High Intensity Low Frequency Sound Used for Cleaning Purposes

Abstract

Low frequency sound has been used for the continous cleaning of boilers and also for cleaning museum textiles. New methods are presently being developed for other types of cleaning. For the cleaning of industrial boilers a powerful 20 Hz infrasound generator (sold under the tradename Infrafone) has been developed. The maximum acoustic power output is in the region of 1000 W. The Infrafone consists of an exigator and a quarter wave resonance tube. It operates on air from a blower and the infasound is generated by the exigator pulsating the air into the resonance tube. When the Infrafone is adapted to a cavity (i.e. the economizer of a boiler, the air preheater or any other part of the boiler) the dimensions of which are smaller or in the same range as the wave length of the infrasound, an almost homogeneous infrasonic pressure is established in all parts of the cavity. The sound pressure level inside a boiler depends on the area of the chamber walls and the acoustic power emitted from the generator. At an infrasonic pressure level of 130 dB – 135 dB (depending on operational conditions) most soots are sufficiently activated not to deposit on tubes and walls. The infrasound even has the capacity to break the bonds between particles and metallic surfaces, thus preventing the particles from adhering to heat exchanging surfaces. Up to the present some four hundred Infrafones have been installed in different types of boilers. As well as oil-fired boilers these include boilers fixed by coal, bark, garbage, peat and black liquor. For the removal of insecticides from museum textiles a 50 Hz sound is used. The sound is generated by a loud speaker adapted to a Helmholtz resonator. The textiles are placed in the resonator and within five minutes about 98 per cent of the insecticide is removed. The sound pressure level is in the region of 140 dB.