Dry fog equipment is a relatively new type of spray equipment at present. It is used in large places such as World Expos. Its main function is to increase the air humidity to make the air temperature approach the dew point temperature on the enthalpy value line, so as to achieve a cooling effect. Effect.
The use of dry fog equipment in the paint shop is relatively high, and no water film is allowed to form on the body. Therefore, a two-fluid nozzle must be used, that is, the compressed air fluid is used to further atomize the water fluid to achieve the atomization effect. At present, there are mainly the following two places in the application area: the area that needs to increase the environmental humidity and dust removal, and the area that needs to rely on water spray to reduce the ambient temperature. area.
The principle of reducing static electricity generation by relative humidity is roughly: the higher the relative humidity, the better the static electricity effect. Because the water molecules in the air are conductive, they can release the static electricity to a certain extent, thereby reducing the voltage in the air, so the relative humidity is relatively low. In winter, it is easier to generate static electricity than in summer, which is directly related to the air humidity. On the premise of not changing the air temperature, increasing the relative humidity is the only way to solve this problem.
The body of the painting workshop is not allowed to have a water film in the dust removal station. First of all, the water film is easy to adhere to the impurities in the air. After the water film is dried, the impurities will remain on the body and cause defects. At the same time, if the water film remains, it will be mixed with the sprayed chemicals. Many unforeseen problems arise.
At present, the areas where this equipment needs to be added include: various grinding areas in the paint shop. In the grinding area, the external defects of the car body need to be cleaned up, there will be a lot of dust, and static electricity needs to be removed immediately; because the vehicle storage area is relatively open, air supply and exhaust are required. The amount is relatively small and the residence time is long, and it is also suitable to increase the relative humidity to ensure the cleanliness of the environment.
1 Currently feasible engineering solutions and equipment operation mechanism 1) Try to choose an area with air supply and exhaust, there is no clear limit on the amount of air supply and exhaust, as long as it can ensure that the ambient air does not reach saturated humidity.
2) Select a suitable atomizing humidifier according to the environmental area, and the spray particle size should not exceed 8 μm. Each atomizing humidifier nozzle can control the environmental humidity of about 9 m2, and configure a reasonable number of nozzles according to the humidified area. Relatively speaking , the humidifier should be equipped with nozzles around it as much as possible, which is the most economical.
3) The arrangement position of the atomizer should not be on the top of the body as much as possible to prevent the body from getting wet when the atomizer is aging and leaking, and try to arrange it in the channel and other areas. Of course, this will also increase the number of nozzles.
4) Install a temperature and humidity detection device in the environment to monitor changes in ambient temperature and humidity in order to control the start and stop of the humidifier. Generally, we will set 65% as the condition for opening the humidifier and 80% as the condition for closing the humidifier. The installation position of the temperature and humidity detection device is very important for the stable operation of the equipment. It needs to be installed in a place where the humidification effect is moderate in the middle area as much as possible. It was found in the field reconstruction that the smaller the particle size of the humidified water particles, the easier it is to be vaporized by the air. The more average the temperature and humidity.
We installed the above equipment at the Lingang factory site. The entire area is about 330 m2 and the height is 3 m. After installing the dry fog dust removal equipment, the cleanliness of the vehicle has been significantly improved, and the problem of dust floating has been significantly solved. On the basis of smearing vaseline on the surface of the chamber to adhere the dust, the pass rate of the first-time submission of the product is further improved. According to incomplete statistics on site, the increase is about 5%, which is very significant for the reduction of the entire production cost.
2 Precautions for using dry mist spray
1) All spray water on site must use DI water, that is, secondary pure water. Because the nozzles are very fine and easily blocked, and the equipment is not continuously turned on, it may stop using in recent months, so the water quality is very important.
2) All nozzles must be kept away from the body and possible travel positions. It is inevitable to find water leakage during on-site debugging.
3) Take water pressure and air pressure as the main inspection parameters. During the on-site debugging process, the water pressure is not adjusted (estimated by the power station), no secondary booster pump is added, and the spraying effect is completely tested by adjusting the air pressure, so the inspection of the air pressure is very important. There will be a large deviation in the size of the water particles.
4) All pipes should be stainless steel pipes. Some production lines will use UPVC pipe as the transport carrier of pure water, but in actual use on site, it is found that the material is relatively brittle and hard, and there is a risk of rupture in the event of a collision.
5) At present, the domestic dry fog equipment is mostly imported, and the Japanese brand equipment is used on site. According to the experience in the procurement process, the cost of purchasing a single piece of such equipment is nearly 4 times the converted price of the overall purchase, so in one When investing, you should try to consider the number of purchases to reduce subsequent changes. Another function of dry fog cooling in the coating workshop is to reduce the temperature of the workpiece in the high-temperature area, quickly cool the surface temperature of the workpiece, and achieve the construction permit environment of the subsequent work. Also a big improvement. At present, it is mainly used at the outlet of the drying room to cool the surface of the car body. The cooling principle of dry fog equipment is roughly to absorb the heat of the body surface by using the vaporization potential of pure water to achieve the effect of cooling the body surface. The role of the car body, in principle, pure water cannot wet the car body, which is slightly different from the reason for spraying wet dust. The probability of damage and expand its contamination area, causing defects such as shrinkage holes in the subsequent process.
