve 2 is sealed; When the entire system stops running, all electric control valves are closed.
It is worth mentioning that although the above-mentioned exhaust system solves the problem of regular exhaust, on-site debugging of such a system may still encounter another problem: when the system is running normally, if the electric valve 3 is not tightly sealed and the residual pressure of the air conditioning unit is high, it may cause fresh air to be sucked back into the air conditioning unit through the exhaust pipe. The main way to prevent this problem is to ensure the selection of valves from high-quality manufacturers, in addition to designing pipelines reasonably.
2.3 Precautions for irregular ventilation settings in clean rooms
The irregular exhaust system during room operation has not been included in most designs
Taking one step into consideration, in addition to installing a one-way valve to prevent backflow, it is recommended to install an electric regulating valve that automatically opens and closes with the start and stop of the exhaust fan. In this way, on the one hand, it can prevent a large amount of processed air from flowing out through the exhaust duct under the action of indoor pressure difference when the fan is not running, causing energy waste; On the other hand, it can reduce the noise generated by the large amount of outdoor air flowing out through the exhaust outlet during non operating time. As the author conducted debugging on the second floor of a purification production workshop, the planar pressure distribution of the sterile room and the cross-sectional layout of the roof exhaust fan are shown in Figures 4, 5:
Due to the short exhaust duct and the absence of an electric sealing valve on the exhaust duct, when the exhaust fan is not running, a large amount of indoor air flows out through the exhaust duct under the action 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). From any angle, setting the exhaust of the clean room in this way clearly does not meet the requirements.
The issue of buffer room 3
The setting of buffer rooms is not only to prevent pollutants from entering the clean room, but also to compensate for pressure differences. The buffer room is good for maintaining negative pressure in the clean room and positive pressure externally. A purification room with strict requirements often has two or more buffer rooms, but there are currently the following issues related to buffer rooms.
3.1 The buffer room is not equipped with a supply air outlet but only a return air outlet
The buffer room that enters the clean area through the non clean area is only equipped with a return air outlet, not a supply air outlet. This will inevitably lead to two shortcomings: firstly, although the buffer ensures negative pressure indoors, it is difficult to ensure positive pressure outdoors; Secondly, the buffer room belongs to a quasi clean area, and without air supply, it is difficult to ensure the cleanliness of the quasi clean area solely by compensating for the air leakage through the door gap of the changing room. So it is recommended to provide an appropriate amount of air supply to the buffer room.
3.2 No buffer room is set up at the emergency exit leading to the outside of the clean corridor
Regarding the issue of setting up buffer rooms at emergency exits. Different designers have different opinions, but the author suggests adding buffer rooms from a debugging perspective. Figure 6 shows a practical example of a certain project. From the perspective of pressure difference, the pressure difference between the clean corridor and the outdoor corridor is as high as 50Pa. Under such a high pressure difference, the door gap at the emergency exit makes a very loud noise, and if this door is opened, it will cause the entire clean corridor to release pressure, and some rooms in the clean room will experience pressure backflow. If a buffer room is set up and air is supplied to it, this situation will be completely avoided. It is worth noting that the opening direction of the buffer room door should not face towards the side with higher pressure (i.e. clean profile), but should be the same as the opening direction of the emergency exit door.
Problem with regulating valve 4
4.1 Ordinary air volume control valve
Due to different manufacturers, there are significant differences in the quality of valves. Many debugging problems on site are caused by ineffective valve opening and closing. For example, during the debugging of an electronic production workshop, an air conditioning unit had a constant air volume no matter how the air supply valve was opened. After inspection, it was found that the blades of this valve were misaligned, forming a 90 degree angle with each other. Whether fully open or fully closed, there were always half open and half closed. Another issue regarding the valve is the lack of position markings for opening and closing, making it difficult to determine whether the valve is open or closed. Only through testing can it be known, which poses challenges for the future management of Party A. Therefore, it is recommended that Party A fully consider the convenience of future management and maintenance when selecting valve manufacturers.
4.2 Fire Control Valve
At present, most purification air conditioning system units are equipped with fire control valves at the outlet, which theoretically play a role in fire prevention and can also adjust the supply or return air volume of the unit. However, in actual debugging, it was found that the adjustment function of most current fire control valves is weak, because the gear adjustment used (usually 5 or 6 gears) is difficult to ensure that the adjusted air volume meets the design requirements.
For example, during the debugging of an air conditioning unit in a certain purification workshop, the fire control valve of the unit's air supply main was set to level 3, which resulted in a lower air volume, but set to level 4, which resulted in a significantly higher air volume. Similarly, there is also a problem with the small adjustment amount of the fire control valve on the return air main. In order to ensure the relative pressure difference between two different purification systems, it is necessary to further adjust the air volume of one of the air conditioning units in the case of a small range of fresh air adjustment. However, the relative pressure difference caused by opening and closing the return air fire control valve is too large to meet the design, specifications, and actual on-site requirements. Of course, this situation is also closely related to the regulating flow characteristics of the valve, but due to the limitation of the gear, the regulating flow characteristics of the valve itself become worse.
At the same time, it was found during debugging that the phenomenon of ineffective opening and closing of fire dampers is also common. Some fire dampers can only be fully opened or fully closed, and cannot be tightened when in other gears, losing their regulating function. Therefore, the author believes that if allowed on site, it is recommended to separate the fire damper and the regulating valve. It is suggested to use a continuously adjustable regulating valve for the regulating valve, and it is not recommended to use a non continuous regulating valve with fewer gears.
5 Problems with air conditioning units
Debugging has found that some air conditioning units blindly pursue compactness in structure, blindly shortening the distance between the fan outlet section and the filter section without taking other remedial measures (such as installing a flow equalization plate at the fan outlet), resulting in the treated air not being able to diffuse in time, making the air filtration rate of the entire section of the medium efficiency filter at the fan outlet extremely uneven. This not only affects the filtering effect of the filter, but also greatly shortens the service life of the filter.
At the same time, poor overall sealing performance of the unit is currently an extremely common phenomenon. Some air conditioning unit power cables (such as motor power lines and fan power lines) pass through the chassis, and the connection with the chassis board is not tightly sealed, or even not treated at all; At the same time, during on-site debugging, it was found that there was severe air leakage around the maintenance door of the air conditioning unit, and the phenomenon of noisy noises from the unit maintenance door occurred from time to time. Therefore, it is recommended that manufacturers strictly control the quality, and the maintenance door should not only meet the requirements during operation, but also ensure the tightness of the air leakage rate measurement method specified in the national standard GB/T1494-93 [1] at a positive pressure of 700Pa. The author encountered a situation where the filter unit was unable to conduct an air leakage rate experiment due to severe air leakage during an air conditioning unit experiment in a certain factory (the pressure inside the unit could not reach 700Pa).
On site, phenomena such as water in the surface cooler of the air conditioning unit, unit nameplate air volume greater than the nameplate air volume of the fan inside the unit, lack of coarse filter screen at the fresh air intake, lack of differential pressure gauge in the unit filtration section, and unreasonable placement of the maintenance door also occur. As long as engineering technicians and manufacturers pay attention to the problems that arise and make efforts to improve, we believe that the purification of air conditioning will have a great improvement.
Article source: Hunan Purification Engineering http://www.0571pet.cn/
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