Acoustic cleaning

Acoustic cleaning is a maintenance method used in material-handling and storage systems that handle bulk granular or particulate materials, such as grain elevators, to remove the buildup of material on surfaces. Acoustic cleaning apparatus, usually built into the material-handling equipment, works by generating powerful sound waves which shake particulates loose from surfaces, reducing the need for manual cleaning.
History and design.
An acoustic cleaner consists of a sound source similar to an air horn found on trucks and trains, attached to the material-handling equipment, which directs a loud sound into the interior. It is powered by compressed air rather than electricity so there is no danger of sparking, which could set off an explosion. It consists of two parts:
The overall length of acoustic cleaner horns range from 430 mm to over 3 metres long. The device can operate from a pressure range of 4.8 to 6.2 bars or 70 to 90 psi. The resultant sound pressure level will be around 200 dB.
There are generally 4 ways to control the operation of an acoustic cleaner. 
An acoustic cleaner will typically sound for 10 seconds and then wait for a further 500 seconds before sounding again. This ratio for on/off is approximately proportional to the working life of the diaphragm. Provided the operating environment is between −40 C and 100 °C, a diaphragm should last between 3 and 5 years. The wave generator and the bell have a much longer life span and will often outlast the environment in which they operate.
The older bells which were made from cast iron were susceptible to rusting in certain environments. The new bells made from 316 spun steel have no problem with rust and are ideal for sterile environments such as found in the food industry or in pharmaceutical plants.
Acoustic cleaning began in the early 1970s with experiments using ship horns or air raid sirens. The first acoustic cleaners were made from cast iron. From 1990 onwards the technology became commercially viable and began to be used in dry processing, storage, transport, power generation and manufacturing industries. The latest technology uses 316 spun stainless steel to ensure optimum 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.
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:
Health and safety.
The introduction of acoustic cleaners has been a significant improvement in many areas of health and safety. For instance in silo cleaning - the previous solutions tended to be intrusive or destructive. Air cannons, soot blowers, external vibrators, hammering or costly man entry are all superseded by noninvasive sonic horns.
An acoustic cleaner requires no down time and will operate during normal usage of the site.
Taking the example of silo cleaning a little further, there are two typical problems.
Bridging.
This is when the silo blocks at the outlet. Previously the problem was addressed by manual cleaning from underneath the silo which in its turn introduced significant risk from falling material when the blockage was cleared. An acoustic cleaner is able to operate from the top of a silo through in situ material to clear the blockage at the base.
Rat holing.
Compaction on the side of a silo. This not only reduces the operating volume in a silo but it also compromises quality control by disrupting the first in first out cycle. Older material compacted on the side of a silo can also start to degrade and produce dangerous gases. An acoustic cleaner will produce sound waves which will make the compacted material resonate at a different rate to the surrounding environment resulting in debonding and clearance.
Advantages of acoustic cleaners.
These advantages mean that the financial payback is often very quick.
It is also possible to compare acoustic cleaners directly to alternative solutions.