Cooling tower noise management
Cooling tower noise refers to the cooling tower operation process of water pumps, pipelines, valves and tower intake, exhaust, deceleration noise, drenching noise and radiation noise to the outside.
The cooling tower exit noise is 5-10dB (A) higher than the entrance noise, and the spectrum is characterized by a low-frequency-based continuous spectrum, which is low-frequency noise.
When the circulating hot water in the cooling tower falls down from the shower device, the shower generated by the collision with the water in the water catching tray at the bottom of the tower belongs to the high-frequency noise, and the size of the shower is related to the height of the shower and the water flow per unit of time.
The cooling tower as a whole noise is a low-frequency-based continuous spectrum, there is no obvious noise peak, generally between 31.5 and 2000 Hz, the noise level of 55 to 85dB (A).
Large cooling tower noise belongs to the medium and high frequency steady state noise, the sound source of the "nominal sound level" of about 80db (A), cooling tower noise management objectives should in principle be the noise interference with the sound receiving point noise level control in the noise and the local environment relative to the national standard range.
The cooling tower noise source mainly consists of the following four parts:
1) fan intake and exhaust noise;
2) water shower noise;
3) fan reducer and motor noise;
4) cooling tower pumps, piping and valve noise.
Evaluation indicators
Currently, the cooling tower has two different evaluation indicators noise. One is the cooling tower design and manufacturers of national product standards GB/T 7190.1-2008 and GB/T 7190.2-2008 "glass fiber reinforced plastic cooling tower". This standard specifies the user's national standards GB3096-2008 "sound environment quality standards" for different circulating water volume and model products. These standards specify the maximum sound level in different environmental areas. If the enterprise produces cooling towers in accordance with the maximum limits of GB/T 7190.1-2008 and GB/T 7190.2-2008, all the products cannot meet the requirements of night noise ≤ 45~50dB(A) in GB3096-2008 in the area of class II, and only a few low-tonnage and ultra-low-noise cooling towers can meet the requirements of the class A Regional night noise standard requirements Therefore, according to the national environmental noise standard GB3096-2008, the cooling tower around the residents and government environmental protection departments require cooling tower users to pollute the cooling tower noise management.
Noise measurement
Small and medium-sized (single tower cooling water ≤ 1000m3/h) and large (cooling water 1000m3/h) glass fiber reinforced plastic cooling tower noise measurement according to the following methods, see GB/T 7190.1-2008 "small and medium-sized glass fiber reinforced plastic cooling towers," and GB / GB / T 7190.2
The noise measurement of large glass fiber reinforced plastic cooling tower is carried out according to the following methods.
Preparation
Measurement of cooling tower air outlet, air inlet and shell noise, should be measured at each point of measurement of the A sound level and the center frequency of 31.5 Hz ~ 8 kHz octave band sound pressure level. Measurement should be carried out during the normal operation of the cooling tower. Measurement should be carried out before the background noise test, the measurement of the surrounding environment must be quiet. The background noise should be at least 10dB(A) lower than the cooling tower noise, otherwise the measured value should be corrected. The microphone shall be equipped with a windshield during the measurement. When the wind speed exceeds 5 meters per second, stop the measurement.
Measurement method
(1) When measuring the cooling tower air outlet noise at the measurement point location, the measurement point is selected in the direction of 45 degrees away from the air outlet, which is twice the diameter of the air outlet of the air conduit. When the diameter of the air outlet is greater than 5m, the measurement distance is 5m.
When measuring the cooling tower air inlet noise, the measurement point should be selected in the direction of the air inlet. According to the horizontal distance of the tower wall, the tower diameter should be doubled. When the tower diameter is less than 1.5m, the measurement distance is 1.5m:When the tower is square or rectangular, the equivalent diameter of the tower is used as the measurement distance:A and B are the side lengths of the tower. The location of the measurement point is shown in Figure 1. When measuring the cooling tower noise, the horizontal distance between the measuring point and the tower is two times the diameter of the tower.
(2) When measuring the height and measuring the noise of the cooling tower inlet, the measuring point is 1.5
(2) Improve the water distribution or water collection system to reduce the water spray noise;
(3) Set up anechoic facilities around the cooling tower;
④Keep the cooling tower away from the noise of sensitive areas.
Governance
Noise reduction measures for fans
(1) Increase the impeller diameter, reduce the fan speed, reduce the circumferential velocity. According to the characteristics of the cooling tower and energy saving requirements, increase the impeller diameter, reduce the outlet dynamic pressure, so as to realize the requirements of energy saving and noise reduction. Reduce the circumferential speed is also one of the effective ways to reduce fan speed noise.
(2) large arc transition wide blade shape similar to a rounded rectangle, suitable for low speed, to meet the requirements of high efficiency and noise reduction.
(3)The airflow disturbance of the airfoil blade is smaller than that of the flat blade of equal thickness, especially at high airfoil and high rotational speed, it has higher lift coefficient and larger impact angle, which is conducive to the reduction of cyclic disturbances and tail vortex, and it can realize a larger amount of noise reduction and has good aerodynamic performance.
(4) Adoption of mean-flow contraction profile line maximizes the realization of mean-flow velocity field, minimizes the vortex at the inlet of the fan, and ensures the normal operation of the axial fan.
(5) The clearance between the outer edge of the fan blades and the casing should be constant, otherwise uneven disturbances and periodic pulsating noise will occur. Therefore, the radial runout of the outer edge should be controlled.
6 The end faces of the fan blades should be in the same plane, otherwise turbulent noise will be formed.
⑦Improve the machining and installation precision of the whole machine parts, and at the same time reduce the fan noise.
⑧ Improve the rotor balance accuracy, first static calibration, then dynamic balance calibration, in order to reduce the resulting vibration and fan blade interference noise.
Measures to reduce water overflow noise
(1) increase the thickness of the filler, improve the arrangement of the filler;
(2) the "snowflake" suspended between the filler and the water surface of the water catching tray (because of its shape similar to a snowflake, formed by the lateral pressure of the diameter of the high-pressure polyethylene), which can reduce the difference between the water droplet
(3) laying polyurethane porous foam on the receiving surface. This is a cooling tower specializing in new material noise reduction. It not only has the flexibility of ordinary foam, but also has the permeability of porous water permeability, can reduce the impact of falling water noise.
(4)Increase the parabolic radial sound insulation board at the air inlet, which does not affect the air inlet, and does not directly radiate the noise of falling water to the outside.
Setting sound barriers
In cooling tower noise control engineering, sound barriers are a commonly used noise reduction measure. However, the use of sound barriers around cooling towers can bring a series of problems. Attention must be paid to the following three points:
(1)Generally speaking, adding sound barriers will affect the normal air intake and cooling effect of the cooling tower, which depends on whether the cooling tower initially selected has sufficient capacity, otherwise it should be used with caution.
(2) Generally speaking, sound barriers for cooling towers can only be set on one side, and at most they can be arranged in an L-shape. If the noise affects the general public, the effect of setting the barrier is not ideal.
(3) The height and width of cooling tower sound barriers are generally large, and most of them are installed in high places with high wind pressure. During construction, consideration should be given to whether the original building is solid and whether there is an installation location.
Adding muffling equipment
The increase of air inlet and outlet device muffling also affects the ventilation effect, so the device muffling ventilation resistance is less than the device muffling.