![]() ![]() When feedwater enters the boiler, it enters between the heat-transfer surface and the surface of the boiling water. Under normal conditions, the steam bubbles tend to be cooled to saturation temperature as they rise through the water. Because of this higher pressure, the steam bubbles produced at the heat-transfer surface will either leave the boiler slightly superheated or be cooled to the saturation temperature of the water as it rises through the water. Because of the heat of water, the pressure at the heat-transfer surface is slightly higher than the pressure at the surface of the water. Steam bubbles are produced at the heat-transfer surface, rising through the water and then leaving the water surface to enter the steam system. In a simplified explanation of boiler operation, a hot heat-transfer surface is covered with water. While entrainment cannot be completely prevented, it can be minimized by proper boiler and steam system operation. Basically, both high- and low-efficiency boiler operation can produce - or not produce - excessive entrainment. This entrainment, while damaging to the steam system, is independent of boiler efficiency. Since steam is produced by the rapid boiling of water in high-heat flux boilers, it can entrain (or draw in and transport) water as it escapes from the water surface. This water hammer will gradually - and sometimes catastrophically - loosen pipe fittings and supports. This can lead to erosion, vibration, and water hammer. And if a puddle of water is allowed to accumulate in steam pipes, it will eventually be picked up by the high-velocity steam and accelerated to near-steam velocity, increasing chances of it crashing into elbows, tees, and valves. They can erode pipe fittings and rapidly eat away at valve seats. (For example, steam at 100% quality contains no liquid water and appears as a 100% clear gas, while steam at 90% quality contains 90% steam by weight and 10% water by weight in the form of a fog, cloud, or droplets.) Water droplets in high-velocity steam can be as abrasive as sand particles. Steam quality is a measure of the amount of liquid water contaminating the steam. These problems are most frequently caused by low steam quality, often called "wet steam" or "carry-over." Specific problems can include frequent boiler shutdowns from low-water level, damaged steam pipes and valves due to water hammer, vibration, corrosion, erosion, reduced capacity of steam heaters, and overloaded steam traps. ![]() In spite of these advantages, many steam users experience system safety problems, premature equipment failures, and poor steam system efficiency. Compared to other heat delivery and distribution systems, steam is less expensive to operate and is 100% recyclable. Steam can also be delivered to users with conventional piping and valve equipment that is inexpensive, is readily available, requires little maintenance, and has a long service life. Other advantages include the safe, nontoxic and nonflammable characteristics of steam plus its ability to deliver heat at a constant, controlled temperature. With a latent heat of vaporization (or condensation) as high as 1,000 BTU per pound, it takes very little steam to carry a large amount of energy. (6 printed pages)īoiler efficiency measures how much combustion energy is converted into steam energy, while steam quality measures how much liquid water is present in the steam produced.Ī major benefit of using steam as a heat transfer medium is the large amount of heat released when it condenses into water. Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. Boiler Efficiency and Steam Quality: The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies ![]()
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