The total heat content of the substance comprises three components namely, heat of water or Sensible Heat, heat of evaporation or Latent Heat and superheat – the additional heat imparted to the dry saturated steam to make it superheated called Super Heat. It is also know as enthalpy of water, enthalpy of super heated steam
Enthalpy of Water:
The amount of heat absorbed by one kilogram of water being heated from the freezing point (0 Degree C) to the boiling point is known as the Enthalpy of the Saturated Water (sensible heat of water). It is denoted by the symbol ‘h’.
Sensible heat of 1 kg of water at 0 Degree C is zero and sensible heat of 1 kg of water at 100 Degree C is 4.187 x 100 = 418.7 kJ/kg.
Enthalpy of Evaporation:
The enthalpy of evaporation (or Latent heat) is defined as the amount of heat required to convert one kilogram of water at the saturation temperature tsat corresponding to its pressure into steam at the same temperature and pressure. Latent heat varies with the pressure. Its value decrease with increase of pressure. Its value at 1 bar absolute pressure is 2258 kJ/kg. Value of Latent heat corresponding to any pressure can be directly obtained from the steam tables.
Enthalpy of Dry Saturated Steam:
It is the sum of Enthalpy of Water and Enthalpy of Evaporation (i.e., Latent Heat). It is defined as the quantity of heat required to raise the temperature of one kilogram of water from freezing point to the saturation temperature and then convert it into dry saturated steam at that temperature and pressure. It is denoted by the symbol ‘Hs’.
Thus, Hs = h + L
This value ‘Hs’ can be directly obtained from the Steam Tables corresponding to given value of pressure or temperature of Steam.
Enthalpy of wet steam:
It is the amount of heat required to raise the temperature of one kilogram of water from freezing point (00C) to the boiling point ‘tsat’ and then to convert the boiling water into wet steam. Obviously, such wet steam shall not have absorbed full quantity of Latent Heat, the content of which will be proportional to the fraction of Dry Saturated Steam in the Wet Steam.
Enthalpy of Water:
The amount of heat absorbed by one kilogram of water being heated from the freezing point (0 Degree C) to the boiling point is known as the Enthalpy of the Saturated Water (sensible heat of water). It is denoted by the symbol ‘h’.
Sensible heat of 1 kg of water at 0 Degree C is zero and sensible heat of 1 kg of water at 100 Degree C is 4.187 x 100 = 418.7 kJ/kg.
Enthalpy of Evaporation:
The enthalpy of evaporation (or Latent heat) is defined as the amount of heat required to convert one kilogram of water at the saturation temperature tsat corresponding to its pressure into steam at the same temperature and pressure. Latent heat varies with the pressure. Its value decrease with increase of pressure. Its value at 1 bar absolute pressure is 2258 kJ/kg. Value of Latent heat corresponding to any pressure can be directly obtained from the steam tables.
Enthalpy of Dry Saturated Steam:
It is the sum of Enthalpy of Water and Enthalpy of Evaporation (i.e., Latent Heat). It is defined as the quantity of heat required to raise the temperature of one kilogram of water from freezing point to the saturation temperature and then convert it into dry saturated steam at that temperature and pressure. It is denoted by the symbol ‘Hs’.
Thus, Hs = h + L
This value ‘Hs’ can be directly obtained from the Steam Tables corresponding to given value of pressure or temperature of Steam.
Enthalpy of wet steam:
It is the amount of heat required to raise the temperature of one kilogram of water from freezing point (00C) to the boiling point ‘tsat’ and then to convert the boiling water into wet steam. Obviously, such wet steam shall not have absorbed full quantity of Latent Heat, the content of which will be proportional to the fraction of Dry Saturated Steam in the Wet Steam.
If ‘x’ is the per unit content of Dry Saturated Steam in Wet Steam then,
Hwet = h + x.L kJ/kg.
The term ‘x’ is called Dryness Fraction and its value lies between 0 and 1. This is further explained below:
Wet steam is a mixture of water particle in suspension and dry steam. Dryness fraction of wet steam is a ratio of the mass of actual dry steam to mass of wet steam containing it.
If, ms= Mass of dry steam contained in the steam considered and
m = mass of water in suspension in the steam considered;
then, dryness fraction , x = ms/(ms + m)
Enthalpy of Superheated Steam:
It is defined as the amount of heat required to convert one kg of water at freezing temperature (0 Degree C) into superheated steam at a given pressure and temperature above saturation temperature. It is denoted by symbol Hsup.
Hsup = h + L + Cp (tsup – tsat) kJ/kg
or
Hsup = Hs + Cp (tsup – tsat) KJ/kg
Wet steam is a mixture of water particle in suspension and dry steam. Dryness fraction of wet steam is a ratio of the mass of actual dry steam to mass of wet steam containing it.
If, ms= Mass of dry steam contained in the steam considered and
m = mass of water in suspension in the steam considered;
then, dryness fraction , x = ms/(ms + m)
Enthalpy of Superheated Steam:
It is defined as the amount of heat required to convert one kg of water at freezing temperature (0 Degree C) into superheated steam at a given pressure and temperature above saturation temperature. It is denoted by symbol Hsup.
Hsup = h + L + Cp (tsup – tsat) kJ/kg
or
Hsup = Hs + Cp (tsup – tsat) KJ/kg
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