The Physical Aspect of Steam Generation at High Pressures and the Problem of Steam Contamination

Abstract

The constitution and the physical properties of boiler water are investigated with reference to the operating pressure. The process of evaporation at a submerged heating surface is examined and it is shown how nuclear boiling is the normal mode of evaporation, but that under certain conditions film boiling may take place. The size of the steam bubbles generated bears a definite relationship to the characteristics of the heating surface and the operating pressure. During migration from the heating surface to the free water level, the steam bubbles experience rapid growth at low pressures, but a negligible amount of growth at high pressures. The effect of bubble size on circulation is discussed. At high pressures the steam bubbles are very small and the presence of salts in the boiler water tends to inhibit their coalescence. Small steam bubbles throw up minute drops of water upon disruption into the steam space. The height to which the drops rise above the water level depends on the operating pressure and the rate of steam release; the drops become entrained with the steam at a certain critical rating, causing wet steam. Foaming is another cause of wet steam and is found to depend on the physical state of the suspended solids in the boiler water. Mechanical priming and methods of preventing steam contamination by moisture and solids are also discussed.