Shandong zhangqiu yi tai machinery manufacturing co., LTD
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Abstract: according to the characteristics of the roots blower, combined with scientific research for noise control at home and abroad, put forward on the design and manufacture methods of reducing the noise of a roots blower works, and lists some specific examples to illustrate.
Keywords: roots blower noise eddy current
1 the introduction
Roots blower suction, exhaust air volume is very big, and every time have a mutation phenomenon, resulting in a large noise, known as "tiger" of mechanical products, especially in the case of high pressure especially, and the air volume is larger, the higher the pressure, the faster the speed, the greater the noise, and modernization of production and hope roots blower can provide higher pressure and larger volume. In order to improve the fan performance, to reduce the noise pollution, meet environmental protection requirements, engineers have tried all kinds of countermeasures. This text set about from the noise source, the design and manufacturing has put forward some methods to reduce noise.
2 noise analysis
Roots blower noise mainly includes mechanical noise and aerodynamic noise, and the aerodynamic noise including rotation noise and vortex noise. Mechanical noise mainly include gear noise, bearing and pipeline vibration noise, etc. Rotation noise is in the rotation of the impeller over the narrower the channel exit, along the circumferential pneumatic pressure and airflow velocity has a big change, make periodic suction, exhaust and instantaneous constant volume compression of air velocity and the pressure pulsation, the aerodynamic noise. Eddy current is also called the turbulence noise, which is caused by turbulent boundary layer and its from air pressure pulsation. On the one hand, the impeller rotates, the surface form vortex, the eddy current in split surface produced vortex noise; On the other hand, the high pressure gas to the low pressure area through clearance leakage and through the orifice, corners also produces eddy current noise. These noise combined with fan inlet volume of helmholtz resonance, makes the roots blower noise reached the unbearable degree.
Three structural design
3.1 the reflux hole design
In case the rotor center to open a certain stage of the wind the U shape of the hole, can reduce the pressure of the outlet end, in the impeller and casing, which is formed by the wall vessel is about to enter closed state, can make the high pressure gas has a small part of the outlet back into the vessel, and make the vessel with the outlet chamber formed certain pressure balance. Continue at the same time, when the impeller rotates, vessel volume smaller, pressure increases, but also makes the airtight vessel before a large amount of exhaust gas by reflux hole walkthrough, so can reduce the vortex noise "blind Angle" gas, but also reduce the exhaust due to pressure too release the impact of noise. This is the current domestic is increasingly developed "countercurrent cooling technology. Inlet reflux hole hole should be connected to the "blind Angle", and export direction should be consistent with exhaust direction; The size of the hole also shoulds not be too large, generally take 10 ~ 15 mm, and the Angle between the delta also should be less than 20 °, otherwise it will caused by excessive discharge in air volume can not meet the requirements.
3.2 design shaped into the outlet
Traditional roots blower into the outlet for the rectangular, inhale, the impeller outer circle into the sealing area at the same time, make the gas shut down suddenly, when the exhaust impeller outer round and open at the same time, the high pressure gas release suddenly, makes the suction and discharge gas can produce high noise and large vibration. Designs into the outlet abnormity, breathed in the seal and discharge of the open based on open area from maximum to zero and from zero to maximum, are based on the gradient, thus delaying the into the vent gas pressure difference rate of change, have the effect of cutting cyclical impact exhaust noise, thus make the low noise and smooth. Alien mouth forms a lot, from the perspective of easy fabrication, the most commonly used is the diamond or oblique mouth mouth, orifice Angle is related to the size and length of the casing. Usually, tuyere, casing short, the slope can be big, appropriate design of oblique mouth, make simple; On the other hand, the appropriate design of the diamond.
