Fan-jet nozzles

It is necessary to know how the optimum drop size can be achieved. The drop size will be governed by the equipment used, particularly by the type and size of nozzle and by the conditions of pressure and liquid output under which the nozzles are operated. It will be governed also by the physical properties of the spray fluid, such as density, surface tension, and viscosity. For fan-jet nozzles, one of the commonest types of atomizing equipment used in pesticide sprays, Yeo (25) has shown that a simple expression can be used to relate the various parameters governing drop size. For static or slow-moving nozzles, this takes the form ... 

Although it may seem reasonable that an increase in viscosity of the spray fluid should increase the drop size, there is little fundamental information on the relationship between the drop size of the sprays and the viscosity of the spray liquid. Moreover, the information that is available is conflicting. Besides the work of Yeo and Dorman, where a viscosity term was found unnecessary when working with liquids having relatively low viscosities, other workers have found that a function of viscosity was necessary to describe drop size (16,17, 20), but the value of this function has varied from v01 to v106 (where v is the kinematic viscosity of the liquid). More recently, Dombrowsld and Johns (9) have examined the breakup of sheets of viscous liquids formed from fan-jet nozzles and have derived a theoretical expression for the size of drops produced. The expression is very complex and includes viscosity terms, but it is difficult to use in a practical fashion to predict drop size and its variation with a particular physical parameter of the spray fluid. 

Quantitative data on hollow-cone nozzles, which are also important in pesticide sprays, appear to be even more scanty than the data for fan-jet nozzles. Fraser (16) states that the mean diameter for cone nozzles is proportional to v0 2, and Knight (19) has produced an expression ... 

The sprays were formed from a series of hollow-cone nozzles (supplied by Delavale-Watson, Widnes, Lancs, England) and a series of ceramic tipped, fan-jet nozzles (supplied by E. A. Allman Ltd., Chichester, Sussex, England). Their characteristics are listed in Table I. 

Figure 1 shows the variation in volume median diameter of drops formed from a typical hollow-cone nozzle (Spraycone WG 4008) with increase in viscosity from 1 to about 100 cp. To maintain a constant emission rate, the operating pressure was increased as the viscosity was increased. The variation of drop size with viscosity is similar to that already reported using fan-jet nozzles (II), and the results can be considered in terms of three viscosity ranges ... 

Thus, the changes in drop size which accompany changes in viscosity (Figure 1) appear to be related to changes in the shape and size of the spray sheet. The relationship between the various properties that govern sheet dimensions may be found from a dimensional analysis in a manner similar to that published for fan-jet nozzles (12). 

Figure 5. Fan-jet nozzles spraying liquids of different viscosities...

Table III. Range of Parameters Used in Deriving Equation 4 for Fan-Jet Nozzles...

This expression gives reasonably good results for the size of drops produced from ligaments using various fan-jet nozzles operating under various conditions 11). 

Practical Application of the Theory. The practical difficulty in using Equations 3 and 4 is in measuring the sheet dimensions I and a for hollow-cone nozzles r and 6 for fan-jet nozzles. Unlike the remaining parameters in Equations 3 and 4, these dimensions and angles cannot be measured in the field. However, they can be measured from flash photographs in the laboratory using a nozzle design similar to that to be used in the field. From the laboratory results, a plot of the appropriate Vi function vs. 1/Re can be drawn, where Re is calculated from ... 

Table VI. Effect of Nozzle Type and Water-to-Oil Phase Ratio on Drop Size and Emission Rates for Water-in-Oil Emulsions Applied through Fan-Jet Nozzles...

The principle of operation of the impinging jet nozzle resembles that of the fan spray nozzle with the exception that two or more independent jets are caused to impinge in the atmosphere. In impact atomisers, one jet is caused to strike against a solid surface, and for two jets impinging at 180o(34), using SI units ... 

Since Equation 3 for hollow-cone nozzles can also be reduced to Equation 5, the latter appears to be a general expression that can be applied to the sheet breakup of all types of fan-jet and hollow-cone nozzles. The function of Re appears to take the same form in all cases, but its value may vary with different designs of nozzle. 

Martin [87] provides design correlations for multiple-slot and round jets besides recommendations for the spatial arranganent of jet nozzles on the basis of maximizing heat transfer per unit fan energy. The optimal ratio of the pitch of the nozzles compared with the distance above the surface (0.7), which is recommended by Martin, is close to the reported critical value at which jet-to-jet interactions start influencing the heat transfer at the stagnation point under the jet axis [84]. 

FIG. 14-87 Charactersitic spray nozzles, a) Whirl-chamber hollow cone, (h ) Solid cone, (c) Oval-orifice fan. (d) Deflector jet. (e) Impinging jet. (/) Bypass, (g) Poppet, (h) Two-flnid. ( ) Vaned rotating wheel. 

The Majac jet pulverizer (Ho.sokawa Micron Powder Sy.stems Div.) is an opposed-jet type with a mechanical classifier (Fig. 20-55). Fineness is controlled primarily by the classifier speed and the amount of fan air dehvered to the classifier, but other effects can be achieved by variation of nozzle pressure, distance between the muzzles of the gun barrels, and position of the classifier disk. These pulverizers are available in 30 sizes, operated on quantities of compressed air ranging from approximately 0.6 to 13.0 mVmin (20 to 4500 ftV min). In most apphcations, the economics of the use of this type of jet pulverizer becomes attractive in the range of 98 percent through 200 mesh or finer. 

A number of techniques have been evolved to disperse liquids in gases in the form of fine droplets. The various atomizing techniques are jet injections, fan sprays, centrifugal nozzles, twin fluid atomizers, impinging jets, and rotary... 

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