Ultrasonic cleaning of heat exchanging nuclear power plant equipment
The method for ultrasonic cleaning of surfaces of heat-exchange equipment is examined. Schematic description of the ultrasonic cleaning mechanism is presented. When ultrasound is applied to the surface of heat exchange equipment filled with heat conducting medium, then descaling of the surface takes place under the effect of shock waves emerging when cavitation air bubbles collapse. In addition, multiple bubbles not associated with cavitation phenomena are formed in the liquid. These bubbles penetrate the pores, fissures and gaps between the scale and the surface of heat exchanging equipment. The bubbles intensively vibrate under the action of ultrasonic oscillations also causing disintegration of the upper layer of the scale. Ultrasonic cavitation in a liquid depends on its density, viscosity, temperature, molecular weight, compressibility, concentration of gas, presence of microscopic impurities, frequency and intensity of ultrasonic oscillations, static pressure, etc. It is shown that activity of the cavitation process can be influenced in the required direction by purposefully changing some of these factors. Quality and rate of cleaning are determined by the acoustic power and frequency of the vibrations, by the temperature and composition of the working solution.
High frequency oscillation accelerates chemical and physical processes occurring in the liquid. Generators of UZG type with PMS-type magnetostriction converters can be used for the purpose. Ultrasonic transducer converts electrical oscillations applied to it into mechanical oscillations of the same frequency. Frequency bands from 18 kHz to 44 kHz with oscillation intensities from 0.5 W/cm2 to 10 W/cm2 are used in the majority of installations. The upper boundary of the frequency band is predetermined by the mechanism of formation and destruction of cavitation bubbles: at very high frequency bubbles do not have sufficient time to collapse which reduces the micro-impact produced of cavitation. Cleaning of most insignificantly small pores is achieved due to the fact that forces acting on the impurity particles are more or less uniformly distributed within the volume of the washing solution. Quality of ultrasonic cleaning is by far batter than that achieved using other methods. Special advantage of ultrasonic cleaning is associated with its high productivity and low manual labor expenditures; by easy maintainability and relative simplicity of equipment and possibility to replace flammable and expensive organic solvents with safe and cheap aqueous solutions of alkaline salts.
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