1. Introduction to condensation examples
1. 1 High Temperature Example
A factory φ 4.5m × l2.96m cement mill is a cyclone mill with a cyclone separator. The end of the mill is dusted with a flat bag filter. When the grinding system is placed in the room, the convection between the mill and the outside air is weak, the system has low air leakage, and the exhaust gas treatment system has no external insulation.
During production, it was found that the pressure difference between the inlet and outlet of the bag filter is large; the temperature of the milled cement is up to 170°C (the upper limit of the design is 120°C); the negative pressure at the feed inlet of the mill is not guaranteed, and ash is often thrown out due to paroxysmal positive pressure. . The grinding stoppage inspection revealed that almost all of the milled quilting joints were cemented; the flat bagged dew condensation was severe. The system has large ventilation resistance, which impairs the air-cooling effect of the mill. In addition, the temperature of the clinker entering the mill is high, and the high temperature alarm of the hollow shaft at the end of the grinding is frequent during the day. In the days, the grinding can only be performed at the time from 0 to 1l. The rest of the time had to stop grinding and cooling because of the high temperature. Later, the capacity of the bag filter was expanded and the cold air was leaked. The filter bag was replaced with water-repellent material, and the water spray was stopped before it passed the inspection and acceptance. The subsequent thermal test results: Taiwan production 120t / h; into the clinker temperature of 110 °C; grinding head static pressure -15 ~ 20Pa; disable grinding head water spray; bag filter inlet temperature 120 °C, outlet 92 °C, the standard condition outlet air flow 9882m3 / h; out cement mill temperature 140 °C, grinding tail gas temperature 127 °C, air volume 12480m3 / h; cement finished product temperature 116 °C; grinding tail cylinder shell 120 °C. Production has been maintained by constantly changing filter bags.
1.2 Low-temperature examples
The T factory φ 3m × llm cement mill is also a cyclone mill with a cyclone separator. It uses an airbag-type bag filter. The clinker is purchased from outside, and the clinker content is equivalent to the atmospheric temperature in winter and slightly higher in summer than 15 °C. The grinding waste gas treatment system has no external insulation layer. The mill is semi-open, with the same indoor and outdoor temperatures. The exhaust gas temperature during normal operation is as low as 18 °C. In winter and spring seasons, bag dust collectors often stick bags, and the exhaust fan current drops from above 75A to 60A under normal conditions; the wind speed inside the nose hood of the inlet belt is reduced from about 0.5m / s to 0.2m / s. During the grinding operation, the bag dust collector top shell was inspected and the temperature of the housing chamber was found to be equal to the atmospheric temperature. The upper cover of the air box was opened, and the clean air room was covered with water on four walls. The inside of the cover plate became wet; the local place was filtered; Bags are almost filled with damp cement dust; filter bags are difficult to extract due to the hard cement; cleaning the bag and found that the entire bag surface is covered with wet ash, a paste or hard state, the bottom of the bag is mostly hard Shell; filter bag near the shell due to low temperature, condensation plug more serious, especially in the corners.
2. Reason analysis
2.1 Stop grinding and condensation
For the exhaust gas high-temperature N plant, the moisture in the mill mainly comes from the material brought in and the water from the grinding head. The plant’s slag and gypsum both have an average moisture content of 8%, and the amount of water carried into the mill is 1. 517t / h, the amount of water in the grinding head is 1. 500t / h, the total of two items of moisture is 3. 017t / h. Due to the high temperature of the cement (140 °C), the air entering into the mill is offset by the cement and the water removed. 3. The moisture of 017t / h is totally injected into the exhaust gas at 92 °C. Under the standard condition, the outlet volume of bag filter is 9882m3 / h, then the moisture content of wet gas is 305g / m3. The corresponding dew point is 75 °C. If other conditions remain unchanged and only changes in grinding water injection are used, the corresponding dew point is 59 °C. According to the general requirements at home and abroad, the dew point design value of the bag filter is 60 ~ 70 °C, both of which are generally in line with the requirements.
