Acoustic Cleaning - Operation and Performance

Operation and Performance

The majority of acoustic cleaners operate in the audio frequency range from 60 hertz up to 420 Hz. However a few operate in the infrasonic range, below 40 Hz, which is mostly below the human hearing range, to satisfy strict noise control requirements. There are three scientific fields which converge in the understanding of acoustic cleaning technology.

• Sound propagation. This relates to an understanding of the nature of the sound waves, how they vary and how they will interact with the environment.
• Mathematics of the environment. Materials science, surface friction, distance and areas familiar to a mechanical engineer.
• Chemical engineering. The chemical properties of the powder or substance to be debonded. Especially the auto adhesive properties of the powder.

An acoustic cleaner will create a series of very rapid and powerful sound induced pressure fluctuations which are then transmitted into the solid particles of ash, dust, granules or powder. This causes them to move at differing speeds and debond from adjoining particles and the surface that they are adhering to. Once they have been separated then the material will fall off due to gravity or it will be carried away by the process gas or air stream.

The key features which determine whether or not an acoustic cleaner will be effective for any given problem are the particle size range, the moisture content and the density of the particles as well as how these characteristics will change with temperature and time. Typically particles between 20 micrometres and 5 mm with moisture content below 8.5% are ideal. Upper temperature limits are dependent upon the melting point of the particles and acoustic cleaners have been employed at temperatures above 1000 C to remove ash build-up in boiler plants.

It is important to match the operating frequencies to the requirements. Higher frequencies can be directed more accurately whilst lower frequencies will carry further, and are generally used for more demanding requirements. A typical selection of frequencies available would be as follows:

• 420 Hz for a small acoustic cleaner which might be used to clear bridging at the base of a silo.
• 350 Hz will be more powerful and this frequency can be used to unblock material build-up in ID (induced draft) fans, filters, cyclones, mixers, dryers and coolers.
• 230 Hz. At this frequency, the power involved is sufficient to use in most electricity generation applications.
• 75 Hz and 60 Hz. These are generally the most powerful acoustic cleaners and are often used in large vessels and silos.