Spray patterns

Aerosol products are hermetically sealed, ensuring that the contents caimot leak, spill, or be contaminated. The packages can be considered to be tamper-proof. They deUver the product in an efficient manner generating Httie waste, often to sites of difficult access. By control of particle size, spray pattern, and volume deUvered per second, the product can be appHed directiy without contact by the user. For example, use of aerosol pesticides can minimize user exposure and aerosol first-aid products can soothe without applying painful pressure to a wound. Spray contact lens solutions can be appHed directiy and aerosol lubricants (qv) can be used on machinery in operation. Some preparations, such as stable foams, can only be packaged as aerosols. 

The actuator contains the final orifice and a finger pad or mechanical linkage for on—off control. The spray pattern is largely affected by the constmction of the actuator, particularly by the chamber preceding the orifice. Actuators are often termed mechanical breakup and nonmechanical breakup depending upon the complexity of this chamber. Mechanical breakup actuators are of more expensive two-piece constmction. Actuators are usually molded from polyethylene or polypropylene the breakup insert may be almost any material, including metal. 

The spray pattern is also important. A soHd cone pattern mixes less well than a hoUow cone, sheet, or multiple jet patterns. Drops that enter along the edge of the spray pattern nearest the exit have less time to heat up and evaporate. Drops that enter along the edges of the spray pattern nearest the walls may not fully evaporate before hitting the walls, resulting in erosion and corrosion of the brick. 

Because high quaHty, low cost, and optimum performance are required for spray equipment, improved analytical and experimental tools are iadispensable for increasing productivity ia many competitive iadustries. In most iastances, it is no longer adequate to characterize a spray solely on the basis of flow rate and spray pattern. Information on droplet size, velocity, volume flux, and number density is often needed and can be determined usiag advanced laser diagnostic techniques. These improvements have benefited a wide spectmm of consumer and specialized iadustrial products. 

Hquid is fed into center of spinning disk, cup, or wheel, and spreads out toward rim produces a 360° spray pattern and relatively uniform drop size used in spray drying and cooling appHcations... 

Patternation. The spray pattern provides important information for many spray appHcations. It is directiy related to the atomizer performance. For example, in spray drying, an asymmetric spray pattern may cause inadequate Hquid—gas mixing, thereby resulting in poor efficiency and product quaHty. Instmments that provide quantitative information on spray patterns are therefore essential for many processes. The pattern information must be able to reveal characteristics such as skewness, degree of pattern hoUowness, and the uniformity of Hquid flux over the entire cross-sectional area. 

Hollow Sprays. Most atomizers that impart swid to the Hquid tend to produce a cone-shaped hoUow spray. Although swid atomizers can produce varying degrees of hoUowness in the spray pattern, they aU seem to exhibit similar spray dynamic features. For example, detailed measurements made with simplex, duplex, dual-orifice, and pure airblast atomizers show similar dynamic stmctures in radial distributions of mean droplet diameter, velocity, and Hquid volume flux. Extensive studies have been made (30,31) on the spray dynamics associated with pressure swid atomizers. Based on these studies, some common features were observed. Test results obtained from a pressure swid atomizer spray could be used to iUustrate typical dynamic stmctures in hoUow sprays. The measurements were made using a phase Doppler spray analyzer. 

Because of the complexity of designs and performance characteristics, it is difficult to select the optimum atomizer for a given appHcation. The best approach is to consult and work with atomizer manufacturers. Their technical staffs are familiar with diverse appHcations and can provide valuable assistance. However, they will usually require the foUowing information properties of the Hquid to be atomized, eg, density, viscosity, and surface tension operating conditions, such as flow rate, pressure, and temperature range required mean droplet size and size distribution desired spray pattern spray angle requirement ambient environment flow field velocity requirements dimensional restrictions flow rate tolerance material to be used for atomizer constmction cost and safety considerations. 

Though spray requirements differ from one appHcation to another, the spray pattern or shape appears to be a sensible criterion for selecting Hquid atomizers for certain processes. Table 2 Hsts a variety of appHcations that are based on the pattern of the spray. 