At the exhaust gas temperature of 92 °C, the cause of dew bagging by the dust collector is:
At the beginning of grinding, the mill is still hot, the residual moisture evaporates, and the exhaust fan has stopped and the air volume is extremely low, so it is easy to dew condensation. After the shutdown, the whole system is cooling down, and the bag filter is located at the rear end of the bag. The heat capacity is not large. The air box bag filter used by many factories is relatively bulky and has a large cooling area. The original temperature is the lowest and the temperature drop is fast, which is easy to reach the dew point. In addition, the filter bag has a full-filtering “interception” effect on the tiny droplets (water mist) in the air flow, so that the moisture concentrates in this precipitation; if it meets the entire line from the feed to the exhaust fan, the interlock jump stop or stop the water stoppage time If the amount of advance is insufficient and the exhaust fan is shut down prematurely, the moisture in the mill is more than when the grinding is stopped normally; or the poor atomization of the water spray and the delayed evaporation of water will increase the condensation. Especially in the winter, the temperature drops quickly, and the drop is large. When the dew point is reached, water is precipitated. Set the dew point to 80 °C, the saturated moisture content under standard conditions is 716g / m3 dry gas, down to 10 °C when it is 9. 8g / m3 dry gas, the precipitation of water is about 706g / m3 dry gas, and down to 60 °C saturated moisture content of 196g / m3 dry gas, down to 30 °C to 35. 1g / m3 dry gas, that is, the more the temperature is lowered, the more thoroughly the water is precipitated. Therefore, the winter dew condensation is particularly sensitive to bag filter without outer insulation layer. Stop grinding for a long time, after the open-heating period, there is a process from the dew point. In case of improper operation, the condensation of the filter bag is still possible in the initial stage of grinding.
The effect of stopping the grinding and cooling on the condensation can be roughly estimated through the total amount of water, ventilation, and temperature of the mill, but it is more difficult to do detailed and accurate, because the amount of water evaporating is difficult to determine and can only be accumulated by the experience.
In summary, the negative impact of stoppage grinding also exists in low-temperature exhaust gas types.
2.2 The amount of ventilation in the mill
The amount of ventilation in the mill is a very important operating parameter in the grinding process. A mill has its optimum range of internal ventilation for the mill under certain conditions, and the value of the optimum airflow range is quite different. Hodbank’s multinational company believes that: grinding air ventilation area should have a circulating wind speed of 1. 0 to 2 . Om / s; grinding larger diameter, air flow can be lower, such as φ 4. 8m mill is 0. 45m3 / kg cement, and φ3. 2m mill is 0. 9m3 / kg cement. Lude recommends multi-warehouse grinding for 0. 3m3 / kg cement. It was also suggested that: To strengthen ventilation, take 0. 40 to 1. 20m3 / kg cement. N plant calibration data, wind speed within the mill is 0. 3hn / s, equivalent to 0. 10m3 / kg cement. The air volume is obviously low, and the dew point rises significantly under certain conditions. According to the above data conditions, if the moisture content is constant, the air volume will be increased to 4 times the measured value, and the dew point will be reduced from 75 °C to 45 °C. Calculated under the standard atmospheric pressure condition, the former contains 232g/m3 more moist air than the latter, and the saturated moisture content at the dew point of 45°C is only 76g/m3 of humid air. Therefore, an effective way to solve the condensation is to increase the amount of ventilation inside the mill. Huaxin Cement Factory φ 2 . In the 7m × 4m secondary cement mill, after the water was sprayed in the mill, it was found that there was a small amount of steam condensation in the air pipe and bag filter to make the cement coarse, and the ventilation volume was increased from the original 2000 to 2 500m3/h to 4000m3/h. Coarse grain disappears.
The amount of ventilation in the mill is maintained by an exhaust fan. Because there is a series of leakage points after the grinding to the front of the exhaust fan, each leakage point is slightly relaxed and plugged, and the ventilation volume in the mill is greatly affected.