The working principle of the dry fog cooling equipment is roughly based on compressed air as the power source, and the pure water is impacted and sheared, so that the pure water at the outlet reaches a particle size of 10-30 μm, and the particle size and flow distribution are relatively uniform. The requirements for the use of dust removal equipment in this environment are slightly lower, mainly because the absorption capacity of water particles in high temperature areas is much higher than that in normal temperature environments. This design can also ensure that the amount of pure water dispersed on the surface of the vehicle body is uniform and fine. Uniformity is also critical here, and there must be no local flow too high. Because of the premise that the body cannot be wetted, once unevenness occurs, the global flow rate must be reduced, and the cooling effect will be greatly reduced. The economic analysis of dry fog cooling equipment is as follows: if an old line without chilled water is retrofitted, compared with the one-time investment of dry fog cooling equipment, the difference between the two is not much, and the cost of dry fog equipment may be slightly higher by 5%~ 10%. Chilled water includes: upper refrigerating unit, refrigerating station room, chilled water pipe equipment, air supply bellows renovation, temperature control system, etc. The increase of power distribution and compressed air has little impact on the overall investment, and the cost of dry fog operation is roughly 30% of the cost of chilled water, which can be recovered in a relatively short period of time.
In the actual debugging process, some limitations in the control of the device were also found: the dry fog cooling system has an opening temperature, that is, an infrared detection device is installed at the entrance to sense whether the surface of the vehicle body reaches the limit that needs to be started, and the water volume is often based on the lowest temperature. The limited temperature adjustment ensures that no water film will appear on the surface of the vehicle body in this state, which also affects the effect of cooling the high temperature vehicle body. Especially in the case of a large temperature difference in one day, the lack of compatibility of the equipment appears. The work of adjusting the water pressure valve greatly restricts the use effect of the equipment, which is also a difficult problem to solve, because for two Fluid nozzles do not simply increase the water volume to improve the dehumidification effect. It is necessary to adjust the air volume and air pressure together to ensure that the particle size of the water particles is still within the control range. The membrane is not allowed, and the precision requirements are too high, and the current on-site adjustment cannot be satisfied, so only part of the dehumidification effect can be sacrificed to ensure process stability.
3 Experience of installation and arrangement
1) The dry fog cooling equipment needs to be in an area with good ventilation. This is different from the dust removal requirements, because the cooling is still carried out by a large amount of water, and the dust removal is only necessary to maintain the environment. Therefore, the equipment needs to be equipped with a humidity sensor at the exhaust air. If the relative humidity exceeds a certain limit value (85% set on site), the pure water pipeline of the equipment will be closed unconditionally.
2) The installation position of the temperature and humidity detector is very important. If the injected pure water is directly sucked away by the exhaust air without sufficient heat exchange, the detection value of the humidity sensor at the exhaust air may be higher than the real situation, so that It will also cause the pure water to be turned off and affect the effect.
3) It is best to install rubber and plastic insulation for the main pipes of the water and gas lines, because when the equipment is running, the medium in the pipeline is updated quickly, and the temperature of the medium is relatively low, which is a relatively low temperature inside the chamber. point, condensation is likely to occur.
4) The cooling performance of dry fog is significantly enhanced with the increase of the surface temperature of the vehicle body, but at the same time, the possibility of condensation should also be considered, and it should not be installed at the entrance of the strong cooling chamber.
5) There are two types of dry fog: annular arrangement and strip arrangement, which are determined according to different machine operation forms and rhythm requirements. The ring arrangement is suitable for continuous machine equipment, which can be divided into several sections, with 1 to 2 rings as a control unit, the sensor switch detects the position of the body, and sprays and closes in sections. In the annular structure, the air pressure and water pressure of the nozzles at each point are the same, and if the same type of nozzles are arranged, the flow rate of each nozzle is also the same.
The strip arrangement is suitable for the intermittent running of the roller bed spray, and the effect of the equipment will be affected if the rhythm is too fast. Several ordinary water pipes are used in the strip arrangement, and the starting and stopping basis of the roller bed is based on whether there is an induction occupancy. Several water pipes are individually controlled, and the air pressure and water pressure are individually adjusted. If the same type of nozzles are arranged, the flow rate of the nozzles on each water pipe Consistent.
6) The machine operation rhythm has a huge impact on the dry fog equipment. If it is required to be continuous through, the rolling bed where the dry fog equipment is located and the front and rear rolling beds should run at a slow speed to avoid fast rolling beds; method, the tempo is recommended not to exceed 27 JPH, which will limit the spray time and the effect is limited.
7) When the spraying is stopped, the position of the vehicle body is fixed, and the nozzle positions can be appropriately arranged for the high temperature area according to the characteristics of the vehicle body; it can ensure that each nozzle will not directly spray pure water into the air, reducing the number of inductive switches The possibility of direct contact with water mist, since each pipeline can adjust the flow rate, the flow rate can be increased for the area with relatively high temperature at the bottom of the sled, and there is no need to think too much about whether it will be wetted or not. It can reduce the flow of the nozzle near the exhaust port at the top, and avoid the direct contact of the switch with water mist. However, it is inevitable that the cooling will not be uniform.
When the local temperature decreases and the water volume remains the same, a water film may appear.
8) The annular spray is relatively uniform. In order to prevent the water vapor from being directly pumped away, the upper nozzle can be appropriately moved down, or the surrounding structure can be changed together with the designer of the strong cooling chamber. For strong cooling, the air is drawn from the top. The exhaust port can be set at the top of the bottom sloping roof. First, the water mist can travel the longest path. At the same time, there will be a larger cavity at the top, and there will be an air deceleration. The mixing process further reduces the amount of liquid water in the air.
Dry fog equipment is suitable for use in painting workshops in principle. With the reduction of equipment costs, the application prospects are still very broad.