G88

In calculating the impact point of spray, one should recognize that the spray angle closes in as the spray moves away from the nozzle. This is caused by loss of momentum of the spray to the gas.

TABLE 14-19 Atomizer Summary

Types of atomizer

Design features

Advantages

Disadvantages

Pressure.

1. Hollow cone.

h. Grooved core.

a. Oval or rectangular orifice (see Fig. 14-87c). Numerous variants on cavity and groove exist.

4. Nozzles with wider range of turndown.

Sonic.

Rotary wheels (see Fig. 14-87i) disks, and cups.

Ultrasound.

Flow a(AP/pi)1/2. Only source of energy is from fluid being atomized.

Liquid leaves as conical sheet as a result of centrifugal motion of liquid. Air core extends into nozzle.

Centrifugal motion developed by tangential inlet in chamber upstream of orifice.

Centrifugal motion developed by inserts in chamber.

Similar to hollow cone but with insert to provide even distribution.

Liquid leaves as a flat sheet or flattened ellipse.

Combination of cavity and orifice produces two streams that impinge within the nozzle.

Liquid from plain circular orifice impinges on curved deflector.

Two jets collide outside nozzle and produce a sheet perpendicular to their plane.

A portion of the liquid is recirculated after going through the swirl chamber.

Conical sheet is developed by flow between orifice and poppet. Increased pressure causes poppet to move out and increase flow area.

Gas impinges coaxially and supplies energy for breakup.

Gas generates an intense sound field into which liquid is directed.

Liquid is fed to a rotating surface and spreads in a uniform film. Flat disks, disks with vanes, and bowl-shaped cups are used. Liquid is thrown out at 90° to the axis.

Liquid is fed over a surface vibrating at a frequency > 20 kHz.

Simplicity and low cost.

High atomization efficiency.

Minimum opportunity for plugging.

Smaller spray angle than 1a and ability to handle flows smaller than 1a.

More uniform spatial pattern than hollow cone.

Flat pattern is useful for coating surfaces and for injection into streams.

Minimal plugging.

Different liquids are isolated until they mix outside of orifice. Can produce a flat circular sheet when jets impinge at 180°.

Achieves uniform hollow cone atomization pattern with very high turndown (50:1).

Simplest control over broad range.

High velocities can be achieved at lower pressures because the gas is the high-velocity stream. Liquid-flow passages can be large, and hence plugging can be minimized.

Similar to two-fluid but with greater tolerance for solids.

The velocity that determines drop size is independent of flow. Hence these can handle a wide range of rates. They can also tolerate very viscous materials as well as slurries. Can achieve very high capacity in a single unit; does not require a high-pressure pump.

Fine atomization, small size, and low injection velocity.

Limited tolerance for solids; uncertain spray with high-viscosity liquids; susceptible to erosion. Need for special designs (e.g., bypass) to achieve turndown.

Concentrated spray pattern at cone boundaries.

Coarser drops for comparable flows and pressure drops. Failure to yield same pattern with different fluids. Small clearances.

Coarser drops.

Extreme care needed to align jets.

Waste of energy in bypass stream. Added piping for spill flow.

Difficult to maintain proper clearances.

Because gas is also accelerated, efficiency is inherently lower than pressure nozzles.

Similar to two-fluid.

Mechanical complexity of rotating equipment. Radial discharge.

Low flow rate and need for ultrasound generator.

At some low flow, pressure nozzles do not develop their normal pattern but tend to approach solid streams. The required flow to achieve the normal pattern increases with viscosity.

Two-Fluid (Pneumatic) Atomizers This general category includes such diverse applications as venturi atomizers and reactor-effluent quench systems in addition to two-fluid spray nozzles. Depending on the manner in which the two fluids meet, several of the breakup mechanisms may be applicable, but the final one is high-level turbulent rupture.

As shown by Table 14-20, empirical correlations for two-fluid atom-ization show dependence on high gas velocity to supply atomizing energy, usually to a power dependence close to that for turbulent breakup. In addition, the correlations show a dependence on the ratio of gas to liquid and system dimension.

Further differences from hydraulic nozzles (controlled by sheet and ligament breakup) are the stronger increase in drop size with increasing surface tension and decreasing gas density.

The similarity of these dependencies to Eq. (14-190) led to a reformulation with two added terms that arise naturally from the theory of power dissipation per unit mass. The result is Eq. (14-198) which is labeled power/mass in Table 14-20.

D32 = 0.29 ^ p1/velocity)1^1 + j0^Dno22lej0.4 (14-198)

where o = surface tension Pg = gas density L/G = mass ratio of liquid flow to gas flow D nozzle = diameter of the air discharge

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

This is remarkably similar to the empirical two-fluid atomizer relationships of El-Shanawany and Lefebvre [J. Energy, 4, 184 (1980)] and Jasuja [Trans. Am. Soc. Mech. Engr., 103,514 (1981)]. For example, El-Shanawany and Lefebvre give a relationship for a prefilming atomizer:

D32 = 0.0711(c/pG)0-6(1/velocity)12(1 + L/G)(DnOzzle)0-4(pi/pG)01

+ 0.015[(|1l)2/(o X pL)r(D„ozzle)0-5(1 + L/G) (14-199)

where |L is liquid viscosity. According to Jasuja,

D32 = 0.17(c/pG)045(1/velocity)0-9(1 + L/G)05(Dnozzle)055

+ viscosity term (14-200)

[Eqs. (14-198), (14-199), and (14-200) are dimensionally consistent; any set of consistent units on the right-hand side yields the droplet size in units of length on the left-hand side.]

The second, additive term carrying the viscosity impact in Eq. (14199) is small at viscosities around 1 cP but can become controlling as viscosity increases. For example, for air at atmospheric pressure atomizing water, with nozzle conditions

D nozzle = 0.076 m (3 in) velocity = 100 m/s (328 ft/s) L/G = 1

El-Shanaway measured 70 |m and his Eq. (14-199) predicted 76 |m. The power/mass correlation [Eq. (14-198)] predicts 102 |m. The agreement between both correlations and the measurement is much better than normally achieved.

Rotary Atomizers For rotating wheels, vaneless disks, and cups, there are three regimes of operation. At low rates, the liquid is shed directly as drops from the rim. At intermediate rates, the liquid leaves the rim as threads; and at the highest rate, the liquid extends from the edge as a thin sheet that breaks down similarly to a fan or hollow-cone spray nozzle. As noted in Table 14-19, rotary devices have many unique advantages such as the ability to handle high viscosity and slurries and produce small droplets without high pressures. The prime applications are in spray drying. See Masters [Spray Drying Handbook, Wiley, New York (1991)] for more details.

Pipeline Contactors The correlation for droplet diameter based on power/mass is similar to that for two-fluid nozzles. The dimension-less correlation is

D32 = 0.8(c/pG)0-6(1/velocity)12(Dpipe)0-4 (14-201)

(The relation is dimensionally consistent; any set of consistent units on the right-hand side yields the droplet size in units of length on the left-hand side.)

The relationship is similar to the empirical correlation of Tatterson, Dallman, and Hanratty [Am. Inst. Chem. Eng. J., 23(1), 68 (1977)]

Predictions from Eq. (14-201) align well with the Tatterson data. For example, for a velocity of 43 m/s (140 ft/s) in a 0.05-m (1.8-inch) equivalent diameter channel, Eq. (14-201) predicts D32 of 490 microns, compared to the measured 460 to 480 microns.

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