Atomize

Atomization , also called spraying , is the breaking up of a liquid into fine droplets as an aerosol ( mist ) in a gas (usually air). The resulting aerosol is also spray or spray called. This can either consist of drops that all have the same diameter - monodisperse spray - or contain drops of different sizes. In this case one speaks of a polydisperse spray. Atomization technology deals with industrial applications .

The term atomization is misleading insofar as dust is usually understood to mean the finest solid particles, while atomization is mostly applied to liquids and less often to solids.

application

Liquids are usually atomized with the aim of creating a large reactive liquid surface, for example in carburetors and air humidifiers . This favors processes of mass and heat exchange, such as those that take place in evaporation processes (drying technology, spray drying) and combustion processes. An atomizing component for such combustion processes are diesel and gasoline injectors of internal combustion engines.

Liquids are also atomized in order, for example, to ensure uniform surface coatings in painting technology or to generate sufficient impulse transmission for cleaning tasks ( high-pressure cleaner ). An atomizer in spectroscopy is used for the chemical analysis of substances. Atomization also takes place in inkjet printers and when extinguishing water is used. Pesticides are often applied to plants and soil by means of atomization. Atomization is also used for flue gas treatment, for cooling gases or surfaces, among other things in the form of cold spray .

In addition to many other applications, there are numerous medical or cosmetic sprays that are used to directly wet the skin with an active ingredient or are inhaled as an aerosol, for which there are atomizers (inhalers) of different designs as well as small spray cans with a structurally determined amount of the aerosol released per impact ( Dosing spray).

While in some applications very fine aerosols are generated, in some a spray jet with a high proportion of liquid acts , which can then not necessarily be regarded as atomization.

requirements

For atomization and thus for the formation of new free surfaces, energy or work must be supplied to the liquid. This serves to overcome the effective surface tension . For the work to be done, the following applies as a first approximation ${\displaystyle \sigma }$

${\displaystyle W_{\sigma }\approx \sigma \cdot A_{S}}$

${\displaystyle W_{\sigma }}$ = Work; = Surface tension; = Spray surface ${\displaystyle \sigma }$ ${\displaystyle A_{S}}$

Generation of monodisperse sprays

A simple way of producing monodisperse droplets is to let liquid in the earth's gravitational field drip off from a capillary with an internal diameter D with an acceleration g and a very low flow velocity . In the case of non-wetting between the liquid and the capillary material, this results in a theoretical drop diameter of ${\displaystyle x}$

${\displaystyle x={\sqrt[{3}]{\frac {6\cdot D\cdot \sigma }{\rho \cdot g}}}}$

In practice the drops are slightly smaller than the calculated ones; This is due to the fact that the detachment of the drop from the capillary does not take place in the ideal spherical shape, but rather the drop constricts somewhat. A certain part of the liquid flows back into the capillary. This draining of liquids from capillaries is used, for example, when dosing liquid medicaments from dropping bottles. The generation of small droplets with a high volume flow is hardly practicable with this method. However, graduation towers are a historical application .

Atomizer types

Depending on the type of energy supply, atomizer nozzles can be divided into the following classes:

• Single-substance pressure nozzles

The energy source is the liquid to be atomized itself. This is fed to the single-substance pressure nozzle under pressure. Depending on the type of nozzle, a liquid jet or a liquid lamella is generated at the nozzle mouth. The drop formation starts at a certain distance from the nozzle mouth. Typical representatives of this type of nozzle are turbulence, flat jet, impingement and hollow cone pressure nozzles.

• Two-substance or pneumatic nozzles

These nozzles work on the principle of a jet pump . Energy is supplied by a gas or steam flowing at high speed. The liquid to be atomized can be fed in almost without pressure. Some of these nozzles are self-priming. Two-substance nozzles are divided into nozzles with an internal mixture and nozzles with an external mixture. A typical representative of this design is the airbrush .

• Mechanical atomizers

Here, rotating disks are often used to impart the required energy to the liquid. Also include ultrasonic to this group.

• Special atomizer

Atomizer based on the principle of electrostatic atomization; rarely found in practice.

The technique that deals with the atomization of liquids is the atomization technique .

literature

• Thomas Richter: Atomizing liquids . expert-Verlag, Renningen, 2004, ISBN 3-8169-2309-7

Web links

• Nozzle basics on a nozzle manufacturer's website