On the structural design and construction of ventilation pipes: most ventilation pipes are hoisted horizontally, which makes them bear approximately uniform load in the vertical direction under the action of their own weight. However, due to the asymmetric installation of various accessories (such as tee, elbow, tuyere, valve, inspection port, etc.). ) On the side of the air duct, they will bear some torsion besides gravity. When conveying airflow, the air duct should bear uniform internal pressure (positive pressure air duct) or external pressure (negative pressure air duct). There are other factors, such as asymmetric bending caused by the opening in the side wall of the air duct; Temperature effect will occur when conveying non-warm air flow. 1 Reasonable use of different plate thicknesses according to actual engineering In order to reduce the weight of the air duct itself, while ensuring the normal operation of the equipment, the smallest plate thickness should be selected as far as possible according to the size and pressure grade of the air duct to be used in actual engineering design. At present, a large number of hot-dip galvanized steel plates and steel belts are used in metal air ducts, and the matrix is cold-rolled low-carbon structural steel plates with tensile strength >: 270N/mm2 and elongation >; Within σ= 0.6 mm ~1.2 mm, the thickness tolerance is 0.07 mm ~ 0.1.3 mm. In the past, it was considered that the industrial technology, testing equipment and testing methods in developed countries were more advanced than those in China. Therefore, under the same size and pressure grade, the thickness of domestic air ducts is generally about 20% larger than that of foreign standards, so the consumption of plates and the weight of air ducts themselves will increase accordingly. However, with the improvement of industrial technology in China, domestic manufacturers are fully capable of mass production of high-quality plates with uniform thickness, so the minimum thickness of air duct should be as close as possible to foreign standards to avoid wasting plates. The thickness specified in GB50243-2002 is close to the thickness of high-pressure air duct in foreign standards, which is guaranteed in technology and practice, but obviously not economical and reasonable. From the design point of view, a system can completely adopt several duct thicknesses corresponding to different pressure levels according to the compression conditions at different positions. The end of the positive pressure air duct system or the front end of the negative pressure air duct can adopt the low pressure air duct, and the medium pressure or high pressure air duct can be adopted near the fan. Practice has proved that the investment can be saved under the condition of ensuring the stability of the system by reasonably configuring the air duct. 2 Choose the appropriate connection mode of longitudinal seam For rectangular air duct, the joint corner bite is suitable for any pressure level, but in recent years, the button-type bite has been widely used and has gradually replaced the joint corner bite because it can save a flanging process. However, it should be noted that the buckle is only used for low-pressure and medium-pressure air ducts. For vertical seams and sealed vertical seams, rivets, self-tapping screws (spacing 150 mm ~ 300 mm), spot welding and indentation can be used for fastening. The longitudinal sealing vertical seam can not only enhance the bending strength of the air duct and the stiffness of the panel wall, but also make full use of the width of the panel when making a rectangular air duct with a larger size, thus saving metal materials and manpower. This method is especially suitable for rectangular air ducts with long side size >: 1.5 m, especially for smoke exhaust ducts, which are not suitable for purification systems due to the high airflow noise. 3. Choose appropriate transverse connection mode to save metal consumption. The allowable pressure of air duct depends not only on the wall thickness, but also on the type of connection and the reinforcement method of panel wall. Transverse connection is not only a means to extend the length of the air duct, but more importantly, it plays a role in enhancing the stiffness of the air duct and keeping the transverse dimension of the air duct accurate. The commonly used detachable transverse connection is angle steel flange connection, but the flange connection has been widely used in medium-sized air ducts with medium and low pressure (the long side of rectangle is less than 1.6 m). In recent years, splint connection and socket connection have been applied in some public buildings in China. The author thinks that the five connection methods, such as flat splint, vertical splint, S-shaped crimping splint, vertical seam and integral flange with splint, have simple technology and good structural performance, and can generally be processed on site or prefabricated in the factory without complicated molding tools, which is more suitable for the construction status of air ducts in China. At present, all the existing bite machines have blade heads, which can be made even on manual folding machines; Although the socket connection has good manufacturability, the stiffness of the air duct in this way is not ideal. If the reinforcing hoop is used at the joint at the same time, the material will increase, so it is more reliable to use it only in small-sized air ducts. In addition, the special-shaped flange