Shape

• Purity level, that is, residual gas levels

• Production costs

Gases have a lower heat-carrying capacity than water so the metal stream does not quench as rapidly in a gas-atomized system. The residence time of a liquid droplet in the atomizing chamber is longer in a gas system than a water atomizer. This additional time permits the driving forces to minimize the surface free energy by forming a sphere. Although this effect is somewhat alloy dependent, fine gas-atomized powders are generally more spherical than water-atomized products. Because most of the commercial fine gas-atomized powder systems are vacuum or atmosphere controlled, the typical values for oxygen and nitrogen are lower than water-atomized powders. For applications that do not reduce the soluble gas levels in process, this is a critical difference. The third difference is production cost. On a momentum transfer basis, water is significantly less expensive than gas. Water systems do not require the special plumbing associated with gases and their subsequent recovery. In addition, the elimination of a vacuum cycle allows water-atomized systems to be configured to have a shorter cycle time than gas-atomized systems.

Typical vacuum-gas systems include a 1 to 2 h vacuum cycle, a 1 to 2 h melting time, and a 1 to 2 h atomization time. Cooling time for the atomized powder is additional. A typical 500 kg water-atomized heat can turn around in melt and atomize in 2 h. With dual tundishes, it can be run on a semi-continuous basis.

All of these reductions in equipment mean that the capital costs and resultant depreciation are lower for water-based systems compared with gas atomizers. An excellent treatment of these variables has been performed by Dunkley (Ref 9). The remaining factors of labor, yield, and overhead tend to be producer specific.

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