This mixture of water in the liquid state tiny droplets and gaseous state steam is called wet steam. Steam played a vital role in the industrial revolution.
The modernization of the steam engine in the early 18th century led to major breakthroughs such as the invention of the steam locomotive and the steamboat, not to mention the steam furnace and the steam hammer. The latter is not a reference to water hammer found in steam piping, but rather to a steam-powered hammer used to shape forgings. Nowadays, however, internal combustion engines and electricity have often replaced steam as a power source. Even so, steam is still being widely used in electrical power plants and for some large scale industrial applications.
Steam is now mostly known for its heating applications, as both a source of direct and indirect heat. The direct steam heating method refers to processes where steam is in direct contact with the product being heated.
The example below shows Chinese dumplings being steamed. A steaming basket is placed over a pot of boiling water. As the water boils, steam rises into the basket and cooks the food. In this setup, the boiler pot and steaming vessel basket are combined together. The principle behind steaming food is that by allowing steam to come in direct contact with the product being heated, the latent heat of steam can be directly transferred to the food, and the water droplets formed through condensation can supply moisture.
In industry, the direct steam heating method is often used for cooking, sterilization, steam smothering, vulcanization and other processes.
Cogeneration plants create steam in a boiler much like other power plants, but the leftover steam can be transferred to nearby buildings for heating instead of the usual condensing done in its Rankine cycle. New York has a few of these plants which contribute to its tremendous steam heating system. Fossil Fuels. Nuclear Fuels. Acid Rain. Climate Change. Thanks for reading Scientific American. Create your free account or Sign in to continue.
See Subscription Options. Discover World-Changing Science. Get smart. The gaseous state is the mostdiffuse state in which the molecules have an almost unrestricted motion, and the volume increases without limit as the pressure is reduced.
The critical point is the highest temperature at which water can exist. Any compression at constant temperature above the critical point will not produce a phase change. Compression at constant temperature below the critical point however, will result in liquefaction of the vapour as it passes from the superheated region into the wet steam region.
The critical point occurs at Above this pressure the steamis termed supercritical and no well-defined boiling point applies. How can steam be formed from water without adding heat? Flash steam occurs whenever water at high pressure and a temperature higher than the saturation temperature of the low-pressure liquid is allowed to drop to a lower pressure.
Conversely, if the temperature of the high-pressure water is lower than the saturation temperature at the lower pressure, flash steam cannot be formed. In the case of condensate passing through a steam trap, it is usually the case that the upstream temperature is high enough to form flash steam. The amount of energy in one kilogram of condensate at saturation temperature at 5 bar g is kJ. In accordance with the first law of thermodynamics, the amount of energy contained in the fluid on the low-pressure side of the steam trap must equal that on the high-pressure side, and constitutes the principle of conservation of energy.
Consequently, the heat contained in one kilogram of low-pressure fluid is also kJ. This excess heat boils some of the condensate into what is known as flash steam and the boiling process is called flashing. Therefore, the one kilogram of condensate which existed as one kilogram of liquid water on the high pressure side of the steam trap now partly exists as both water and steam on the low-pressure side. The amount of flash steam produced at the final pressure P2 can be determined using Equation 2.
The case where the high pressure condensate temperature is higher than the low pressure saturation temperature. The proportion of flash steam produced can be thought of as the ratio of the excess energy to the enthalpy of evaporation at the final pressure. The case where the high pressure condensate temperature is lower than the low pressure saturation temperature.
Consider the same conditions as in Example 2. As this enthalpy is less than the enthalpy of one kilogram of saturated water at atmospheric pressure kJ , there is no excess heat available to produce flash steam.
The condensate simply passes through the trap and remains in a liquid state at the same temperature but lower pressure, atmospheric pressure in this case.
Should the lower condensate pressure have been less than this, flash steam would have been produced. The principles of the conservation of energy and mass allow the flash steam phenomenon to be thought of from a different direction. Consider the conditions in Example 2. This can be illustrated schematically in Figure 2.
The total mass of flash and condensate remains at 1 kg. The principle of energy conservation states that the total energy in the lower-pressure state must equal the total energy in the higher-pressure state. Therefore, the amount of heat in the flash steam and condensate must equal that in the initial condensate of kJ. Therefore, according to the steam tables, the enthalpy expected in the lower-pressure state is the same as that in the higher-pressure state, thus proving the principle of conservation of energy.
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