What Does Dry Steam Mean?
Dry steam is steam that is at the temperature of saturation, but does not contain water particles in suspension. It has a very high dryness fraction, with almost no moisture.
Commercially, dry steam contains not more than one half of one percent moisture. The presence of moisture in steam causes deposition, corrosion and reduction of life expectancy of boilers or other heat exchangers. Therefore, in heating applications, dry steam is preferable, because it has a better energy exchange capacity and does not cause corrosion.
Corrosionpedia Explains Dry Steam
Dry steam is saturated steam that has been very slightly superheated. It results when water is heated to the boiling point and then vaporized with additional heat. If this steam is then further heated above the saturation point, it becomes superheated steam. In dry steam, all the heat which is present is used to transform the water into steam; therefore no micro drops are present.
When steam is produced in a boiler or steam generator it usually has some moisture present, which comes from the water from which it is generated. The presence of any such moisture is sufficient to render the steam "wet." The clean/pure steam generators used in the pharmaceutical industry are designed to produce "dry" steam, a clear, colorless vapor with no entrained moisture.
Steam dryness has a direct effect on the total amount of transferable energy contained within the steam, which affects heating efficiency and quality. The steam dryness fraction is used to quantify the amount of water within steam. If steam contains 10% water by mass, it's said to be 90% dry, or have a dryness fraction of 0.9.
Moisture in steam increases the amount of corrosion. Moisture traveling at high steam velocities erodes valve seats and fittings, a condition known as wiredrawing. It also causes erratic operation of control valves. Pure steam cannot carry any impurities, but water can. These impurities increase scaling of pipework and heating surfaces.