Sonic horn placement and sizing: how many horns and where to mount them

Sonic horn placement starts with a fouling map, not a catalogue coverage radius. A horn with a strong nameplate output can work well into an open bay and do nothing useful at a surface hidden behind a tube bank, catalyst layer, baffle or filled corner. Distance weakens the field, internals reshape it, and the process gas changes how sound propagates. Whatever the rating, a horn firing into a dead corner or behind an obstruction is not covering the deposit that matters.

There is no honest universal answer to "how many sonic horns?" Count follows the acoustic zones that need cleaning. The acoustic cleaning system must reach each target while allowing for mounting, compressed air, maintenance and personnel exposure. The basic mechanism is covered in how sonic horns work. Here, the question is how to turn it into a defensible layout.

Characterise the deposit before sizing anything

Do not buy hardware before the deposit and problem zone have been characterised. Record where material first appears, how quickly it matures and which operating condition makes it worse. Use photographs, process trends and cleaning records to distinguish the initiating zone from places where released material lands later.

The deposit must also be physically suitable. A sonic horn is strongest on dry, friable and loosely bonded particulate, especially when it fires often enough to prevent a mature layer forming. It is weak or useless on sticky, wet, molten, sintered or hard-bonded material. Placement cannot compensate for the wrong application, and adding more horns only adds cost and noise. Where moisture or low-melting ash changes the bond, start with the deposit analysis described in our guide to sticky ash in biomass and waste-to-energy boilers.

Define the symptom at the same time. "Ash in the boiler" is not a zone. "The lower third of the second-pass outlet bundle plugs after six weeks and raises draught loss" is. Precision gives the designer a boundary and the pilot a pass or fail result.

Vessel volume is an input, not a sizing formula

Gross vessel volume is useful, but horn sizing by vessel volume alone is unreliable. Two vessels with the same capacity can behave differently. An open silo with a changing fill level and a vessel containing tube bundles, catalyst modules, filter bags and leakage paths do not present the same reach or number of zones.

Divide the equipment into acoustically connected spaces. A zone is a volume in which a horn has a credible path to the surfaces that foul. Solid partitions, deep bundles, closed dampers, packed material and narrow passages can separate zones. Openings can connect them, but connection does not prove equal pressure on both sides. Draw each zone on a dimensioned section and mark:

• the surfaces where fresh deposit forms;
• the shortest and longest paths from a possible horn location;
• tube banks, plates, bags, beams, ducts and other obstructions;
• normal and upset gas flow, temperature and moisture;
• changing boundaries such as silo fill level or isolated compartments;
• doors, platforms, lifting routes and maintenance access.

This exercise turns "one 2,000 cubic metre vessel" into a set of actual coverage duties. It also exposes dead corners before steel is cut.

Frequency and geometry set effective reach

A frequency figure does not define a coverage radius. Lower frequency means a longer wavelength, which can help sound diffract around some objects and cross large open volumes. A low-frequency acoustic cleaner is not immune to attenuation, shadowing or weak zones, and lower is not automatically better.

Separate source output from the field at the deposit. Sound power and sound pressure are different quantities: power describes the source, while sound pressure level is local to a measurement point. A near-bell value cannot be carried unchanged to a remote corner.

In an ideal free field, the inverse-square law warns that doubling distance from a point source reduces sound pressure level by about 6 dB. A vessel is not a free field. Reflections alter the local field, internals scatter energy, openings leak it, and gas absorption adds acoustic attenuation. Standing waves can put a strong region beside a weak one, while resonance in one bay proves nothing about the next.

Temperature changes sound speed and wavelength, while gas composition, suspended dust, turbulence and flow alter propagation. Near field and far field behaviour also makes a mouth measurement unrepresentative of the wider vessel. Assess operating conditions, not ambient-air data copied from another installation.

Published field work in a circulating fluidised bed boiler found substantial pressure close to the cleaners but an insufficient field at the relevant heated surfaces. The lesson is not a universal decibel threshold. Layout, deposit and relevant measurement locations matter more than an impressive near-horn number.

How many horns does a zone need?

Assign each horn a duty: which zone and surfaces must it reach? Test one horn where it has an open path across the zone. If a bundle, partition or long distance separates the far surface, create another duty instead of assuming spare output will find a route.

Multiple horns are commonly justified when:

• one vessel contains physically separated bays or compartments;
• a long gas path leaves the far end outside credible reach;
• dense internals place critical surfaces in acoustic shadow;
• a silo needs sidewall coverage at high fill and a separate cone or outlet duty;
• one mounting point cannot serve both the deposit and safe maintenance access;
• measured or pilot results show a repeatable weak zone.

Do not multiply a nominal radius across a drawing and call the overlaps complete. Coverage inside reflective, obstructed equipment is not circular. Nor should size alone add horns: an open volume may need fewer units than a smaller compartmented one. Use the minimum count that gives each zone a credible path, then prove it in service.

Equipment changes the map. Boiler tube packs and casing divisions can create separate duties. A baghouse plenum, bag compartment and hopper are not automatically one zone. ESP screens interrupt propagation, as does an SCR catalyst layer. In a silo, a top mount may address upper sidewalls while a cone location addresses bridging near discharge. These are hypotheses, not standard layouts.

Place each horn into an open acoustic path

The mouth should discharge into useful gas space with a clear first path towards the fouling zone, not immediately at a wall, beam or casing pocket. Low-frequency sound can bend and reflect, but diffraction does not erase an obstruction. A horn beyond a deep screen or bundle may be more credible than a larger one on the convenient side.

