Acoustic cleaner causes a sound pressure pulse which will detach the particles from the surfaces. The deposits adhering to the surface are subjected to a force greater than that retaining it. The particles with different masses are subjected to alternating sound waves. Because the particles have different masses, the distance they travel is a little different. When the sound impact is repeated with appropriate frequency the particles go out of phase with each other and break apart. The required frequency of impacts depends on the amount of deposits, their natural vibration frequency and adhesiveness. When the impact on the deposit is frequent enough, the deposit comes of and the surface is clean.
Compressed air driven acoustic cleaners do not need a lot of maintenance. However, the maintenance is very important to the best efficiency of the cleaner. When the membrane of the generator is worn it has to be turn over or changed. If the sound pressure of the cleaner decreases 3 dB the sound power level is cut to half. The generator contact surfaces also have to be re-machined. This does, however, not have to be as frequently as the change of membrane. NSCD® doesn’t need a lot of maintenance. The condition of the gas valves and all the gaskets have to be regularly checked and the igniters have to be change in a certain period of time.
The acoustic cleaner uses normal work air. The valve group of the cleaner has a filter/water separator to take out the impurities and water out of the work air.
The NCSD® uses LP gas usually propane. The supply pressure of the gas is 2…4 bar
NCSD® can make 1…15 pulses per second. One pulse uses about 0.4g of gas per pulse. In addition, NCSD® uses circa 80 Ndm3 of compressed air while running for burning air and cooling.
The air supply pressure should be 6 bars. While sounding the pressure can decrease to 4 bars.
NI-250 and NI-250/90: 20 Ndm3/s while sounding, 2 Ndm3/s continuously for cooling the horn. NI-100 and NI-100/90: 40 Ndm3/s while sounding, 2 Ndm3/s continuously for cooling the horn. NI-60 and NI-60/90: 40–50 Ndm3/s while sounding, 2 Ndm3/s continuously for cooling the horn.
The outside noise level can be reduced to under 85 dB measured from one meter distance of the generator by using a proper sound insulation box. Nirafon can provide these boxes or instructions and drawings to make proper sound insulation boxes. The effect of the sound insulation boxes is, however, not the same if there are some openings in the walls of the applications the cleaners are installed or it is not sufficiently insulated.
The outside noise level caused by acoustic cleaner depends on the structures where it is installed. When installed to boiler wall the outside noise level is around 100 dB. The outside noise level of the NCSD® does not exceed 85 dB and is not disturbingly audible outside the boiler.
When the NSCD® is operating, the controlled explosions happen inside the combustion chamber of the cleaner. If, however, there is a gas leakage in the gas valves and the gas gets into the boiler, the amount of gas will be very small and will only burn inside the furnace causing no risk of explosion. It is not possible to big amount of gas getting into the boiler rapidly causing the risk of explosion. The pressure of the controlled explosion inside the combustion chamber is around 5 bar. Nirafon always supplies a strength calculation report of the combustion chamber with the NSCD®. In some cases the gas pulse cleaners have been working since 1999 and no signs of explosions inside the furnace have been detected.
The maximum temperature of the generator is around 200 ˚C. When the cleaner is installed into a hot position, the cooling air into the cleaner has to always flow to the cleaner.
The maximum temperature for the pneumatic equipment is 50 ˚C.
Nirafon horns are made of SS2342 (AISI316) acid proof steal, and in the higher temperature areas the mouth cone of the horn is usually made of G-X15-CrNiSi229. These materials can take the temperatures up to 1000–1050 ˚C. The materials can take these temperatures in conditions where oxygen is present. The conditions where the horns are located, oxygen is not usually present and the materials can take even higher temperatures. Also the horn is usually installed into an opening in membrane wall, and the temperature near the wall is not as high as it is further away from the wall. At Helsinki Energy Salmisaari power plant in Helsinki Finland, the horns are located in a temperature area of 1400 ˚C and have been there since 1999. The mouth cones of the horn inside the boiler are still in perfect condition.
Nirafon acoustic cleaners use only acoustic frequencies (audible to human ear 16…16000Hz). The natural frequency of boiler structures is below this acoustic level. Frequencies below 20 Hz could cause the structures to resonate because of the sound and cause some risk to the structures, open bolts, cracks in welds etc. Acoustic frequencies which Nirafon uses, however, do not have any harmful effect to the structures. Compared to steam soot blowing and water cleaning acoustic cleaning does not have the same harmful effects to the heat transfer surfaces as steam soot blowing; no erosion, which is caused by the ash particles in the steam wearing the surfaces or erosion caused by water.
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