2.3 Temperature changes
In recent years, there have been more and more grinding stations in the country, and the clinker temperature is almost the same as the atmospheric temperature. The exhaust gas temperature is low, and condensation is particularly easy, showing that it is very sensitive to changes in the total water content entering the mill.
With regard to bag dedusters, the value of dew point was not taken into consideration, and it is usually set at 60 to 70 °C at home and abroad. Grinding station practice proved difficult to reach such a high temperature. Because the exhaust gas temperature is only about 70 °C, through the pipeline, plus cold air incorporation, the bag filter temperature is below 60 %. In addition, the discussion of the condensation of high-temperature exhaust gas in front of it has also fully exposed its limitations and has little practical significance. Therefore, it is necessary to re-explore control indicators and design data.
3. Solution
1) Regularly check the total moisture content of the incoming materials, and pay close attention to the rain and damp conditions of clinker, limestone, and gypsum, and pay attention to the weather. Once the changes are found, remedial measures should be taken. For example, the T plant will treat the leakage of the storage tanks as a complete treatment to ensure that limestone, gypsum, and wet slag are not added to the plant due to leakage of water, and the limestone moisture content is ≤ 1% and the gypsum moisture content is ≤ 3%. Strictly control the slag into the grinding water, and strictly prohibit the direct discharge of wet residue. A drainage ditch shall be set up at the joint storage wet slag dump site to reduce the original moisture of wet slag as soon as possible. In the formulation of control indicators, it can be different in winter and summer.
In the low temperature and rainy season, the proportion of each mixed material is adjusted under the premise of quality permitting to reduce the total moisture entering into the mill. For example, fly ash can be used to replace part of the slag.
2) Properly increase the amount of ventilation within the mill and explore the optimum range of air volume. The negative pressure of grinding head shall be included in the inspection item during the inspection.
The filling volume of a single grinding body should be suitable to avoid affecting the ventilation area of the hollow shaft. The clogging of quilting should be promptly cleaned. It is forbidden to mix materials such as iron slag into the grind to avoid clogging of quilting. To maintain the ventilation capacity within the mill stable at a good level; – Once there is a change, the exhaust fan can still be used to adjust the amount of ventilation to maintain the best ventilation, such as the T factory ordered a difficult to block the quilting of the compartment board and the output board , Stable ventilation area.
3) Filter bag cleaning wind pressure is maintained at 0. Above 5MPa, oil-water separation is required, and air-bags must be drained in time.
Always inspect the interior of the baghouse to open and close the air duct, and control the sweeping air. Check the degree of clogging of the filter bag and find that it is slightly blocked. Take it out in time to dry it and beat it to clear it. Ensure that the ventilation capacity is normal, and avoid the blockage and continue to operate to form a vicious cycle.
4) Regularly inspect the air leakage of the exhaust gas treatment system, implement complete plugging, and perform necessary external insulation on the exhaust gas exhaust system.
5) Do a good job in production scheduling. When the moisture content is high, continue to operate as much as possible to reduce the number of grinding stops and stop grinding time. Stop grinding to avoid the low temperature periods such as late afternoon and morning, and pay special attention to the low temperature season.
6) cold grinding season boot, try to avoid excessive temperature material moisture into the mill; pay attention to control the feed rate; circle flow mill can increase the amount of powder back to increase the wind temperature inside the mill.
Before stopping the grinding, extend the stop time as much as possible, and stop the mixture of high moisture content as soon as possible. After stopping the grinding, properly extend the time of internal exhaustion of the mill, and at the same time, do a good job of transposing the grinding body.
7) The mill with inner water spray should ensure good atomization. Stop the water a quarter of an hour before stopping grinding. Avoiding the evaporating of moisture evaporates the filter bag.
By adopting the above measures, T plant will control the total moisture entering the mill in winter. Below 5%, the limestone dosage is up to 5. At 8 %, the internal combustion control index of cement lost to ignition was 100 %; the air temperature of the mill exhaust fan was increased by more than 7 °C, the minimum was 25 °C, and the fluctuation range was reduced; the ventilation resistance of the bag filter was stable for a long time.