As shown in Figure 6, desirable fixative properties superior to PVP homopolymer can be specified by judicious selection of the amount of vinyl acetate. Hair sprays are limited in the molecular weight of the resin because if they are too high the resulting viscosity of the formulation will result in a poor (coarse) spray pattern. Increasing the VP/VA ratio causes properties to increase in the direction shown by the arrows. 

Deposits sometimes block fuel nozzles and distort fuel spray patterns, lea ding to skewed temperature distribution with the possibiUty of burnout of turbiae parts by a "hot streak" exhaust. These deposits are sometimes associated with metal-containing particulates, but ia geaeral are another manifestation of fuel iastabiUty. 

For proper use of the equations, the chamber shape must conform to the spray pattern. With cocurrent gas-spray flow, the angle of spread of single-fluid pressure nozzles and two-fluid pneumatic nozzles is such that wall impingement wiU occur at a distance approximately four chamber diameters below the nozzle therefore, chambers employing these atomizers should have vertical height-to-diameter ratios of at least 4 and, more usually, 5. The discharge cone below the vertical portion should have a slope of at least 60°, to minimize settling accumulations, and is used entirely to accelerate gas and solids for entty into the exit duct. 

Another type of distributor, not shown in Fig. 14-64, is the spray nozzle. It is usually not recommended for hquid distribution for two reasons. First, except for small columns, it is difficult to obtain a uniform spray pattern for the packing. The fuU-cone nozzle type is usually used, with the need for a bank of nozzles in larger columns. When there is more than one nozzle, the problem of overlap or underlap arises. A second reason for not using spray nozzles is their tendency toward entrainment by the gas, especially the smaller droplets in the spray size distribution. However, some mass transfer in the spray can be expected. 

Effect of base polymer polydispersity on ease of forming a uniform spiral spray pattern from the corresponding adhesives... 

The spray pattern of the oil/steam leaving the nozzle tips tends to be flat (fan spray)... 

In conventional spraying paint is forced under pressure to the spray gun, where it mixes with air and, forced through a small orifice, atomises. Airless spray is created by forcing paint at extremely high pressures through an accurately designed small hole. Rapid expansion as it leaves the gun produces an extremely fine and very even spray pattern. No air is mixed with the paint before it leaves the gun, so avoiding dry spray . A wetter, heavier... 

Air assisted airless spray This concept is a combination of air spray and airless methods. Paint can be atomised with full spray patterns at low pressures. Turbulence is reduced significantly and overspray is minimised. 

A field test was conducted by spraying a commercial oil spill-dispersant (Corexit 9527) from aircraft [696]. Test objectives were to determine the efficiency of delivering the dispersant to a selected target using a large aircraft and to compare various measurement systems for droplet size and spray pattern distribution. The results indicated that aerial flights up to 46 m can produce droplet sizes and swath widths that would be operationally effective for an oil spill. 

A friend developed a unique spray that holds very constant velocity and consistent spray pattern regardless of flow, thereby overcoming turndown problems in other spray nozzles. This is very useful in quench or desuperheating service. Figure 15-1 illustrates this unique spray. 

Spray patterns and densities can be adjusted real-time. Affected by wind at long distances. 

Fan Spray 100-1000 High-pressure painting/coat ing, Annular combustors Good atomization, Narrow elliptical spray pattern High supply pressure... 

Rotary Atomization Spinning Disk 10-200 Spray drying. Aerial distribution of pesticides. Chemical processing Good mono-dispersity of droplets. Independent control of atomization quality and liquid flow rate Satellite droplets, 360° spray pattern... 

A hollow-cone spray can be generated via a simplex atomizer. The spray pattern varies depending on the injection pressure. At very low pressures, liquid dribbles from the nozzle orifice. With increasing pressure, the liquid emerges from the orifice as a thin,... 

Fan spray atomizers have been widely used in the spray coating industry (Fig. 2.5), in some small annular gas turbine combustors, and in other special applications that require a narrow elliptical spray pattern rather than the normal circular pattern. In particular, fan spray atomizers are ideal for small annular combustors because they can produce a good lateral spread of fuel, allowing to minimize the number of injection ports. 

In flashing liquidjet orflashing injection atomization f181 a high-pressure liquid with dissolved gas flashes, shattering the liquid into small droplets in a fairly regular spray pattern. Flashing even... 

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