Falling water noise of cooling tower and its prevention measures
Falling water noise of cooling tower and its prevention measures
1. Detection of cooling tower falling water noise
The measured noise and spectrum of some natural ventilation cooling towers are measured at a height of 1.2 m[1] at a distance of 5m from the bottom edge of the air inlet, that is, the edge of the pool of the general inverted T-shaped tower base.
2. Sound source characteristics of cooling tower falling water noise
Attributes of sound source: The noise source is the huge circular water surface under the falling water area, which is the steady-state water noise generated by the collision between the large-area continuous liquids of the cooling falling water in the tower and the pool water. It is one of mechanical noise, aerodynamic noise, and electromagnetic noise. A special kind of noise outside.
Instantaneous speed of falling water impact: 7-8 m/s[2]
Sound source sound level: around 80 db(a).
Spectrum: The audio distribution is a peak-shaped curve dominated by high frequency (1000-16 000 Hz) and intermediate frequency (500-1000 Hz) components; the peak is located at around 4 000 Hz.
Speed of sound: c=340 m/s.
Wavelength: λ=c/f; 1.36m (250 hz) ~ 0.02 m (1 000 hz), mainly 0.085 m (4 000 hz).
3. The influence range of cooling tower falling water noise
3.1 Distance attenuation law of sound waves
The attenuation characteristic of falling water noise with distance conforms to the law that hemispherical wave attenuates with the expansion of energy distribution in the process of propagation. db.
The sound source of falling water noise is a built-in circular water surface, and the sound waves in the cavity propagate outward through the air inlet, so the air inlet can be regarded as the edge of the sound source. The sound wave does not immediately attenuate according to the distance attenuation law of the "point sound source". For each doubling of the sound energy attenuation by 3 db), in the transition area of the distance attenuation law, only when the sound receiving point (measurement point) is moved to the rear beyond the annular air inlet of the cooling tower can be regarded as a "point", the sound wave will not be detected. Begins to decay according to the distance decay law of "Point Sound Source". Therefore, in the range other than "point sound source", as long as the sound level of a certain measuring point is known, the sound level of any point can be obtained according to the above formula.
3.2 The cooling tower is the starting position of the "point sound source"
According to the existing measured data of distance attenuation, the law of each starting position d (the air inlet is regarded as the edge of the sound source) is analyzed, and the starting position d when the cooling tower is regarded as the "point sound source" can be estimated by the following formula:
d=a1/2/4
In the formula: a——the cooling tower area, m2.
Taking the cooling tower of 2 000 m2 (Yihua Power Plant) - 9 000 m2 (Wujing Power Plant), which is common in my country at present, as an example, the starting position of the "point sound source" is point d (starting from the bottom edge of the air inlet), They are 11.18 m and 23.72 m respectively. It can be seen that basically all cooling towers can be regarded as "point sound sources" for noise measuring points located 25 m away from the tower (from the bottom edge of the air inlet).
3.3 Assessment of cooling tower noise impact range
Although the sound level of cooling tower noise is not very large in industrial noise, and its sound energy is also attenuated by 6 db with each doubling of distance ("point sound source"), due to its large sound source, it is The attenuation starting distance of the noise is relatively far (25m), and it has tripled to 200m, which is only 18 db lower than that at 25m, so its influence range is much larger than that of general industrial noise. Still taking a cooling tower of 2 000-9 000 m2 as an example, the measured sound levels at a distance of 25 m (the measurement point outside the "point sound source", starting from the bottom edge of the air inlet) are 71.7 and 77.ldb (a) , if according to the distance attenuation law of "point sound source", that is, the sound energy attenuates by 6 db for each doubling of the distance, the sound level at 50 m should be 65.7 and 71.ldb (a) respectively; the sound level at 100 m should be 59.7 and 65.ldb(a) respectively; the sound level at 200 m should be 53.7 and 59.ldb(a) respectively, and the sound level at 220 m should be calculated as 52.9 and 58.3 db(a) respectively . This is an approximate estimate of the noise impact range (strength), which includes the range of various types of towers commonly seen today. With this method, we can evaluate the sound levels of various tower types (single towers) according to the measured sound levels at 10-25 m (the starting position of the "point sound source" corresponding to the size of each tower and its tower size) at the far measuring point. The range (velocity) of noise influence. But this is just a simple and rough evaluation method under ideal conditions. And the influence of other sound sources, the actual distribution and attenuation law of various cooling tower noises will be outstanding. According to the actual measurement of the falling water noise of the 9 000 m2 cooling tower of Wujing Power Plant [4], the noise value measured at the sound receiving point 220 m away from the tower is 55.4-58.3 db(a) (another test result is 61.9 db(a), which is estimated to be affected by the downwind), which is in good agreement with the result of our estimation of 58.3 db(a) at 220 m based on the measured sound level at 25 m. Figure 2 shows the range of influence of cooling tower noise. As can be seen from Figure 2, due to the huge sound source of the cooling tower, the noise level attenuation is very slow within the range of 10-25 m from the air inlet, and the theoretical value of the sound level attenuation within the distance from the "surface sound source" is zero. However, for a general sound source with a small scale (about 1m), since there is no attenuation form of "surface sound source" and "line sound source", the sound level of the sound source is attenuated by the "point sound source" at the beginning. The rate drops rapidly.
4. The basic way and control method of cooling tower noise control
The noise of large cooling towers belongs to medium and high frequency steady-state noise, and the "nominal sound level" of the sound source is about 80 db(a). Within the national standard for noise corresponding to the local environment.
