Steam Volume, Steam Quality, Superheated Steam

Steam Volume- How is it important in steam plant?

If 1 kg (mass) of water (which is 1 liter, by volume) is all converted into steam, the result will be exactly 1kg (mass) of steam. However, the volume occupied by a given mass depends upon its pressure. At atmospheric pressure 1kg of steam occupies nearly 1.673 cubic meters(m3). At a pressure of 1bar abs, that same 1 kg of steam will only occupy 0.1943 m3. Thus, steam should always be generated and distributed at rated boiler pressure and used at possible low pressure.The volume of 1kg of steam at any given pressure is termed its Specific Volume (symbol Vg).

Steam Quality

In practice, steam often carries tiny droplets of water with it and cannot be described as dry saturated steam. Nevertheless, we find that it is usually important that the steam used for process or heating is as dry as possible.

Steam quality is described by it’s "dryness fraction" - the proportion of completely dry steam present in the steam being considered. The steam becomes "wet" if water droplets in suspension are present in the steam space, carrying no specific enthalpy of evaporation. "Wet steam" has a heat content substantially lower than that of dry saturated steam at the same pressure. The small droplets of water in wet steam have weight but occupy negligible space. The volume of wet steam is, therefore, less than that of dry saturated steam.

Volume of Wet Steam = Volume of Dry Saturated Steam * Dryness Fraction

Dryness fraction of the steam depends upon the steam boiler design and capacity. For example, Coil type low capacity boilers produce about 30 to 50 % wet steam which is not desired in steam heating applications.

Superheated Steam

As long as water is present, the temperature of saturated steam will correspond to the figure indicated for that pressure in the Steam Tables. However, if heat transfer continues after all the water has been evaporated, the steam temperature will again rise. The steam is then called "superheated", and this "superheated steam" can be at any temperature above that of saturated steam at the corresponding pressure. Superheated Steam is totally dry and follows the gas laws.

Saturated steam will condense very readily on any surface which is at a lower temperature and gives up the enthalpy of evaporation which, as we have seen, is the greater proportion of its energy content. On the other hand, when superheated steam gives up some of its enthalpy, it does so by virtue of a fall in temperature. No condensation will occur until the saturation temperature has been reached, and it is found that the rate at which we can get energy to flow from superheated steam is often less than we can achieve with saturated steam. Even though the superheated steam is at a higher temperature, superheated steam because of its properties, is the natural first choice for power steam requirements, whilst saturated steam is ideal for process and heating applications.

Steam Generation

The chemical energy, which is contained in coal, gas or other boiler fuel, is converted into heat energy when the fuel is burned. That heat energy is transmitted through the wall of the boiler furnace to the water. The temperature of the water is raised by this addition of heat energy until saturation point is reached – it boils. The heat energy which has been added and which has had the effect of raising the temperature of the water is known as the liquid enthalpy.

At that point of boiling, the water is termed Saturated Water. The water in our boiler is now at saturation (boiling) point at 100 °C. Heat transfer is still taking place between the furnace walls and the water. The additional enthalpy produced by this heat transfer does not increase the temperature of the water. It evaporates the water, which changes its state into steam. The enthalpy that produces this change of state without change of temperature is known as the enthalpy of evaporation.




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