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How does it work?This question has been asked many times when discussing the benefits of acoustic cleaning. Those who have heard or felt high intensity sound intuitively think that it somehow works by shaking the tubes or heating elements. Those involved with acoustic cleaning know otherwise.
How Does it Work?Sound waves travel through flue gas (compressible media) like waves travel in the ocean. The water moves up and down as a result of the wave passing by, but there is no transverse movement of the water. Sound waves are pressure oscillations (from plus to minus pressure values), traveling at the speed of sound. The flue gas molecules, when exposed to these pressure waves, migrate toward the low pressures. Since numerous pressure cycles per second travel past any given point, the molecules oscillate back and forth. The amplitude of this displacement is very small, but increases as frequency drops. This scrubbing action and particle displacement is responsible for two effects: The first effect is prevention of particles settling on surfaces. The particles are kept in suspension and are carried away with the flue gas. The second effect is the removal, over time, of particles that have already accumulated on the heating surfaces. The gas flow velocity will carry the loosened deposits away. Since the forces are miniscule, they affect only the small ash particles and have little or no effect on internal structures.There are two phenomena that need to be considered when selecting an operating frequency for an Acoustic Cleaner. Displacement vs Frequency:As explained above, the gas particle displacement is greatest at lower frequencies. This displacement is proportional to the inverse square of frequency. When we compare 200 Hz sound with 20 Hz sound it can be calculated that the displacement is 100 times larger at 20 Hz than at 200 Hz if the sound pressures are equal. When compared with 75 Hz the displacement at 20 Hz is still 14 times larger at equal sound pressures. Reflectiveness vs Frequency:As frequency gets lower the wavelength of the sound gets longer. As wavelengths increase, so does the sound’s ability to reflect off of rigid surfaces. Sound reflected on itself causes reverberation and an increase in the sound pressure within an enclosure. This translates into significantly larger areas that can be covered with a single Acoustic Cleaner Cleaning Effectiveness:Since the sound field is uniform throughout the target cleaning area, the cleaning is more uniform as well. As is indicated, cleaning with low frequency sound is volume cleaning (through several tube banks, or the entire air preheater rotor). The backside of the tube is equally clean as the front side of the tube close to the source. In general, WaveMasterTM Acoustic Cleaners do not need to be close to the area to be cleaned. At frequencies higher than say 60 Hz only the frontal surface of the first few rows of tubes are exposed to high intensity sound, but can not penetrate through the tube bank without loosing cleaning capability. While oversimplified, this explanation provides a basis for understanding how low frequency sound is used to clean ash and soot particles from heat exchangers in boiler systems.
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Technology Explained
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