Check where material will collect after installation. A clean location may receive falling ash or become buried at normal silo fill. Gas flow must carry released material to a legitimate exit, or cleaning one surface may create a new deposit downstream.

Avoid relying on a single predicted acoustic impedance match or vessel mode. Fabrication tolerances, temperature and deposit growth change the boundary conditions. Where a critical corner remains doubtful, design a measurement point or inspection method for it and leave room to revise the layout.

Design the mounting nozzle as part of the horn

The sonic horn mounting nozzle is part of the acoustic and mechanical design. It should provide a smooth, open transition through the insulation and casing without becoming a shelf for ash or powder. A long narrow neck, abrupt internal step, blind pocket or horizontal ledge can restrict coupling and collect the very deposit the system is meant to prevent.

Keep the path as direct as the pressure boundary allows and orient surfaces to shed material where practical. Check expansion, erosion, corrosion, insulation, condensation and casing load. Hot or corrosive service may need different nozzle material or stand-off from an ambient silo. Preserve pressure integrity and required isolation.

Design for removal. A serviceable connection should expose the driver, diaphragm, valve and mouth without vessel entry or dismantling unrelated plant. Allow tools and lifting, protect walkways, and define isolation. An inaccessible horn will eventually become an unreported dead zone.

Size the air and sequence with the layout

Acoustic coverage assumes the horn receives the specified compressed air pressure and flow while firing. Static header pressure before a pulse proves little. Long small-bore lines, restrictive valves, wet filters and an undersized receiver can make the pressure collapse during the event.

For several horns, calculate instantaneous and recovery demand. A cycle controller or sequencer can fire zones separately so the header supports each pulse and so operators can tune the cleaning interval by process condition. Simultaneous firing should be a deliberate, verified requirement, not a default. Record firing pressure or another health signal so a closed valve or failing diaphragm is found through predictive maintenance, rather than after fouling returns.

Plan maintenance access and noise together

The best acoustic position may be a poor human position. Review platforms, ladders, access doors, routine rounds and outage work before fixing the nozzle. Keep the horn and valve package reachable without exposing technicians to hot surfaces or requiring awkward lifting. Interlock or isolate firing where personnel could be inside the equipment or working close to an open path.

Noise planning must address exposure outside the process, not only useful sound inside it. Measure leakage at the horn body, nozzle, access doors and nearby workstations under real firing conditions. Do not compare an unweighted source rating with a worker's daily A-weighted exposure. UK guidance sets lower and upper daily or weekly exposure action values at 80 and 85 dB(A), while EU Directive 2003/10/EC and OSHA 29 CFR 1910.95 govern other jurisdictions.

Control the source and path before relying on hearing protection. A sound attenuation enclosure, sealed openings, remote firing, access control and a sequence that avoids occupied periods may all be relevant. Low-frequency leakage is difficult to stop with a token screen, so attenuation must be designed and then measured.

Prove one zone before rolling out

A staged single-zone pilot turns a proposed layout into evidence. Choose a recurring, measurable problem with a suitable deposit. If the mature layer is hard, restore a clean baseline so the pilot tests prevention rather than recovery cleaning.

Agree the baseline and acceptance criteria. Measure the relevant pressure, temperature, fan load, hopper interventions, cleaning hours, photographs or run length. Record load, fuel, feed and moisture so favourable operation is not mistaken for cleaning performance.

Install one zone with permanent-quality air delivery, controls, mounting and noise protection. Confirm firing pressure, inspect at the next safe opportunity, and tune the least intensive effective cycle. Run long enough to include the condition that causes fouling. A short clean-fuel trial cannot prove a wet-fuel campaign.

If the result is poor, inspect the chain before buying more horns: deposit suitability, acoustic path, nozzle condition, air delivery, sequence and removal route. The pilot may show that a second mounting position is needed. It may also show that acoustic cleaning is the wrong method. Either result is more valuable than a full rollout built on an assumed coverage radius. Only after the zone meets its agreed target should the team extend the map and complete the acoustic cleaning payback for the remaining plant.

A practical placement and sizing check

Before approving the layout, confirm that:

• the deposit is dry, friable and still weakly bonded when the horn fires;
• each horn has a named zone and target surfaces;
• drawings include operating fill level, internals and changing boundaries;
• frequency and reach are assessed at process temperature and gas conditions;
• no critical surface is accepted behind an untested obstruction;
• the mounting nozzle has no ledge, blind pocket or inaccessible wear point;
• firing pressure and receiver recovery are verified at the horn;
• maintenance, lifting, isolation and personnel access are workable;
• occupational noise is assessed and controlled before commissioning;
• a single-zone pilot has measurable acceptance criteria and a stop decision.

The bottom line

The answer to how many sonic horns is not hidden in vessel volume or a nameplate decibel figure. It comes from dividing the equipment into real fouling zones, tracing credible acoustic paths, allowing for frequency, distance, internals and gas conditions, and giving each unresolved zone its own duty. One well-placed horn can outperform several convenient ones. One badly placed horn can do nothing, whatever its rating.

Start with the deposit, not the hardware. Reject sticky, wet, molten, sintered and hard-bonded duties that placement cannot rescue. Design the nozzle, air supply, access and noise controls with the acoustic layout. Then prove a single characterised zone against plant data before scaling. That is how coverage becomes an engineering result rather than a sales claim.