Acoustic cleaning is a maintenance method used in material-handling and storage systems that handle bulk granular or particulate materials, such as

grain elevator A grain elevator is a facility designed to stockpile or store grain. In the grain trade, the term "grain elevator" also describes a tower containing a bucket elevator or a pneumatic conveyor, which scoops up grain from a lower level and deposi ...

s, to remove the buildup of material on surfaces. Acoustic cleaning apparatus, usually built into the material-handling equipment, works by generating powerful

sound wave In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

s 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 An air horn is a pneumatic device designed to create an extremely loud noise for signaling purposes. It usually consists of a source which produces compressed air, which passes into a horn through a reed or diaphragm. The stream of air cause ...

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 acoustic driver. In the driver, compressed air escaping past a

diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...

causes it to vibrate, generating the sound. It is usually made from solid machined

stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...

. The diaphragm, the only moving part, is usually manufactured from special aerospace grade titanium to ensure performance and longevity. *The bell, a flaring horn, usually made from spun 316 grade stainless steel. The bell serves as a sound

resonator A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator ...

, and its flaring shape couples the sound efficiently to the air, increasing the volume of sound radiated. 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: *The most common is by a simple timer *

Supervisory control and data acquisition Supervisory control and data acquisition (SCADA) is a control system architecture comprising computers, networked data communications and graphical user interfaces for high-level supervision of machines and processes. It also covers sensors an ...

(SCADA) *

Programmable logic controller A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity t ...

(PLC) *Manually by

ball valve A ball valve is a flow control device which uses a hollow, perforated and pivoting ball to control fluid flowing through it. It is open when the ball's hole is in line with the flow inlet and closed when it is pivoted 90-degrees by the valve ...

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

rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...

ing 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 horn A horn is a sound-making device that can be equipped to motor vehicles, buses, bicycles, trains, trams (otherwise known as streetcars in North America), and other types of vehicles. The sound made usually resembles a "honk" (older vehicles) o ...

s or air raid sirens. The first acoustic cleaners were made from

cast iron Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuri ...

. 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 The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

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 Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors. Overview The main areas of noise mitigation or abatement are: transportation 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 Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...

distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...

and areas familiar to a mechanical engineer. *

Chemical engineering Chemical engineering is an engineering field which deals with the study of operation and design of chemical plants as well as methods of improving production. Chemical engineers develop economical commercial processes to convert raw materials in ...

. 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 Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...

, 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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

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 Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, crops, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as ...

and the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

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 Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its s ...

applications. *75 Hz and 60 Hz. These are generally the most powerful acoustic cleaners and are often used in large vessels and

silo A silo (from the Greek σιρός – ''siros'', "pit for holding grain") is a structure for storing bulk materials. Silos are used in agriculture to store fermented feed known as silage, not to be confused with a grain bin, which is use ...

Health and safety

The introduction of acoustic cleaners has been a significant improvement in many areas of

health and safety Occupational safety and health (OSH), also commonly referred to as occupational health and safety (OHS), occupational health, or occupational safety, is a multidisciplinary field concerned with the safety, health, and welfare of people at wo ...

. For instance in

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

*Repetitive use during operations means that there are fewer unscheduled shut downs. *Improved material flow by the elimination of hang-ups, blocking and bridging. *Minimisation of cross contamination by ensuring complete emptying of the environment. *Improved cleaning and reduction of health and safety risks. *Increased energy efficiency. Reducing the buildup on heat exchange surfaces results in lower energy usage. *Extended plant life. Aggressive cleaning regimes are avoided. *Ease of operation. It is easy to automate the horns either at regular intervals or to tie the sounding in to changes in their environment such as pressure or flow rates. *Importantly they prevent the material buildup problem from occurring in the first place. These advantages mean that the financial payback is often very quick. It is also possible to compare acoustic cleaners directly to alternative solutions. * Air cannons. These are well established but are expensive with limited coverage thus requiring multi unit purchase. They are also noise intrusive and have a high compressed air consumption. *Vibrators. These are easy to fit to an empty silo but can cause structural damage as well as contributing to powder compaction. *Low friction linings. These are very quiet but are expensive to install. Also they are prone to erosion and can then contaminate the environment or product. *Inflatable pads and liners. Again these are easy to install in an empty silo. They help side wall buildup but have no impact on bridging. They are also hard to maintain and can cause compaction. *Fluidisation through a 1 way membrane. This can help already compacted material. However they are expensive and difficult to install and maintain. They can also contribute to mechanical interlocking and bridging.

Specific applications

Boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central ...

s. Cleaning of the heat transfer surfaces. *

Electrostatic precipitator An electrostatic precipitator (ESP) is a filterless device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit. In con ...

s. Acoustic cleaners are being used for cleaning hoppers, turning vanes, distribution plates, collecting plates and electrode wires. *

Super heater A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There are ...

s, economisers and air heaters. *Duct work. *Filters. Acoustic cleaners are used on reverse air, pulse jet and shaker units. They are effective in reducing pressure drop across the collection surface which will increase bag life and prevent hopper pluggage. Generally they can totally replace the both reverse air fans and shaker units and significantly reduce the compressed air requirement on pulse jet filters. *ID fans. Acoustic cleaning helps to provide a uniform cleaning pattern even for inaccessible parts of the fan. This maintains the balance of the fan. *

Kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...

inlet. Acoustic cleaners help to prevent particulate buildup at the

kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...

inlet and this will minimise nose ring formation. *Mechanical pre Collectors. Acoustic cleaners help prevent buildup around the impellers and between the tubes. *Mills. Acoustic cleaners help maintain material flow and also prevent blockages in the pre grind silos. They also help prevent material buildup in the downstream separators and fans. *Planetary Coolers. Acoustic cleaners help prevent bridging and ensure complete evacuation. *Precipitator. Acoustic cleaners help clean the turning vanes, distribution plates, collecting plates and electrode wires. They can either assist or replace the mechanical rapping systems. They also prevent particulate buildup in the under hoppers which would otherwise result in opacity spiking. *Pre heaters. Used in towers, gas risers, cyclones and fans. *Ship cargo holds. Used both to clean and de aerate current loads. * Silos and hoppers. To prevent bridging and rat holing. *Static cyclones. Acoustic cleaners will work both within the cyclone and with the associated duct work.

External links

{{commons-inline, Acoustic cleaners Acoustics Audio engineering Cleaning tools Cleaning methods