How can an infrasound generator keep an exchanger clean ?

The operating principle of this device, is to create a low frequency sound wave in order to make the gasses oscillate, and thus not allowing particules stick to the surfaces they contact.

How does the infra sound generator work ? (see scheme)


Principle :

A sound wave in a gas ( which is what we are concerned about) is a disturbance of the gas pressure moving forward with a velocity which depend on the gas characteristics.

Infrasound waves used in soot cleaning have their frequency in between: 17 Hz and 30 Hz depending on the installation.

But the aim is not only to produce a low frequency sound, but also a low frequency sound with a very high power.

For example, a loudspeaker could produce low frequency sounds but with quite a low power to them , and would not stand the typical high temperatures found in a boiler.

The infrasound generator is basically a "wind instrument" working in the same way, which is basically to cause the wave in a tube to cycle forward and backward at the tube's correct resonance. The tube is generally called a " ¼ wave length" resonance tube.

How to create a sound wave in a tube : (see scheme)

We take, for example, a tube with a theoretic infinite length, open on one side and closed on the other one.

On the closed side we connect a device which produces air pulses, for instance a piston is moved up and down by a crankshaft and can open and close the slots . When the slots are open the air can leak from a surge tank connected to the tube.

Phase A : before starting, the piston keeps all the slots closed, and the pressure in the surge tank is much higher than the pressure in the tube which is still at the atmospheric pressure.

PhaseB : the motor has started and rotated the crankshaft so the piston is going to open the slots to let air leaking from the surge tank to the tube: a small high pressure appears at the beginning of the tube.

Phase C : The motor has rotated 90° and the slots are fully open, and the maximum flow is leaking through the slots to build a localised high pressure at the beginning of the tube.

Phase D :the slots are now almost closed, the flow is decreasing.

Phase E :the piston has totally close the slots and the pressure at the beginning of the tube will come back to the atmospheric pressure.

This wave will move forward in the tube with a velocity called the " sound velocity", depending on the physical properties of the gas.

If the motor rotates at the accurate speed and if the surge tank is fed, waves will take place in the tube, and far from the piston, the pressure will be sinusoïdal , positive and negative around the average pressure (atmospheric one) as shown on the sketch.