3.3 rotor cascade design method
Impeller generally as a whole and the shaft coupling, if the impeller along the axial into several segments, are concatenated rotor. Each section of the impeller with the same leaf type, diameter, and even the same length. Concatenated, two adjacent impeller circumferential stagger certain Angle (90 ° staggered, lobes 3 leaf stagger 60 °), set the partition between and within the chassis or impeller, lie between the corresponding period, each piece of work is similar to a single fan. Because of the paragraphs in the working process of the impeller has a certain time, reduce air pulse, and compared with the length of a single impeller total exhaust flow rate is constant and pulse becomes more stable, noise is relatively low.
3.4 twisted impeller design
Tooth roots blower impeller generally parallel to the axis, the straight tooth shape, so processing, testing is more convenient, but with the development of processing technology, still should be designed as a twisted impeller, namely inclined dentate, because this will increase the length of the line of action. Twisted blade roots blower work smoothly and small gas pulsation, low noise, and work with internal compression process, compared with the straight blade roots blower with high efficiency, low energy consumption, is a traditional roots blower alternatives.
3.5 the impeller curve of CAD design method
In the heart of roots blower impeller parts, the surface shape is critical, gas is through two mesh on the surface of the impeller, suction and exhaust. In order to make the mesh properly, the impeller impeller curve is generally designed involute and cycloid envelope or circle. Based on the design and manufacturing technology, the traditional impeller generally designed as simple line, through mathematical method to calculate various parameters, including the center distance, base circle, pressure Angle, initial meshing Angle, etc. With the development of computer and numerical control technology, CAD design software and function of nc programming software is becoming more and more powerful, should make full use of the software resources, the impeller curve segment, combination design, get rid of the previous single curve, through CAD simulation, simulation, guarantee the impeller under any circumstances when mesh can be periods of relatively fixed. Because this kind of combination curves in modern CNC machine programming, processing is not difficult. Uniform impeller clearance can not only greatly improve the stability, lower noise, such as air volume, vibration, but also can guarantee life important mechanical properties.
4 manufacturing precision
Improve the means of the increased cost of product, but in order to meet the required performance, to improve the accuracy of corresponding aspects. Here is to meet the low noise performance should improve accuracy are put forward.
4.1 the impeller surface quality and balance
Impeller surface quality mainly depends on the material and the processing quality. For small impeller, generally choose to cast steel or nodular cast iron, cast and shaft, the impeller choose HT200, roughness in Ra3. 2, on the nc machine tool processing, take small feed, lower roughness can be obtained; Rotor balance at least should guarantee the G6. 3, best to G5. 6.
4.2 bearing accuracy
Bearing as wearing parts, general enterprises are unwilling to improve the precision to make the product cost increase, so often do more harm than good. Because of the low precision bearings produce larger vibration and friction, and as a whole machine assembly benchmark, on the performance of the whole machine and other components have crucial influence service life. Foreign fan bearing precision generally at least equivalent to a C grade standard of our country.
4.3 the gear precision
Gear clearance, motion accuracy and precision of tooth to directly determine the uniformity and stability of impeller meshing, tooth surface roughness and friction of one of the main sources of noise. Therefore, according to the requirements of national standard gear accuracy should guarantee the above magnitude 7, and general mechanical processing plant gear processing, testing means are not strong, the precision can't meet the requirements. So the gear processing is best and professional gear processing factory cooperation.
4.4 air duct quality
Smooth surface of the air duct can let air pass, is not only beneficial to reduce the loss, but also greatly reduce from the air flow is blocked whistlers, roughness of pipe wall should be reduced as far as possible, therefore, reduce the number of corners; Into the outlet should not be in a rapidly varied flow field, should by the party go round smooth transition. If the system has multiple pipe fittings such as elbow, pipe, etc., is the distance between them should be opened 5 ~ 10 times the pipe diameter.
5 the silencing, sound insulation and vibration isolation measures
In addition to the structure and the manufacturing precision of control noise outside, should be used in bearings, gears, seal high quality lubricating oil, into the outlet, a muffler, the sound insulation cover outside the machine and the supplementary equipment design, conditional place to roots blower fan work in the basement or in water for sound insulation and vibration isolation, etc.
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