A spray is a dynamic collection of drops dispersed in a gas. The process of forming a spray is known as atomization. A spray nozzle is the device used to generate a spray. The two main uses of sprays are to distribute material over a cross-section and to generate liquid surface area. There are thousands of applications in which sprays allow material to be used most efficiently. The spray characteristics required must be understood in order to select the most appropriate technology, optimal device and size.
Formation Spray atomization can be formed by several methods. The most common method is through a spray nozzle which typically has a fluid passage that is acted upon by different mechanical forces that atomize the liquid. The first atomizer was invented by Dr Auphon Euget-Les Bain in France in 1849, at a spa in Euzet, France. A portable atomizer was invented by Jean Sales-Girons in 1858, which he called a pulverisateur. Spray formation has taken on several forms, the most common being, pressure sprayers and centrifugal, electrostatic and ultrasonic nozzles.
Characteristics Spray nozzles are designed to perform under various operating conditions. The following characteristics should be considered when selecting a nozzle:
Pattern Capacity Spray impact Spray angle Drop size
Pattern Selecting a nozzle based on the pattern and other spray characteristics that are required generally yields good results. Since spray nozzles are designed to perform under many different spraying conditions, more than one nozzle may meet the requirements for a given application. Surfaces may be sprayed with any pattern shape. Results are fairly predictable, depending on the type of spray pattern specified. If the surface is stationary, the preferred nozzle is usually some type of full cone nozzle, since its pattern will cover a larger area than the other styles. Spatial applications, in which the objective is not primarily to spray onto a surface, are more likely to require specialized spray characteristics. Success in these applications is often completely dependent on factors such as drop size and spray velocity. Evaporation, cooling rates for gases and solids, and cleaning efficiency are examples of process characteristics that may depend largely on spray qualities.
Each spray pattern is described below with typical end use applications.
Solid stream This type of nozzle provides a high impact per unit area and is used in many cleaning applications, for example, tank-cleaning nozzles (fixed or rotary).
Hollow cone This spray pattern is a circular ring of liquid. The pattern is achieved by the use of an inlet orifice tangential to a cylindrical swirl chamber that is open at one end. The circular orifice exit has a diameter smaller than the swirl chamber. The whirling liquid results in a circular shape; the center of the ring is hollow. Hollow cone nozzles are best for applications requiring good atomization of liquids at low pressures or when quick heat transfer is needed. These nozzles also feature large and unobstructed flow passages, which provide a relatively high resistance to clogging. Hollow cone nozzles provide the smallest drop size distributions. The relative range of drop sizes tends to be narrower than other hydraulic styles. The hollow cone pattern is also achievable by the spiral design of nozzle. This nozzle impinges the fluid upon a protruding spiral. This spiral shape breaks the fluid apart into several hollow cone patterns. By altering the topology of the spiral the hollow cone patterns can be made to converge to form a single hollow cone.
Full cone Full cone nozzles yield complete spray coverage in a round, oval or square shaped area. Usually the liquid is swirled within the nozzle and mixed with non-spinning liquid that has bypassed an internal vane. Liquid then exits through an orifice, forming a conical pattern. Spray angle and liquid distribution within the cone pattern depend on the vane design and location relative to the exit orifice. The exit orifice design and the relative geometric proportions also affect the spray angle and distribution. Full cone nozzles provide a uniform spray distribution of medium to large size drops resulting from their core design, which features large flow passages. Full cone nozzles are the style most extensively used in industry.
Flat spray As the name implies, the spray pattern appears as a flat sheet of liquid. The pattern is formed by an elliptical or a round orifice on a deflective surface that is tangent to the exit orifice. The orifice has an external groove with a contoured internal cylindrical radius, or “cat's eye” shape. In the elliptical orifice design, the pattern sprays out of the orifice in line with the pipe. In the deflector design, the spray pattern is perpendicular to the pipe. There are two categories of flat spray, tapered and even, depending on the uniformity of the spray over the spray pattern. Flat spray patterns with tapering edges are produced by straight-through elliptical spray nozzles. This spray pattern is useful for overlapping patterns between multiple nozzle headers. The result is uniform distribution across the entire sprayed surface. Non-tapered flat spray nozzles are used in cleaning applications that require a uniform spray pattern without any overlap in spray area.
Multiple plume spray Multiple plume sprays are routinely used in automotive injectors. The multiple plumes are primarily used to provide for the optimal mixing of fuel and air so as to reduce pollutant emission under different operating conditions. The multiple plume automotive injectors can have anywhere from 2 to 8 plumes. The precise location of the centroid of these plumes, the individual plume angles, and the percentage split of the liquid amongst the plumes are normally obtained using an optical patternator.
Capacity Spray nozzle manufacturers all tabulate capacity based on water. Since the specific gravity of a liquid affects its flow rate, the values must be adjusted using the equation below, where Qw is the water capacity and Spg is the specific gravity of the fluid used resulting the volumetric flow rate of the fluid used Qf.
… excerpt ends here. Continue reading the full article.
