Ducts

Ducts exposed to dust-laden gas, e.g., goosenecks in recovery boilers, will risk fouling of the inner walls with time. Fouling can obstruct the gas flow through the duct, increasing differential pressure and fan load and possibly also interfering with the flue gas flow distribution into a downstream electrostatic precipitator. In the worst case, the fouling can completely block the duct, forcing manual cleaning.

Large amplitude pulsation of the flue gas

When cleaning, e.g., an economizer using infrasound, the cleaning effect comes from the extra turbulence created in the flue gas when the infrasound oscillates the flue gas back and forth. The same effect is present when keeping ducts clean. Infrasound cleaning uses a low frequency, around 20 Hz. The displacement amplitude of the oscillation is inversely proportional to the frequency, resulting in a large displacement amplitude. Compared to acoustic cleaning using higher frequencies, the large amplitude of infrasound results in a more powerful way of removing ash particles that have settled on surfaces but have not yet sintered to hard deposits.

Vibrating the duct walls slightly

When operating an infrasound cleaner, sound pressure waves travel back and forth in the flue gas duct. The result is a sound pressure typically varying between +200 Pa and -200 Pa around the actual pressure inside the duct. The pressure changes direction as often as the sound frequency, 20-30 times per second. 200 Pa results in 200 N force (~20 kg) per square meter and the effect is a vibration of the duct walls. This short vibration is not harmful to the walls, but helpful in preventing deposits in ducts. It has a similar effect as a rapping device working on a surface, with the difference in the infrasound acting on the whole duct. The sound pressure created in the duct is calculated before the installation of the infrasound cleaner, and the vibrations created are measured after the system is started.

Long cleaning range also covers long ducts.

Infrasound has a long wavelength, resulting in a long cleaning range and the ability to reach beyond bends in the flue gas ducting. This is due to the fact that sound is screened off only by objects of size comparable to the wavelength. For most cases, when cleaning, e.g., an economizer allows for cleaning of a whole back pass using just a single infrasound cleaner. For the same reasons, it is possible to keep clean long ducts, straight or bent, using a single infrasound cleaner. This results in low costs for installation, operation, and maintenance.

Low noise

Compared to audible frequencies, a significant advantage of using infrasound is that the noise generated is lower. Audible sonic horns use sound frequencies of 75-300 Hz. Higher frequencies are more disturbing because the human ear is more sensitive to higher frequencies. That is also why the dB(A) scale is used when measuring noise. The dB(A) scale is the sound pressure level measured in dB and weighted according to the human ear’s sensitivity.