Industrial Sprays and Atomization

It is often stated that the primary reason for breaking-up liquid into droplets is the advantage gained, for various processes, by the resulting increase in the surface area of the liquid; see, for example, Lefebvre^†. This is certainly the case for many processes, particularly those where rapid vaporization of the liquid is required. For example in the combustion of liquid fuels the utilization of sprays is often the only real choice available. However, in other applications this increase in surface area may be either one of several benefits, or an incidental and irrelevant result of the main process. For example, in spray painting the formation of an even surface coating takes advantage of the dispersion of droplets into a nearly homogeneous spatial pattern, which is made possible with several types of spraying nozzles. With suitable choices of droplet sizes and momentum, coatings of the required thickness are achieved with minimal splashing and unevenness. In many industrial applications outside the combustion field, the choice of spraying, as opposed to other unit processes, is not always an obvious one and it requires careful balancing of the pros and cons. As an example, although metal powder production by spraying molten metal is a major industry^†, there are still cases where use of conventional mechanical communition is preferred. As a further example, although fine water sprays are ideal for rapid vaporization there are cases where they are not the automatic choice.

A spray is [1] “a dispersion of droplets with sufficient momentum to penetrate the surrounding medium”. The nozzle or other device that produces the spray is referred to as an atomizer and the surrounding medium is gaseous. Processes utilizing droplets require enough momentum to transport the droplets to where they are utilized or to provide mixing with the gas. We may go outside this definition of a spray to cover other processes that utilize liquid droplets such as; (1) those that use orderly arrays of droplets and (2) those that use a suspension of fine droplets (an aerosol) or droplets falling under the action of gravity.

Today, food processing includes well-established techniques such as membrane filtration, evaporation, and spray drying and fluidized beds. These processes can operate either individually or in a combination depending on the desired end-products. The main requirements for these processes are sanitation, flexibility and adaptive capability and also ensuring the quality and functional properties of many established products. Examples of typical food product ingredients and tailor-made food products are: coconut, cream powder, egg products, cappuccino powder, soy milk powder, flavours, non-dairy creamers, shorteners, gelatin, biscuits, chocolate, pizza, various vegetables, meat (e.g. burgers), cornflakes, etc.

Fire has two faces: one is friendly, it gives light and warmth. It has set in motion the process of civilization. Through its force the human race has developed. But when it gets out of control it shows its other nature. Then fire devours and destroys everything. Its destructive force cannot be subdued. It becomes an incalculable enemy of man. Among historical events, which have shaken the world, there have been disasters like the one in the year 64 in Rome, the Great Fire of London in 1666 and the big bush fires of 1994 in Sydney [1].

Crop protection and fertilization are important to the efficiency of agricultural production. With a constantly growing world population applying pesticides to crops is considered to be indispensable in order to guarantee the supply of food. The technology of crop protection uses spray nozzles for an even distribution of atomized liquid onto the plants. The chemicals to be sprayed can be subdivided into herbicides, fungicides, insecticides, liquid fertilizers and bio-regulators. Improper application in agriculture, however, can cause severe environmental contamination. This necessitates the use of precise and high quality spray equipment.

Steady industrial fuel sprays produced by “burners” are used to generate heat in several types of fixed plant equipment. The utilization and description has been treated in this section under the broad headings of burners, boilers and furnaces. However, the overlapping designs of plant burners and their atomizers, for different applications, should be recognized. For example, a given plant may constitute a burner and a boiler with a particular atomizer design (e.g. pressure jet) to generate heat and power for the refinery and petrochemical industry, or a burner, boiler and furnace may be combined, with the same atomizer, and used in the paper manufacturing industry. Furnaces can be used with a burner, incorporating essentially the same pressure jet atomizer, for iron making, kilns, reforming or cracking. Additionally, burners, with the same atomizers, can be utilized in processes for distilleries. In the “Developed World” natural gas is generally more readily available, it is relatively inexpensive, and is burnt more easily and cleanly than fuel oils. Thus oil firing is often a fall-back option whereby dual-fuel burners may operate with gas most of the time. In the Developing World, and the Third World, oil-firing is, however, often the most convenient option. Furthermore, in the latter areas, emissions legislation is generally weaker, making oil-firing an attractive option.

The share of continuous casting in the world of steel production is steadily increasing. An essential reason for the rapid advance of continuous casting methods in steel-making is the capability of producing slabs, blooms or billets, as a “strand” directly from the melt. This universal method offers great advantages over former casting and rolling of single ingots; advantages are: fewer phases of operation, higher output of raw steel and high productivity. However, technology and quality demands on steel grades are steadily increasing. A constantly growing range of high alloy steels must be cast without cracks whilst giving optimized metallurgical properties.

Metal spray forming, sometimes called spray casting or spray deposition, can be traced back to as early as 1910 and has been used for the production of shapes such as billets, tubes, or “near net shapes”, for various alloys. It is an alternative to conventional methods, including casting (see Section 7.1), ingot metallurgy, electrode remelting processes and powder metallurgy. The process involves spraying the molten metallic particles onto a previously prepared base material to form a coating, which is normally in a controlled atmosphere in a spray chamber and often with an inert gas such as nitrogen. The science behind this technology with its wide industrial applicability is still under investigation and the implementation of full production has not yet proved to be economically viable as a substitute for conventional methods. The application of the science and technology of sprays and atomization plays a pivotal role in the deposition of molten materials in spray forming, and this is reflected in the final product quality. In this section the various process applications of spray forming are briefly described, the principal concern being the properties of the sprays and nozzle (or atomizer) designs. Measurement techniques together with problems and future challenges involved in this fast moving new metal manufacturing technology are also described.