With the help of a T-h diagram, Explain the various stages in the formation of Steam.

1 b]With the help of a T-h diagram, Explain the various stages in the formation of Steam.

Steam, which is gaseous form of pure water, is an excellent working medium in various thermodynamic systems because of its following properties

1) It can carry large quantities of heat
2) It is produced from water which is cheap and readily available
3) It can be used for heating purposes after its duty as working agent is completed.
4) It can be used purely as a heating medium in food processing Industries because of a fast, easily controllable and hygienic method of heating.

Formation of Steam:

In general, steam can be formed by boiling water in a vessel. But to use it effectively as a working or heating medium, it has to produce in a closed vessel under pressure. Steam formed at a higher pressure has higher temperature and can be made to flow easily through insulated pipes from steam generator to point of use. A simple arrangement of formation of steam at constant pressure is shown in figure

Explain the various stages in the formation of Steam
Figure: T-h diagram for Steam formation at constant pressure
  • A = Sensible Heat taken by Ice
  • B= Latent Heat of Fusion
  • C = Sensible Heat taken by Water
  • D = Latent Heat of evaporation
  • E = Sensible Heat taken by Steam
  • hw = Specific enthalpy of water
  • hf = Specific enthalpy of saturated water
  • hfg = Latent heat of evaporation
  • hg = Specific enthalpy of dry saturated steam
  • hsup = Specific enthalpy of super-heated steam
  1. Consider 1 kg of ice at temperature -100C which is below the freezing point. Let it be heated at constant pressure P. The temperature of ice starts increasing until it reaches the melting temperature of ice i.e., 00C and during this course ice absorbs its sensible heat. On further addition of heat, ice starts melting, its temperature remains constant at 00C and it absorbs latent heat of fusion and converts completely into water at 00C.
  2. On further addition of heat, the temperature of water starts rising until it reaches the boiling temperature or saturation temperature corresponding to pressure P. This heat absorbed by water in sensible heat.
    Note: Saturation temperature or boiling temperature increases with increase in pressure
  3. After the boiling temperature is reached, it remains constant with further addition of heat and vaporization take place. The water absorbs its latent heat and converts into dry saturated steam remaining at same saturation temperature. The intermediate stage of water and dry saturated steam is wet steam, which is actually a mixture of steam and water.
  4. If further the heat is added, the temperature of this dry saturated steam starts rising from saturation temperature and it converts into superheated steam. This heat absorbed is again the sensible heat. The total rise in temperature of superheated steam above the saturation temperature is called degree of superheat. We must know here that the saturation temperature, latent heat and other properties of steam remain same at constant pressure but varies with the variation of pressure.

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