Atomizers external-mixing

Atomizers for large boiler burners are usually of the swid pressure jet or internally mixed twin-fluid types, producing hoUow conical sprays. Less common are the externally mixed twin-fluid types (89,90). 

Figure 2.7. Schematic of external-mixing two-fluid atomizers.

A variety of atomizer designs have been developed in an effort to control the droplet size distribution and to increase the yield of fine powders. Gas atomizers used for the atomization of melts may be loosely classified into two primary categories in terms of the interaction mode between a liquid metal and an atomization gas during atomization, i.e., (1) internal mixing and (2) external mixing. 

External-mixing air-assist atomizers. Derived from ethanol (glycerin)-air spray data with initial thickness of flat circular sheet up to 0.7 mm and varied air impingement angles Sampled with oil-coated slides... 

FIGURE 3 Atomization nozzle designs for internal mixing and external mixing... 

Twin-Fluid Air-Blast Atomizer. Twin-fluid atomizers can be divided into internal and external mixing systems. Atomization occurs by passing a high-velocity gas stream over a liquid sheet or by mixing in the form of a Y jet. The gas stream is usually air although steam has been used to improve the injection characteristics of heavy viscous fuels. The air stream is usually derived from the main air flow to the combustor, thus utihzing a portion of the combustor pressure drop. 

The pnenmatic nozzle is an atomizer with internal or external mixing of gas and liqnid. Here atomization is accomplished by the interaction of the liquid with a second fluid, usually compressed air. 

It should be noted that this technique is not the same as external mixing of separately atomized droplets coalescing within the chamber. One reported work in the literature utilized droplets atomized from separate nozzles that were then allowed to collide and coalesce within the spray dryer. Janssens et al. [19] have shown that this approach did not improve the solubility and the extent of dissolution of Intraconazole and Kollicoat IR particles the droplets behave as individual droplets without significant mixing and coalescence within the spray dryer. In the 3- and 4-fluid nozzle technique, the mixing of the two immiscible occurs at the position just prior to atomization under very high shear. 

Gas and hquid can be brought in contact either within the nozzle (internal mix) or outside of the nozzle (external mix). In addition, nozzles can be categorized based on the flow rate of the atomizing gas and the way the gas is brought into contact with the liquid. These variations are airblasting, air-assisting, and effervescent nozzles. The main difference between each of the three is the velocity and quantity of air used in the atomizing process. Another difference relates to when the air is mixed with the liquid stream. 

Imamura T., and Nagai, N., The Relative Performance of Externally Mixed Twin Fluid Atomizers, Proceedings of the 3rd International Conference on Liquid Atomization and Sprays, London, July 1985. 

Liquid waste atomization can be achieved by any of the following atomizers rotary cup single-fluid pressure two-fluid, low-pressure air (internal mix) two-fluid, high-pressure air or steam (internal mix) and two-fluid, high-pressure air or steam (external mix). 

Two-fluid nozzles, also known as pneumatic nozzles, use a compressed gas to atomize the liquid feed. There are different designs of nozzles on the market. The two major groups of two-fluid nozzles are known as an external mixing nozzle and an internal mixing nozzle. 

External mixing nozzle is operated with low liquid pressure. The liquid feed is provided through an inner duct while an atomization gas is fed by an external annular opening around the liquid orifice. On the other hand, internal mixing nozzles take advantage of gas expansion at the nozzle outlet. Part of the pressure energy applied is used to scatter the liquid fragments within and beyond the nozzle oriflce by the sudden gas expansion (Walzel 2011). 

An external mixing atomizer is used with a liquid volume throughput of 50ml/min and an air-liquid (mass) ratio (ALR) between 0.5 and 2. The images shown in Fig. 8.13 are captured by the Real-Time Process Analysis System at the center of the spray cone at a distance of z = 34cm and a distance to the nozzle between 2.5 cm and 20 cm. 

For PVP K90 5 m% solution. As also seen in the shadowgraphic pictures, the amount of filaments in the SEM pictures is higher than for the PVP K30 30 m% solution and the filaments are thinner. Also, the fine particle fraction has the same size distribution for the two investigated temperatures. When comparing the mean size 50,3 for all fine particle fractions (Fig. 19.14), the value for K30 30 m% and K90 5 m% is similar. Mainly for the low-viscous K30 solution (20 m%) there is an effect of the atomizer gas temperature on the droplet size. The drying results imply that the atomization process of the PVP solutiOTis is not enhanced by the hot gas when compared to the conventional external-mixing twin-fluid atomizer. 

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