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Technology-Focused Library

Built-in Type Ultrasonic Generator (Nebulizer Units), NB Series

Small, inexpensive ultrasonic generator units for various air conditioning systems. A combination of drive circuits made from a low-noise, high-efficiency design and a piezo resonator made using our exclusive technological know-how.

The principle behind water turning into mist

Piezo resonators are inserted and fixed to the base of water receptacles built into humidifiers. These resonators apply vibration energy generated from voltage (the frequency is the piezo resonator’s own resonant frequency) supplied by drive circuits directly into the water. During this process, the vibration energy is transmitted in a direction that is perpendicular to the resonant surface of the piezo resonator. By optimizing the water depth, it is possible to generate a water column where energy above the axis of the transmission on the water’s surface is concentrated.

Image that water turning innto mist

A concentrated surface tension wave is generated at the end of that column that is both minute and profuse. The result is that surface tension on the swelled surface is reduced and the water surface is divided into minute regions corresponding to the length of the surface tension waves. Each of these areas become individual particles, are lifted up by the air from a ventilation fan and distributed into the air. This entire process is believed to be the principle for turning water into mist using an ultrasonic resonator.

Piezo resonator’s drive mode and performance determining factors

Nebulizer units are made from round piezo ceramic plates with high mechanical quality coefficient Qm (constant that indicates the acuteness of the mechanical oscillation in the resonant frequency).

Explanation illustration of piezo-electric resonetor's drive mode

After electrodes are formed on both sides of this plate during the plating process and depthwise polarization is performed, impressing the harmonic magnetic field will cause the resonator to function as an ultrasonic resonator using thickness expansion vibration mode.

Performance of a nebulizer unit is represented as mist volume per hour (generally, represented by ml/h), but this size is by and large determined by the following four control factors.

1. Acoustic pressure
2. Resonant frequency of piezo resonator
3. Water depth
4. Drive voltage

Pump performance is represented as pumped water volume per hour, but relative merits are determined by the motor’s number of revolutions and amount of torque. With the Nebulizer unit, the resonator’s resonant frequency (drive frequency) represents the motor’s number of revolutions, and torque is the depth displacement of the resonator, or to put it another way, the “acoustic pressure” of the ultrasonic wave transmitted through the water.

Here, when running the immersed piezo resonator in thickness expansion vibration mode, the acoustic pressure p observed on the center axis of the sound wave transmission route is determined by using the following formula:

Formula that calculates acoustic pressure

For thickness expansion mode, the resonant frequency can be increased by making the piezo resonator thinner. As such, we know that by making the resonator thinner and larger, λ becomes small and πa² becomes large, making it possible to attain larger acoustic pressure.

Thus, it seems that using a large, thin resonator to apply large acoustic pressure to the water surface would produce a larger mist-producing water column. Unfortunately, however, this was not the result in actual humidifier models.

Factors that determine atomization efficiency

To perform the unique phenomenon of atomizing water, humidifiers require a small resonator and a highly-efficient system that enables large atomization volume with only a small amount of power. When TDK first began to research the application of piezo resonators to household humidifiers, no one else in the world was trying to effectively control water atomization. We had to start by looking into the determining factors of atomization volume.

The piezo resonator used in our experiments was created by first determining the size range optimal for designing a reduced size humidifier, while controlling costs during mass production and securing product quality stability. We carefully examined the degree of interdependence of relevant factors and created the maximum number of variations of diameter and thickness within that range. Further, we redid each variation every time we altered the piezo ceramic composition or electrode pattern. In each experiment category, we recorded the measurement differences between old and current materials and designs. Through this overlapping and repetitive process, we were able to move toward understanding which factors determine the rate of efficiency of atomization.

Two factors that emerge from a mountain of data

We strictly fixed conditions for the test tank that served as our humidifier. All samples were run for a single turn every time the drive conditions were switched, and the results were recorded. At one point, we continued to repeat tests to confirm our speculations, presumptions, theories and inspirations by using the same thickness and performing sequential tests using a resonator with a different diameter, in different media, at different media levels, and at different voltages, and recording these results. It was during these tests that it became almost certain that frequency spectrums, a factor in producing the largest atomization efficiency rates, existed in each diameter of the resonator. At almost the same point as this discovery, it became clear that the distance between the surface of the resonator fixed to the bottom of the tank and the surface of the water, in other words, the water depth, was another factor that had a major effect on the increase or decrease in atomization efficiency. We happened upon this discovery as we were consolidating unrelated data — the elements leading to this conclusion just appeared from among the various data.

Know-how related to the world’s first atomization efficiency maximization

In our humidifier model, the state of water swelling at which it is easy for water to be atomized was formed and maintained at a specific frequency spectrum (resonant frequency determined by the thickness of the resonator) corresponding to the diameter of the piezo resonator and at a specific depth. All of our data pointed to the conclusion that by adjusting the applied voltage in this state, we could obtain the maximum atomization efficiency rate for that model.

As for why atomization occurs when the resonator diameter and the fixed frequency spectrum matches the material used, the following explanation can be given. The particle oscillation wave of the medium that is concentrated at the end of a swelled liquid is reflected and diffused at the point where it interfaces with air. It is thought that the wavelength of the minute surface tension wave that is generated by this repetition is determined by the speed at which this repetition occurs. However, maintaining this kind of fragile atomization mechanism requires maintaining the effect factor that determines the swelling of the water column — that is, the water surface acoustic pressure necessary for optimum levels. We have confirmed in our testing that water depth and applied voltage serve as the main factors in determining.

Same diameter resonators driving model

As shown in the configuration example model below, the atomization efficiency where the optimum water level for a resonance frequency (thickness of the elements) becomes the largest (central diagram). However, if a transducer with a different thickness (resonance frequency) is set without changing the water level, the atomization remains a low level (diagrams on the left and right).

Association model of atomization volume and element thickness

Also, the following model is an example of comparing the same oscillators driven by the best frequency. But if the water level is lower than the optimum value (diagram on the right), the denominator in the formula x, which decreases acoustic pressure, becomes small and the water pressure, which is a braking factor of the transducers, also decreases, and therefore the vibration velocity V1 (in accordance with the displacement value of the transducer surface) in the numerator increases. As a result, the acoustic pressure soars greatly and phenomenons which degrade the atomization efficiency such as fractures of the top of the water column are observed. On the other hand, if the water level is too high (diagram on the left), the relation between x and V1 inverts and almost no water column will be generated.

Association model of atomization volume and water level

Actual piezo resonator design procedures

First, atomization performance target values to be assigned to the humidifier are determined. Next, a diameter is determined that takes into consideration surrounding factors such as acoustic pressure, mechanical strength, and drive voltage while also easily allowing for low cost, stable mass production and size reduction. Presently, we are using 20mm, but based on our collective data, the resonant frequency that produces the greatest atomization efficiency at this diameter is fixed at 1600 to 1750kHz, and based on material properties the resonator thickness is approximately 1.2mm. Also based on our data, we have determined that the optimal water level is 40mm. Element impedance during resonance at this water level is near 49Ω. Taking into consideration mechanical strength limits and atomization performance targets of 450mL/h, the maximum allowable power is approximately 19W. As a result, the optimal voltage to drive this resonator set at the optimal water depth of 40mm is approximately 130Vp-p.

The shape and drive conditions of a piezo resonator that produces the maximum atomization performance are set as described above. Refer to the characteristic (please click “Catalog” of the pull-down menu to refer to details of the latest characteristic) and the shapes of products for the latest application results of our world-leading expertise in atomization efficiency maximization (NB-59S-09S apply to the above test case).

Externals image of main products