Analysis of the Methods for Atomization of Fluids
This file describes in a concise manner the main methods of atomization of fluids. The choice of the atomization method is very important since it impacts on the manufacturing costs and on the sizes of the droplets obtained.
All subsequent information was used by the following source: Graco Inc., expert and manufacturer of fluid-handling systems, http://wwwd.graco.com/training/concept_and_theory/Atomization%20v2.pdf
What is a droplet?
A droplet is a small particle of liquid having a more or less spherical shape. Droplets are also known as particles.
Figure 1: A spray stream with a variety of droplet sizes
Does Temperature influence the atomization?
Yes, of course. Surface tension is the property of a liquid that causes droplets and that's the reason why particles are round.
As the temperature of liquid increases, it's surface tension generally decreases. This becomes an important factor when handling certain fluids.
What are the main properties to influence nebulization?
A variety of factors affect droplet size and how easily a stream of liquid atomizes after emerging from an orifice. Among these factors are fluid properties of :
- Surface tension
- Viscosity
- Density
Surface tension tends to stabilize a fluid, preventing its breakup into smaller droplets. Everything else being equal, fluids with higher surface tensions tend to have a larger average droplet size upon atomization.
A fluid’s viscosity has a similar effect on droplet size as surface tension. Viscosity causes the fluid to resist agitation, tending to prevent its breakup and leading to a larger average droplet size. Figure 4 represents the relationship among viscosity, droplet size, and when atomization occurs.
Density causes a fluid to resist acceleration. Similar to the properties of both surface tension and viscosity, higher density tends to result in a larger average droplet size
Figure 2: Viscosity, droplet size, and when atomization occurs
What are the main methods to atomize?
The principal methods are the following five:
- Pressure atomization
- Air atomization
- Centrifugal atomization
- Electrostatic atomization
- Ultrasonic atomization
1. How does pressure atomization work?
Figure 3: A high-velocity water jet that is breaking up by airless atomization
In the airless atomization process, high pressure forces fluid through a small nozzle. The fluid emerges as a solid stream or sheet at a high speed. The friction between the fluid and the air disrupts the stream, breaking it into fragments initially and ultimately into droplets.
What is the source of energy?
The energy source for this form of atomization is fluid pressure, which is converted to momentum as the fluid leaves the nozzle.
What are the factors that influence the airless spray?
Three factors that affect an airless spray include the atomizer orifice diameter, the atmosphere, and the relative velocity between the fluid and the air. Regarding orifice diameter, the general rule is that the larger the diameter or size of the atomizer orifice, the larger the average droplet size in a spray.
What role does the atmosphere play in all this?
The atmosphere provides resistance and tends to break up the stream of fluid. This resistance tends to overcome, in part, the fluid’s properties of surface tension, viscosity, and density. In addition, the air temperature may also affect atomization.
What role does speed play?
The relative velocity between the fluid and the air also affects droplet sizes. The fluid’s velocity is created by pressure in the nozzle. As the fluid pressure increases, velocity increases and the average droplet size decreases. And conversely, as fluid pressure decreases, velocity is lower and the average droplet size is larger.
2. How does air atomization work?
In air spray atomization, fluid emerging from a nozzle at low speed is surrounded by a high-speed stream of air. Friction between the liquid and air accelerates and disrupts the fluid stream and causes atomization.
What is the source of energy?
The energy source for air atomization is air pressure. The operator can regulate the flow rate of fluid independently of the energy source. Stream of fluid passing through an orifice; as it emerges, a high-speed stream of air surrounds the fluid stream.
What are the initial speeds in the two methods?
3. How does centrifugal atomization work?
In centrifugal or rotary atomization, a nozzle introduces fluid at the centre of a spinning cup or disk. Centrifugal force carries the fluid to the edge of the disk and throws the fluid off the edge. The liquid forms ligaments or sheets that break into fine droplets. With rotary atomization, operators can control both the flow rate and the disk speed independently of each other. In most spray coating rotary applications, electrostatic charge is applied to the spray to attract the droplets to a grounded target object. In some types of atomizers, such as bells, shaping air can be added to move the spray forward in an axial direction.
What is the source of energy?
The energy source for rotary atomization is a centrifugal force. With the same rotational speed, at low flow rates, droplets form closer to the edge of the disk than with higher flow rates. The spray pattern tends to move radially away from the disk or cup in all directions (360˚).
4. How does electrostatic atomization works?
Electrostatic atomization exposes a fluid to an intense electric field between the charged atomizer and grounded workpiece. The charge transfers to the fluid and repulsive forces between the atomizer and the fluid tear the droplets from the atomizer and send them toward the work surface.
What is the source of energy?
The energy source for electrostatic atomization is the electric charge that the fluid receives.
What is the difference between electrostatic atomization and electrostatic spray charging?
It is important to understand the distinction between electrostatic atomization and electrostatic spray charging. With electrostatic atomization, electrostatic forces are used to atomize the fluid. In electrostatic spray charging, the spray is usually atomized by airless, air spray, or rotary means, and an electrostatic charge is applied to the droplets as they form to help attract them to the work surface.
5. How does ultrasonic atomization work?
Ultrasonic atomization relies on an electromechanical device that vibrates at a very high frequency. Fluid passes over the vibrating surface and the vibration causes the fluid to break into droplets.
What are its applications?
Applications of this technology include: Medical nebulizers for inhalation therapy; drying liquids, powdered milk for example, in the food industry; surface coatings in the electronics industry
What are its limits?
Ultrasonic atomization technology is effective only for low-viscosity Newtonian fluids.
How can optimal atomization be achieved?
Achieving the desired level of atomization requires maintaining a balance of the fluid viscosity and quantity (fluid flow rate) on one side with atomization energy on the other side.
Figure 5: Balancing factors to achieve the desired atomization
REFERENCE
- Graco Inc., Minneapolis (US) - http://wwwd.graco.com/training/concept_and_theory/Atomization%20v2.pdf
