How to reduce the problem of scaling and blockage in dust-containing air ducts?

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In engineering, dust-containing air ducts, especially horizontal ones, often get clogged with dust, forcing shutdowns to remove the buildup. The problem of dust duct scaling is related to many factors. 　　I. Main causes of scaling in dust ducts: 1. Physicochemical properties of the dust material: molecular weight, viscosity, water absorption, particle size, etc. 　　2. The components present in the air ducts include nitrogen, HCl, SOx, NH3, etc. 　　3. Condition of the inner surface of the duct: Rough surfaces are more prone to scaling than smooth surfaces. 　　4. The inclination angle a of the air duct: a values that are smaller tend to cause scaling more easily than those that are larger. When A=0, horizontal air ducts are most prone to scaling. 　　5. Length of air ducts: The longer the air ducts, the more likely they are to accumulate scale. 　　6. Environmental conditions: temperature, humidity, altitude, etc. 　　7. Air volume and wind speed of the fan: In pipes of the same diameter, since Q = AV, when A remains constant, a lower value of Q results in a lower value of V; the lower V is, the more likely scaling will occur. The scientific design of the air distribution network and the correctness of fan selection have a direct impact on work efficiency and economic benefits. 　　II. Issues to consider in the design of the air distribution system: 1. Determination of the upward airflow velocity V: It is recommended that the designed upward airflow velocity V for the air distribution system in powder processing plants be 18–22 mm/s ； The total air volume Q_total is multiplied by the redundancy factor of 1.2. 　　2. The determination of the riser wind speed should also be adjusted according to the number of risers in the wind network, as well as the difficulty factor associated with adjusting the collectors: V_avg = 22 (x-12)/3 K, where: x – total number of risers; K – difficulty factor for adjusting the collectors. For horizontal elbows, K can be taken as 1–2. 3. Method for correcting the pressure loss in the air distribution system when the wind speed increases. 　　An actual increase in the riser V, as well as a change in the type of receiver, will both lead to an increase in the pressure loss in the air network. Then the pressure loss calculated using the original design method must be corrected. Otherwise, setting P_full to < P_full_real will result in the fan operating conditions not meeting the set value Q, causing the lift pipe V_real to decrease. As the flow velocity of the powder dust decreases, caking gradually increases and thickens. Over time, even dust can clog the pipes. For example, in a flour mill, a set of air distribution systems in the flour processing areas consists of 24 lift pipes, using induced draft receivers. The originally designed wind speed in the riser, V_set, was set at 20 m/s; after applying a safety factor of 1.2, the actual value of V became 20×1.2=24 ； Total air volume Q_total = 22546 m3/s, total pressure P_total = 8200 Pa. The corrected values are as follows: Average actual velocity V_actual = 22 + (x–12)/3; K = 22 × (24–12)/3 = 26. Therefore, Q_total = V_actual/Q_actual × Q_total = 26/24 × 22546 = 24425 m3/h. The corrected total pressure is P_total_corrected = V_actual/Q_actual × 8200 = 26/24 × 8200 = 8883 Pa. 4. Fan selection and design: A proper air duct design requires appropriate fan selection in order to achieve the desired operational results. Therefore, it is necessary to pay attention to: (1) the operating conditions and performance of the fan, as well as issues related to differences in these conditions. Under standard operating conditions, the air density r is 1.2 Kg/m3; the fan’s volume flow rate Q and total pressure P are obtained through calculations. The plant is located in a plain area at an altitude of less than 1,000 meters, allowing for the selection of appropriate fans based on either the P total calculation or the Q calculation. If the plant is located on a plateau where the air is thin and its density is less than 1.2 Kg/m3, the total pressure of the fan will still be P_total_set rather than P_total_configured; in such cases, appropriate corrections must be made. 　　(2) The best choice for fan selection Generally, centrifugal fans with a specific speed of Ns between 25 and 40 can be used in the pneumatic conveying systems of dust factories. Only by choosing the centrifugal fans with the highest operating efficiency, such as high-pressure centrifugal fans like 9-19 and 9-26, can better economic benefits be achieved. 　　5. Duct design and installation: (1) Determination of the duct diameter D ； A = Total Q / V; thus, A = (24425/3600)/20 = 0.339. D = 2×√(–A – p) = 2×√(–0.339 – p) = 0.66 m. 　　(2) Minimize or shorten horizontal air ducts ； The inclination angle of the air duct should be large rather than small. 　　(3) Try to use materials with a smooth inner surface. 　　(4) The carrier in the air ducts contains substances such as ammonia, HCl, H2S, and SOx. It is best to use stainless steel plates. Such as SUS304, SUB316 and other steel plates. 　　(5) Minimize the total length of the pipes and reduce the number of elbows. 　　(6) A certain number of dust cleaning windows should be installed on the air duct network, especially on horizontal ducts, to facilitate the timely removal of dust deposits. 　　(7) Depending on the operating conditions, the fan’s damper should be opened to its maximum position from time to time, so as to use the highest airflow volume for soot blowing. 　　(8) Apply an insulation layer to the pipes to reduce heat loss. The carriers of coiled pipes include hot steam, hot air ducts, cold air ducts, cold liquid pipes, etc., all of which should be equipped with insulation layers. 　　(9) Pipe installation should be neat and aesthetically pleasing ； Prevent leaks, spills, drips, and seeps in the pipelines. ”Gas, powder, liquid. 　　(10) Apply an anti-corrosion layer, such as rust-proof paint. 　　6. Carry out technological innovation for wind networks and ducts.