[MBA_NKBD_EN] Rev.: 10/2015
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Fluid tank
Capillary waves
Ultrasonic transducer
5.3 Functional description - ultrasonic air humidifier
All frequencies above 20,000 Hz are called ultrasonic.
According to their physical nature, acoustic waves consist of mechanical oscillations of compressible
media. These oscillations develop due to the deflection of the particles of a compressible material from
their equilibrium position. Acoustic waves are bound to a medium and thus do not occur in the vacu-
um.
Oscillations develop as a result of a change in pressure. Repeated pressure increase and pressure
reduction produce different acoustic waves.
In order to be able to use ultrasonic waves for air humidification, electrical energy must be converted
into mechanical energy. This takes place in the piezoelectric transducer.
A vibration unit consists of the resonance circuit where the high frequency of
~ 1.7 MHz is produced and the piezoelectric transducer to convert the electrical frequency into a pro-
portional mechanical oscillation. This frequency is not audible for human beings or animals.
The piezoceramic transducers are attached
to the bottom of the fluid tank. During
excitation of the transducer, the water leads
the ultrasonic vibrations to the boundary
layer between water and air. Constant
compression and decompression of the
water gauge over the transducer causes
cavitation in the immediate proximity of the
water surface. Thus, crossing capillary
waves are developed, the finest water
particles of which, the aerosols, are
produced in the wave crest.
fig. 2 - Schematic sketch aerosol production
The aerosols are delivered by the air flow in the humidifier and quickly mix with the ambient air. They
have a small diameter (0.001 - 0.005 mm) and thus form a freely floating mist. The droplet diameter
depends on the surface tension and the density of the medium, but also on the excitation frequency.
The higher the excitation frequency, the smaller is the diameter of the droplets.
Demineralised
water
Point of focusing
Air bubbles
