静压与动压（Static pressure and dynamic pressure）

静压与动压（Static pressure and dynamic pressure） 静压与动压(Static pressure and dynamic pressure) A. static pressure (Pi) The pressure acting on the tube wall as a result of irregular motion of the air molecule is called static pressure. When calculating, the static pressure with absolute vacuum as the zero point is called absolute static pressure. The static pressure at zero atmospheric pressure is called relative static pressure. The air static pressure in the air conditioner is relative static pressure. When the static pressure is higher than the atmospheric pressure, it is positive, and is negative when it is lower than the atmospheric pressure. B. dynamic pressure (Pb) Refers to the pressure generated by air flow, as long as the air flow in the air pipe has a certain dynamic pressure, and its value is always positive. C. full pressure (P, q) Total pressure is the algebraic sum of static pressure and dynamic pressure: Pq = Pi ten Pb, total pressure represents the total energy of the L m3 gas. If the atmospheric pressure is the starting point of the calculation, it can be positive or negative. Total pressure = static pressure + dynamic pressure Dynamic pressure =0.5* air density * wind speed ^2 Residual pressure = total pressure - resistance of equipment within the system For example: air conditioning units are return, primary section, middle section, the heating section, the blower sections, each function section resistance respectively: 20Pa, 80Pa, 120Pa, 20Pa, 100, 50Pa, internal resistance for 290Pa, if the outside afterpressure is 500Pa. Just send fans'total pressure should not be less than 790Pa, if the machine outside the residual pressure of 1100Pa, just send fans'total pressure should not be less than 1390Pa, high pressure for general cleaning unit, the size and power of motor related to the selection of wind pressure. In general, according to the actual needs of the project remaining pressure, high pressure is not a good thing. Air conditioning units or air units will often fan installed in the end, outlet wind speed is high, high dynamic pressure, static pressure, static pressure box installation projects often sound at the outlet, reduce dynamic pressure, increase pressure, and flow, and silencing. Just express one's own superficial understanding of "surge": Generally refers to the blade machinery, when the air pressure along the blade motion increases, when the blade surface boundary layer thickening, airflow from the blade surface no longer along the leaf blade movement, separated from the air flow blocking circulation channel between the blades, the circulation flow cross-sectional area of the airflow body decreases, the flow capacity not through the air decreases and the fluid is piled up in the front of the mechanical blade. The obstruction will be improved with changes in the fluid parameters downstream of the blade - produce - improve, so call surge Total pressure or total pressure is the sum of dynamic pressure and static pressure. Dynamic pressure, that is, the kinetic energy obtained by the passage of fluid through a blade, is the velocity energy head. The static pressure on the impeller, and the usual fluid, is different from the static pressure produced by gravity. The static pressure here is due to the difference in blade pressure between the blade and the front of the blade. Relating to the speed of the blade. It is also related to the geometric size of the inlet and outlet pipe of the blade, which can be understood as the change of the pressure energy caused by the change of the cross section. Of course, the ratio of the total pressure to the hydrostatic pressure is also related to the type of blade. It is often said that the static pressure of a fan should mean total pressure. But in practical considerations, it is common to think of static pressure. Because this part of the pressure difference can be the head is the most favorable, and as far as possible to avoid from the speed can be converted to pressure energy. (I) muffler 1. Resistance Muffler: through sound-absorbing material to absorb sound energy, reduce noise, general micro perforated plate muffler belongs to this type, generally used to eliminate high, intermediate frequency noise. However, due to the structural reasons, it is not suitable for high temperature, high humidity and high speed. 2. An anti silencer: noise by changing the cross section. This is the principle of our commonly used anechoic static pressure tank. Generally lower middle and low frequency noise. There is no specific requirement for the wind system. 3. Impedance composite: of course, combine the two principles of structure. Low, medium and high frequency noise can be eliminated. But the requirements of the wind system are the same as the Resistance Muffler 4, for general civil air conditioning ventilation system, I personally think that the use of impedance composite muffler is good. The resistance muffler has good middle and high frequency noise elimination performance. According to the flow channel geometry is different, can be divided into straight type, type, type, labyrinth and honeycomb plate type, flow type, sound type, baffle elbow type etc.. The resistance muffler is suitable for eliminating middle or low frequency noise or narrowband noise. According to its principle of action, it can be divided into expansion type, resonance cavity type and interference type. Impedance compound muffler, resonance cavity, expansion chamber, perforated screen acoustic filtering device, the muffler good acoustic attenuation characteristics and impedance muffler good low-frequency noise characteristics, so the noise band width, it is the most