connection made of S-shaped double vertical rolled edges and angle steel reinforced double vertical rolled edge plywood is similar to all kinds of plywood and vertical seam connection in the range of low-pressure air duct, but it is rarely used at present because of its complicated processing technology and high cost. Almost all of the five types of flange-less connections recommended above can save angle steel, flange bolts and most (even all) rivets. These auxiliary materials account for about 3 0% of the steel used in the air duct, and can save 25% of the labor input. The air conditioning system project of a national demonstration high school in Chaozhou, designed by the author, adopted the new technology that the air duct is not connected with the flange. Practice has proved that the project has saved a lot of metal materials, the equipment runs well and achieved good economic benefits. 4. Reinforce the panel wall according to the functional requirements, which is one of the decisive factors that determine the use pressure of the air duct. There are mainly two ways, one is to make the plate wall itself into a strong rib shape, and the other is to evenly distribute annular, horizontal or vertical reinforcing ribs between the nodes formed by two connecting flanges or other connection methods. For the reinforcing plate, the most commonly used is the convex cross rib, which can be folded gently with a manual folding machine. This type is mostly used for negative pressure pipes such as smoke exhaust pipes, and folding generally requires a special press. The external reinforcing ribs are used to maintain the cross-sectional shape of the air duct and strengthen the panel wall. The specifications of reinforcing ribs are usually the same as those of angle steel used for flange connection, and light profiles can also be used, and their spacing is the same as the allowable distance of transverse connection. In addition, it is necessary to explain the reinforcement method inside the air duct. Usually, round steel is used as the strut, and both ends are welded on the connecting flange or stiffener with nuts. It is also useful to use flat steel as support, and both ends are fixed to the plate wall or flange with bolts or rivets. If flat steel is used, it should be approved by the designer. When conditions permit, the airflow noise should be tested first, and then put into use after passing the test to ensure that the airflow noise is within the allowable range. 5. Pay attention to the tightness of the connecting parts. Even the most common air duct has certain restrictions on the leakage of air flow, which requires that the longitudinal joints and transverse joints of the air duct should have good sealing performance. Use prefabricated gaskets in the form of coils or sheets between flanges or other connected metal opposing surfaces, which can be rubber sheets, asbestos rubber sheets or polyester materials. In the flue, due to the temperature, attention should be paid to the use of incombustible substances such as asbestos rope. For rectangular duct with long side less than 1, the thickness of gasket is required to be greater than or equal to 3 mm. 5m, and the large air duct is greater than or equal to 4mm. For longitudinal joints and transverse joints, sealant is required to be applied between plates and sections, but in practice, the joint angle bite of longitudinal joints of ordinary low-pressure air ducts is not necessarily applied; More vertical seams are used; Splint connection shall be adopted. The sealant can be extruded into the joint during the assembly of the joint, or applied from the outside after the assembly is completed. Whether the sealing performance of the air duct is qualified or not is evaluated according to the specified maximum leakage. Air leakage test is a compulsory test for high-pressure air ducts. Generally speaking, the leakage test can be carried out in stages. Calculate the surface area of the air duct in advance, and then plug all the openings in the air duct system. The test pressure is taken as the average pressure PM = (P 1+P2)/2 (where P 1 and P2 are the starting points of the test air duct system respectively). For low and medium pressure air ducts, it is usually only necessary to decide whether it is necessary to test according to the construction instructions according to the design drawings. The author thinks that the low-pressure air duct can actually achieve the required sealing performance if the construction unit can make it in strict accordance with the design drawings and construction specifications. Unless there is a special need, there is generally no need to carry out the actual leak test. Conclusion The above is the author's analysis of some problems in the structural design and construction of ventilation pipes combined with engineering practice. By reasonably improving the duct structure, it will undoubtedly save metal materials and manpower investment. In the design and construction of a four-star hotel in Chaozhou, the author took the above measures according to the engineering characteristics. The HVAC project can save 20% of metal materials and exceed 15% of manpower input, which is well received by the owners. Reference [1] Ma Zhiqi. Construction technology of ventilation and air conditioning engineering machinery [M]. China Building Materials Industry Press, 2006, 1. [2] Shao. Atlas of architectural ventilation and air conditioning engineering design [M]. Machinery Industry Press, 2005, 1 1.