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Steam

Steam can be best understood, by understanding the concept of atomic structure and molecular structure of matter.A molecule is the small amount of any element or compound and still possessing all the chemical properties of substance. A molecule themselves are made up of even smaller atoms. Example molecule of water is made from two atoms of hydrogen and one atom of oxygen. Since hydrogen and oxygen are plentiful in the atmosphere, thus water is made from atoms of hydrogen and oxygen are also plentiful. Carbon is the another element found in abundance. Many substances exist in more than one physical states i.e solid, liquid and in vapour phases. Example water exists in liquid, solid(ice) and steam. Phase or state depends on upon the molecular arrangement and its bounding. Molecule excitation is related to the physical state of the substance.

What is Triple Point?

Steam can be best understood, by understanding the concept of atomic structure and molecular structure of matter. A molecule is the small amount of any element or compound and still possessing all the chemical properties of substance. A molecule themselves are made up of even smaller atoms. Example molecule of water is made from two atoms of hydrogen and one atom of oxygen. Since hydrogen and oxygen are plentiful in the atmosphere, thus water is made from atoms of hydrogen and oxygen are also plentiful. Carbon is the another element found in abundance. Many substances exist in more than one physical states i.e solid, liquid and in vapour phases. Example water exists in liquid, solid (ice) and steam. Phase or state depends on upon the molecular arrangement and its bounding. Molecule excitation is related to the physical state of the substance.

What is Ice?

It is solid state, where the molecules are locked together in an orderly fashion and thus can’t move freely as the same is restricted, but can vibrate at its mean position. Heat application accelerates vibration and results in breaking-away of the same from neighbors. As a result, the solid state begins melting down to the liquid state. Melting occurs at O°C at atmospheric pressure with no effect of the change in pressure on it. Application of heat breaks the lattice bond to produce the phase change maintaining the temperature of the ice constant is called Enthalpy of melting or heat of fusion. This is a reversible phenomenon means freezing occurs when the same amount of heat released back to the surrounding. Usually, change of phase (from solid to liquid) density decreases but water are the exception to it and that the reason why ice floats on it.

What is Water?

In liquid phase molecules move freely from but still less than one molecular diameter apart due to mutual attraction and collision taking place. In liquid phase heat, addition increases the temperature of the liquid, as the molecular collision increases. Liquid Enthalpy or Sensible heat (hf) of water:- It is the heat that results in temperature change of water without changing its phase is liquid enthalpy or sensible heat of water.

What is Steam?

Water temperatures increase to its boiling point on the application of heat and at this state molecules having more KE (kinetic energy) temporarily break out from liquid surface and back. Subsequent heating causes much greater excitation and the number of molecules with enough energy to leave the liquid increases. At this stage, molecules leaving the liquid surface to surpass the returning molecules. This is called the boiling point of water which is equal to the saturation temperature of the water. Case-I : Adding more heat at constant pressure Initial state : Let the constant pressure be 2 bars having the available Enthalpy of water (hf) = 504.7 kJ/Kg Final state: After adding heat at constant pressure Enthalpy of saturated steam (hfg) = 2201.9 kJ/Kg Result: Addition of heat at constant pressure results in increasing the enthalpy of saturated steam to 2201.9 kJ/kg from that of 504.7 kJ/Kg, although the temperature of the boiling water and steam is same. Energy content in saturated steam is far greater than that of saturated water. Given below steam saturation curve, clearly showing the increase in steam saturation temperature of with the pressure increase. steam saturation curve Thus it can be concluded that:- Any further increase in temperature above saturation temperature is termed as superheating of steam. Applying the above concept to the boiler in the power plant: Suppose the steam going out of the boiler is in proportion to the rate at which it is being produced in the boiler, then heat addition will accelerate the steam production rate. On the other hand, if we control some of the steam going out of the boiler and then increase the heat input to the boiler, then energy flow into to the boiler is more than the energy flowing out of the boiler. This excess energy balance increases the boiler pressure and in turns the saturation temperature of the steam.

Enthalpy of Evaporation or Latent heat(hfg)

It is the quantum of Heat (energy) required to change water from one state to another i.e steam is called as Entahlpy of Evaporation by maintaining temperature as unchanged for the steam-water mixture. Total energy added during the process is completely utilised in changing the state of water to steam. This is the energy (Enthalpy of evaporation) which is recognized as the most useful form of energy, as it can be extracted back during condensation (steam to water).

Enthalpy of Saturated Steam or Total heat of Saturated Steam(hg)

Total energy or enthalpy of saturated steam is defined as the sum of evaporation enthalpy and water enthalpy. Enthalpy of Saturated Steam (hg) = hf +hfg Where, hg = Total saturated steam enthalpy (Total energy/heat)kJ/Kg hf = Enthalpy of Liquid (sensible heat) kJ/Kg hfg = Evaporation Entahlpy (Latent heat) in kJ/Kg Example: By using the steam table it can easily be established that major share of heat content is lies with saturated steam i.e (Enthalpy of evaportaion). As shown in the given below table out of the Total enthalpy of (2675.5 kJ/Kg), 84 % is enthalpy of evaporation and remaining 16 % contribution is of liquid enthalpy. table

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