Atomization Particle Size

Droplet Size

The size of the atomized droplets produced by a nozzle depends on the nozzle's atomization structure, liquid energy intensity, liquid surface tension, and liquid density. Since the atomized droplet sizes are not entirely uniform, the average droplet size becomes an important metric representing droplet size. In many nozzle applications, droplet size is often critical. For example, it is extremely important in quench towers for exhaust gases. Droplets that are too large cannot evaporate completely within the designated time. Unevaporated droplets entering the bag filter at the end of the quench tower will result in poor dust collection efficiency or complete failure of the dust collector. Conversely, droplets that are too small may not be able to cool the exhaust gas to the predetermined temperature range within the designated time. Excessively hot exhaust gas entering the bag filter at the end of the quench tower can not only burn the filter bags but also potentially cause a fire.

There are four ways to express spray droplet size, among which the Sauter mean diameter (SMD) is the most commonly used. The mean diameter refers to the ratio of droplet volume to surface area, often represented by D32, and expressed in μm (micron). (∗1 φ = 10−3 φ)

1. Sauter Mean Diameter (SMD): Refers to the ratio of droplet volume to surface area, commonly represented by D32. It is the most frequently used reference value in the nozzle industry.

2. Number Mean Diameter: The average value obtained by adding the diameters of all measured droplets, then divided by the total number of the measured droplets.

3. Weight Mean Diameter: The average value obtained by adding the weight of all measured droplets (droplet weight is obtained by converting liquid density, specific gravity and diameter), then divided by the total number of the measured droplets.

4. Number Median Diameter: Refers to the value in the middle of all the measured droplet diameters.

Measurements Method

The average particle size is determined using pure water at 25°C.

The immersion method involves filling a laboratory glass dish approximately six-tenths full with silicone oil. The dish is quickly slid under the nozzle during a spray, collecting the atomized water droplets. Due to the higher density of silicone oil, the droplets float on top. The dish is then rapidly moved to an electron microscope, and the diameter of each droplet is meticulously recorded. The average diameter is the average spray particle size.

This method is extremely time-consuming and has a large margin of error; it is rarely used today to determine spray particle size.