For the bag filter, the selection of the filter wind speed plays a key role in ensuring the dust removal effect, determining the type and area of the filter, and even the total investment of the dust removal system. In recent years, in the bidding and design of some engineering projects, the requirements of the owners and some designers to filter the wind speed have become lower and lower. The reasons may be as follows:
(1) In the past, there were literatures or monographs that emphasized that filtering wind speed could not get too high, so as to avoid increase of resistance and increase of operating costs;
(2) Some designers believe that the lowering of the filtration speed can improve the dust removal efficiency, enhance the cleaning capacity, and prolong the cleaning cycle, thereby prolonging the service life of the filter bag;
(3) At present, some bag filter and filter material manufacturers generally recommend the range of 1.0 ~ 1.5m/min, and some designers and owners often reduce the value to determine the filtration speed, resulting in filtration The wind speed is low.
It should be said that the above reasons are not unreasonable, but if the filtration wind speed is easily reduced, even if the absolute value of the reduction is small, such as 0.1 to 0.25 m/min, thereby increasing the filtration area by about 10%, the investment in equipment will also increase. Nearly 10%. The greater the amount of air to be treated, the more investment that will be necessary and the greater the floor area of the equipment. Obviously this is not economical.
There are many factors that affect the performance and long-term reliable operation of the bag dust collection system. It is also inappropriate to view the above reasons in isolation. In fact, the correct selection of the filtration wind speed is a more complex task. It has a close relationship with the nature of dust, the initial dust concentration of the flue gas, the type of filter media, and the cleaning method.
Currently, there are not many types of filter materials and cleaning methods available for products on the market, and the filter materials and their cleaning methods are easy to determine. As for the initial dust concentration of the flue gas, it is measured or referenced in addition to the information provided by the process. The same type of operating conditions and furnace type, determined by experience. Therefore, the key to correct selection of the wind speed is to clarify the nature of smoke and dust, and secondly, to correctly understand and understand the relationship between the filtration speed and dust removal efficiency, filtration resistance, and cleaning performance. For flue gas and dust properties, it is necessary to clarify the particle size distribution of soot, the chemical composition of dust, and the physical and chemical properties of dust, such as temperature, humidity, bulk density, viscosity, and corrosion.
Objectively speaking, it is difficult to collect these materials in a comprehensive and accurate manner, but as a designer, at least it should have a certain understanding. The relationship between the filtration speed and dust removal efficiency, filtration resistance, and dust removal performance can be understood in three ways:
First, filter wind speed and dust removal efficiency. We know that there are inertia effects (including collision and interception) and diffusion effects from the dust removal mechanism. If the dust dp is fine dust less than 1μm, it can be trapped effectively by means of diffusion effect, and the dust removal efficiency can be improved by appropriately lowering the filter wind speed Vs; if the smoke dust dp is within 5 to 15μm, the inertial effect can effectively trap the fine dust. Increase the filter wind speed Vs can increase η. Practice has proved that for general smoke and dust, increasing the filtering speed Vs has little effect on the dust removal efficiency η.
Second, filter the wind speed and filter resistance. Filtration resistance increases with the amount of dust on the filter material. Different filter materials have different dust holding capacity per filter material area. However, from an engineering point of view, the difference is small. Generally, the dust load of the filter is only considered from the particle size of the dust. The coarse dust with a large particle size is taken from 300 to 1000 g/m2, and the fine dust is taken from 100 to 300 g. /m2. At present, there is no measured data on the relationship among the dust content, filtration speed, and filtration resistance of coal-fired power plant dust. However, as early as the 1980s, there were monographs describing the relationship between dust content, filtration velocity, and filtration resistance of cement dust. When the amount of dust filtration was fixed, the filtration wind speed increased by 1 time and the resistance increased by 25 to 50%. The filtration wind speed increased by 2 times, and the resistance increased by less than 80%. The lower the filtration wind speed, the smaller the percentage increase in resistance. Conversely, when the filtration dust volume is fixed, the filtration speed decreases by 1 time, and the resistance decreases by less than 30%. Obviously, the increase or decrease of the filtration wind speed is not proportional to the increase or decrease of the filtration resistance. If it is simply to use the method of reducing the filtration wind speed to achieve the purpose of reducing the filtration resistance and thus reducing the operating cost, it is not appropriate.
Third, the relationship between filtration speed and cleaning performance. The cleaning performance of dust has a great relationship with the nature of the dust, namely, the viscosity/grain size and the bulk density. The viscosity of the dust is large, the particle size is small, the bulk density is small, the dust removal is difficult, and the filtration speed should be lower, and vice versa. Some people in China have done experiments. For the smooth and dusty dust in talcum powder, under all working conditions, only one blowback is required to remove dust, the filter bag resistance can be restored to the original value, and the secondary dust is almost completely blown off. The amount of back blowing is only 25~30%; and for iron oxide ultrafine dust, it is usually necessary to blow dry repeatedly several times to effectively reduce the filter bag resistance, and it is difficult to restore the original value, the ratio of back blowing is as high as 50~70. %. This shows that for a certain bag filter, dust cleaning performance mainly depends on the nature of dust and dust gas, not all dust, filtration speed is lower, it can enhance the cleaning capacity.
Fourth, in the case of filter bag determination, reducing the filtration speed can lengthen the cleaning cycle, but the service life of the filter bag does not depend entirely on the cleaning cycle. Because when a certain filter wind speed is determined, the filter wind speeds in different parts of the filter bag are also different. Experiments done abroad have found that the difference in the local filtration wind speed on a filter bag can be up to 4 times or even more than 4 times.
In summary, it can be concluded that blindly reducing the filtration wind speed does not fully guarantee the improvement of dust removal efficiency, nor does it necessarily reduce the filtration resistance, and may also cause unnecessary economic losses; only if the nature of dust is fully understood and Based on the characteristics of the system, the structure design of the dust collector body is optimized, and the economic and technical analysis is performed correctly so that the filtration wind speed can be reasonably determined.