Acoustic Cleaners

Acoustic Cleaners are used wherever there is a build-up of dry materials and particulates which need to be cleaned regularly to ensure maximum efficiency, and minimize maintenance and down time. An acoustic cleaner works by generating powerful sound waves which will vibrate the dry materials differently to each other and the surrounding structures.

History and design

An acoustic cleaner consists of two parts.

• The wave generator which takes the compressed air and applies it to a diaphragm. The wave generator is usually made from solid machined stainless steel. The diaphragm within the generator is the only moving part within an acoustic cleaner and there is no danger of sparking. The diaphragm is usually manufactured from special aerospace grade titanium to ensure performance and longevity.
• The bell, which is usually made from spun 316 grade stainless steel. The bell is a resonance section or amplifier and it will tune and direct the sound waves. An acoustic cleaner is powered by compressed air with an operating range of between 4.8 to 6.2 bars or 70 to 90 psi. The resultant sound pressure level will be around 200dB. The overall length of the acoustic cleaner will range from between 430 mm to over 3 metres long. There are generally 4 ways to control the operation of an acoustic cleaner.
• The most common is by a simple timer.
• SCADA (Supervisory Control and Data Acquisition).
• PLC Controllers
• Manually by Ball Valves. 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 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 sonic range from 60 hertz up to 420 Hz. Occasionally there is a requirement to operate in the infrasonic range below 40 Hz. This would apply if there were strict noise control requirements, or there was limited plant access. 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.

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 non invasive sonic horns. An acoustic cleaner requires no down time and will operate during normal usage of the site. If we take the example of silo cleaning a little further, then there are two typical problems.

Bridging

This is when the silo blocks