4.1 Governance Approach
In view of the occurrence mechanism and transmission mode of noise, the control of cooling tower noise can be attributed to two basic approaches: inside the tower and outside the tower. There are three ways of sound wave blocking (sound insulation), sound wave absorption (absorption and attenuation along the way) and distance attenuation (sound energy diffusion). Among them, sound wave isolation supplemented by sound wave absorption is the main means of external treatment. Whether it is the sound source control technology in the tower or the external tower acoustic wave blocking technology that has been applied abroad, the application in my country has just started, so there is a lack of practical application. Experience. The following list summarizes and recommends several cooling tower noise control technologies for engineering reference selection, their respective characteristics and applicability.
4.2 Management of sound sources in towers
The dy-1 cooling tower falling water energy dissipation and noise reduction device is mainly composed of two parts: "supporting frame" and "falling water energy dissipation and noise reduction device". The "supporting frame" can be divided into two types: floating and fixed. The "falling water energy dissipation and noise reduction device" is mainly in the form of a hexagonal honeycomb inclined pipe, with a floor height of 18 cm. It consists of four functional sections: vertical guide section, silent rubbing inclined section, viscous deceleration inclined section, and evacuation, spilling and deflecting section. composition.
4.2.3 Material selection
The floating type falling water energy dissipation and noise reduction device is mainly composed of plastic parts or glass fiber reinforced plastic parts (stressed parts) which are formed by extrusion, injection molding or hot pressing. Its material features are light in structure, easy to handle, easy to install, anti-corrosion and durable.
The material of the upper support frame and noise reducer of the fixed falling water energy dissipation and noise reduction device is the same as that of the floating type, the difference is that the fixed primary and secondary support beams at the lower part are composed of section steel. The anti-corrosion treated section steel (q235) has the characteristics of high strength and good rigidity.
4.2.4 Noise reduction effect
Under the standard test conditions of drop h=6 m and water spray density q=8 t/(m2·h), the comparison of sound level and spectrum test results of cooling tower simulated falling water sound source and noise reduction device is shown in Figure 3 [ 5]. The noise reducer cuts out the high frequency content of the falling water sound source. Floating falling water energy dissipation and noise reduction device, 260 yuan/m2, fixed falling water energy dissipation and noise reduction device, 300 yuan/m2
4.3 Sound wave blocking outside the tower
4.3.1 Noise reduction principle
When sound waves encounter obstacles in the process of propagation, three phenomena of reflection, transmission and diffraction will occur. The sound barrier is to insert a facility between the sound source and the sound receiving point to cut off and absorb the direct sound waves from the sound source to the sound receiving point, so that some sound waves are blocked and reflected, and some sound waves are transmitted through the screen after absorption and attenuation ( It reaches the sound-receiving point by additional attenuation forms such as (minimum) and screen top diffraction, so as to reduce the noise impact of the sound-receiving point and achieve the purpose of noise reduction.
4.3.2 Formal structure
The structure of the sound barrier can be divided into two parts: above-ground and underground. The above-ground part is a giant, continuous panel façade (including diagonal braces) that shields sound waves with a thickness of about 20 cm. Parts are load-bearing and anti-overturning (wind load) foundations.
In principle, the height and width of the barrier are limited to blocking the direct sound waves from the sound source to the sound receiving point. Generally speaking, in order to improve the shielding effect, the height of the barrier is usually not less than 1.3 times the height of the air inlet; in order to avoid affecting the air intake, The distance between the barrier and the air inlet is usually not less than 2 times the height of the air inlet.
4.3.3 Material selection
The above-ground part of the sound barrier, that is, the shielding layer, can be made of brick wall, thin steel plate, aluminum alloy, glass fiber reinforced plastic, polycarbonate plastic, etc. to resist aging. Corrosion-resistant materials; the underground part of the sound barrier, that is, the foundation, is mainly made of concrete and steel.
4.3.4 Noise reduction effect
The diffraction phenomenon of the sound wave encountering the barrier will weaken the sound insulation effect of the sound barrier, and the diffraction ability is related to the frequency of the sound wave, so the noise reduction effect of the sound barrier has a great relationship with the frequency or wavelength of the sound wave. The sound barrier has a very significant shielding effect on high-frequency waves with short wavelengths and is not easy to diffract, and can form a long sound shadow area behind the barrier. Of course, the influence of diffracted sound waves on the sound-receiving point can also be weakened by raising the barrier. Since the sound barrier has an obvious and effective shielding effect on high-frequency sound waves, and the frequency spectrum of the cooling tower falling water noise is mainly composed of medium and high frequency components, the sound barrier is used to isolate and absorb the direct sound waves from the sound source of the cooling tower to the sound receiving point. Noise reduction effect.
The noise reduction effect of the sound barrier can reach about 25 db(a) in the local area close to the barrier in the sound shadow area [3], which is a good solution for the evaluation rules based on the factory boundary test results; The noise reduction level outside the shadow area will bounce back due to the arrival of the intermediate frequency diffraction sound wave, but for the high frequency wave, the attenuation can generally reach 10-15 db(a) [6] (excluding the distance attenuation part) However, since the cooling tower falling water noise still contains intermediate frequency components, its noise reduction effect will be discounted. In this way, for the sound receiving point outside the factory, in order to achieve a satisfactory noise reduction effect, it should be adjusted by increasing the height of the barrier without affecting the air intake.
