The Study of Fabric Drying Using Direct-Contact Ultrasonic Vibration

Abstract

Our existing cloth drying technology is an energy-intensive process, which generally involves blowing hot air across tumbling wet fabrics to facilitate evaporation and moisture removal. To address the relatively low energy efficiency of existing cloth drying techniques, in this chapter, a totally new cloth drying technology is introduced, which uses high frequency ultrasonic vibrations generated by piezoelectric transducer instead of thermal heating to extract moisture in cloth as a cold mist, dramatically reducing drying time and energy consumption. The physical mechanism of ultrasonic fabric drying process in direct-contact mode is first studied. A novel ultrasonic transducer driving method, in which the power supply to the transducer is regulated by a binary modulating signal, is then developed for use in direct-contact ultrasonic drying of fabrics. A demonstration unit is finally fabricated to show the efficacy of the process and its energy saving compared to thermal drying process. 2 dryer technologies [3, 5-10] (including heat, heat pump, vacuum, or microwave drying) requires a heat source to provide a latent heat of evaporation of about 2.5 MJ/kg water. The heat source for a dryer can be electric resistance, natural gas and/or an electric heat pump.The metrics to measure the drying efficiency of a clothes dryer is called energy factor (EF), which commonly has three different types of definitions, depending on how the drying effect is qualified: (1) the mass of moisture removed per energy consumption; (2) the mass of cloth dried per energy consumption; or (3) the latent heat of moisture removed per energy consumption. In the United States, the Department of Energy standard of EF measures the mass of cloth (saturated to certain water content) that can be dried per unit of electricity consumed (lb/kWh) [3]. For gas dryers, it measures the pounds of clothes being dried per equivalent kilowatt-hour of natural gas consumed. Here, we focus on EF (lb/kWh) which is commonly used in the United States. Table 1 summarizes the EF values for the typical types of existing clothes dryers.Recently, for the first time, we have introduced a totally new cloth drying technology and are collaborating with Oak Ridge National Laboratory (ORNL) researchers to develop the world's first direct-contact ultrasonic cloth dryer that could potentially change the concept of residential heat-drying technology [11][12][13][14][15]. This novel approach uses high-frequency mechanical vibrations generated by piezoelectric ceramics instead of heat to extract moisture as a cold mist, dramatically reducing drying time and energy use. On the basis of inverse piezoelectric effect, when an electric field is applied in the polarization direction of piezoelectric ceramics, the ceramics will produce mechanical deformation in a certain direction. When a high frequency electric field is applied, the piezoelectric transducer generates acoustic vibration. If the oscillation frequency is larger than 20 kHz, it generates ultrasonic waves that propagate throu...