commonly used standard series of muffler. Suitable wind speed of 6~8 meters / sec, up to 8~12 meters, can be used alone, can also be used in tandem. Noise elimination effect: low frequency 10 ~ 15dB/m, medium frequency 15 ~ 25dB/m, high frequency 25 ~ 30dB/m, average resistance coefficient is 0.4. According to "air conditioning", muffler elbow, muffler static pressure box belong to a muffler. (two) the function of muffler A muffler is a device that allows air to pass through and effectively prevents or weakens the outward propagation of sound energy. (three) static pressure box The static pressure box is a necessary accessory to reduce the dynamic pressure, increase the static pressure, stabilize the air flow and reduce the vibration of the air supply system. It can make the air supply more ideal. (four) the function of static pressure box; 1, can be part of the dynamic pressure into static pressure, so that the wind blow farther; 2, it can reduce noise 3, the air distribution evenly 4, static pressure box can be used to reduce noise, but also to obtain a uniform static air blowing, reducing dynamic pressure loss. And the function of universal joint. The static pressure box is well applied to the ventilation system, which can improve the comprehensive performance of the ventilation system. Two. Calculation method What NR curve, the acoustic calculation aside, P601 does not say so. Included users speech, for reference only (ranking)! 1, in the design of static pressure box, if according to the prescribed wind speed into the design, the box will be very large; general static pressure box long side should be wide out of the air pipe side 400mm, height should be wide out of the air pipe height 400mm. The figures came from the York design manual, and that was an estimate. 2, static pressure box, general engineer experience is 5~10db (a) /m, impedance composite type (seems to air conditioning ventilation system generally use this) muffler 10~15db (a) /m 3, control wind speed within 2.5, if the volume is too large, you may have to improve the wind speed, the length is generally greater than 1 meters, there is no other mandatory requirements 4, height * depth = static pressure box section area, static pressure box section area * 2.5m/s= fan air volume, as for height and depth how to match, own grasp it ~ ~! 5, use your unit volume L / 3 M / s, you can get a plenum area, then you according to your house if the height is 4 meters, you can set 2 meters high, minus the flexible joint is about 0.5 meters, 0.5 meters high above the left, the static pressure box you only 1 meters high, then you can determine the width, number two, number third is easily identified here, the most important is to look at their own air conditioning room enough, if enough position on the big points as far as possible!! The method of length is the same, you know the width, so your width multiplied by the length does not also have an area? This area is also equal to L/3, but in the design of the courtyard, the wind speed is 2m/s, and if enough space, it is bigger. 6, the static pressure box thickness is greater than 600mm, the airflow speed is less than 2m/s, also pay attention to the location and the location of a point air buffer, previously in Shanghai and East China Institute of grassroots brother, brother told me Mister huadongyuan strongly recommended by the static pressure box, the high cost of resistance, there are three conditions as far as possible the use of underpants general muffler elbow or pipe muffler. 7, East China Institute of CEOs advice I have seen, there are many such designs, I don't know why they don't plenum at first, but now that the static pressure box only metal box, and is not a static pressure box muffler cotton, the cost may not be too tight, if it is a static pressure box muffler cotton stick, it must consider the cost, in addition the muffler should pay attention to flow control, most are controlled within 8m/s, as long as it can reach this value, the noise reduction effect is good. 8, I think it is necessary to sound pressure tank, as a palm tree monk said, do not use the static pressure box when it is difficult to pick out the pipe in the engine room, sound is only the second best, velocity is less than 1.5m/s, the ratio of height to width to less than 1:4; as for resistance, most designers are aware, compared to the wind pipe the resistance calculation, I choose the capacity of fans certainly. 9. The function of the static pressure box is to change the dynamic pressure to the static pressure, and to distribute the air volume evenly 10, I remember that the technical measures of Beijing city the series seems to be said to the total air flow meter, through the three sections of a speed of not more than 1.5m/s 11, I generally choose at 4 meters / sec or so, an example, a shopping mall, wind box 24000 air volume, static pressure box size is 2000*600*600, with noise cotton, air-conditioning operation for a year, the effect can still 12, static pressure box is the main role of voltage regulation, noise reduction. It seems that the wind speed in the static pressure box is not more than 2.5m/s. According to the air volume and the wind speed, the volume of the static pressure box can be determined. The static box is usually made to order. Width direction should not be small, 500mm. Usually the static pressure box size than the indoor machine can be. 13, the use of underfloor air supply, Its static pressure box uses two kinds, one kind uses the mezzanine as the static pressure box, another uses the ordinary static pressure box, then in the static pressure box receives several air pipes to the air outlet. Mezzanine can also do static pressure box?