Spray nozzles fundamentals

How the spraying process works, what are the applications of our products, what are spray nozzles technical features and the production techniques of the spray.

The process of atomization

A liquid spraying process can be described as consisting of two phases, namely:

1. Breaking of the liquid into separate droplets
2. Directing the liquid drops onto a surface or an object, to achieve the desired result.

Modern technology allows for a strict control of different parameters of a liquid spray; for example precise information can be obtained about droplet size spectrum, droplets speed and liquid distribution onto the spray target.

Applications

Spraying a liquid through a spray nozzle can serve different purposes, among which the most important are the following:

Cooling
Heat transfer by spraying liquids onto the products surface for a rapid cooling, such as continuous casting cooling in steelworks.

Humidification
Spray of very little quantities of liquid onto the products surface into special chambers or rooms to raise relative humidity. A typical application is textiles humidification.

Coating
Application of coatings or liquids on the food products surface. For example: oil-spraying on bread.

Food processing
Spray to add specific ingredients or substances to speed up chemical reactions. For ex.: addition of fructose in fruit juices, etc.

Washing
Remove dirt from the product surface spraying liquids at high pressure, like in vehicles pre-wash treatment.

Pollution control
Use of atomized scrubbing liquids to capture particulate matter and/or gaseous pollutants in liquid droplets, like in web scrubbers and spray towers.

Spray nozzles technical features

Several technical features must be taken into account to select the proper nozzle.

1. Nozzle efficiency

A spray nozzle is a device that turns the pressure energy of a liquid flow into kinetic energy. The nozzle efficiency can be defined as the ratio between the energy available at the nozzle inlet and the energy wich is actually used to increase the liquid speed and create the spray, the difference being the energy lost during the process because of friction. Depending on the nozzle type and for a good quality machining, the nozzle efficiency varies between 55% and 95% for the types that are commonly used in industrial processes. What above stated is not valid for air-assisted atomizers which require a much higher energy because of the losses inherent in the energy transfer from compressed air to liquid surface.

2. Droplet size

The droplets size depends on the structure of the atomizer, intensity of the liquids energy, liquid surface tension and density. The size of the atomized droplets is not uniform.
Therefore, the average droplets size becomes an important factor. For example, the droplets size in gas quenching towers is extremely important. If their size is too big, they do not fully evaporate leading to dust bag failure. On the contrary, if the droplets size is too small, it’s not possible to lower the temperature to the desired level and high temperature may cause the dust bags burn out.

There are four ways to express the droplets size: The Sauter Mean Diameter (SMD) is the most commonly used. It refers to the drop volume/surface area ratio and it’s often shown as D32, μm(Micron) unit. (1μm=10-3mm)

1 | ARITHMETIC MEAN DIAMETER
This is a diameter value which, multiplied by the local number of droplets in the sample, equals the addition of all droplets diameters.

2 | SURFACE MEAN DIAMETER
This is a diameter of such a droplet whose surface, multiplied by the total droplets number, equals the sum of all droplets surfaces.

3 | VOLUME MEAN DIAMETER
This is the diameter of such a droplet whose volume, multiplied by the total droplets number, equals the sum of all droplets volumes.

4 | SAUTER MEAN DIAMETER (D32)
This is the diameter of such a droplet whose volume/area ratio, equals the ratio between the sum of all droplet volumes divided by the sum of all droplet surfaces.

3. Spray angle

A spray angle is the angle formed by the cone of liquid leaving a nozzle orifice. The spray angle and the distance between the nozzle orifice and the target surface to be covered determine the spray coverage.

4. Impact force

The impact force is the force generated by the jet of water deflected by the impact surface and its strength is often expressed in kg/cm² or lb/inch². The uniformity of a jet impact force and distribution influence the washing effect.

5. Distribution

Engineers design nozzles with different spray distribution patterns. Patterns can be solid stream, full cone, hollow cone, flat spray, spoon flat fan. The nozzle design aims at the uniformity and impact force of the jet sprayed whether nozzles are used individually or overlapping.

Flat fan
Convex distribution

Flat fan
Even distribution

Full cone
convex distribution

Full cone
even distribution

Hollow cone
concave distribution

Straight
single-point distribution

Techniques for spray production

Many different hydrodynamics techniques can be used to produce a spray and most of them are used today for nozzles to be applied in industrial processes.

Pressure nozzles

This is the simplest type of nozzle where an orifice is opened into a chamber where the liquid to be sprayed is fed under pressure.

A spray is produced through the orifice with spray pattern, flow rate and spray angle depending upon the orifice edge profile and the design of the inside pressure chamber.
Typical pressure nozzles are G series straight nozzles and F series high pressure flat fan nozzles.

Turbulence nozzles

Turbulence nozzles use specially shaped vanes which force the pressurized liquid into a whirl chamber producing its high-speed rotation. This breaks up the liquid which exists the nozzle orifice atomized at high-speed.

Different nozzle structures and flow rates produce hollow cone, full cone and full square cone spray patterns. Typical turbulence nozzles are RA series hollow cone and D series full cone nozzles.

Impact nozzles

Here the desired spray shape is obtained producing an impact of the liquid jet onto a properly designed surface. The liquid jet is subsequently changed into a fluid lamina and then broken into drops with the desired spray pattern after leaving the nozzle edge.

Typical impact nozzles are K series flat fan nozzles, E series spiral full cone nozzles and RC series hollow cone nozzles.

Air assisted nozzles

Depending on the liquid supply, these nozzles are of two types: pressure nozzles and siphon nozzles.

Pressure nozzle

Siphon nozzles

Air assisted atomizers

Air-assisted atomizers use their special design and pressurized gas to atomize a liquid and break it into tiny droplets (the smallest average particle size: 10 micron).

Ultrasonic atomizers

Ultrasonic atomizers are sister products of air-assisted atomizers. The front-end has a titanium ultrasonic generator. It uses the energy of the high-speed impact to produce a high-frequency oscillation that micro-atomizes the liquid droplets. The special design produces tiny and uniform droplets (the average smallest particle size: 7 Micron). The advantages are vital to many applications.

Ultrasonic atomizers have two phases of atomization.
1 | Phase one: liquids mix with pressured air and produce tiny droplets to spray.
2 | Phase two: when the atomized droplets hit the ultrasonic generator they get micro-atomized generating smaller droplets.