1) Atmospheric pressure
At atmospheric pressure - 0 bar gauge or absolute 101.33 kN/m2 - water boils at 100 oC. 419 kJ of energy is required to heat 1 kg of water from 0 oC to the saturation temperature 100 oC.
Therefore, at 0 bar gauge (absolute 101.33 kN/m2) and 100 oC - the specific enthalpy of water is 419 kJ/kg.
Another 2257 kJ of energy is required to evaporate the 1 kg of water at 100 oC to steam at 100 oC. Therefore, at 0 bar gauge (absolute 101.33 kN/m2) - the specific enthalpy of evaporation is 2257 kJ/kg.
The total specific enthalpy of the steam (or heat required to evaporate water to steam) at atmospheric pressure and 100 oC can be summarized as:
hs = 419 + 2257
= 2676 kJ/kg
= 2676 (kJ/kg) / 3600 (s/h) = 0.74 kWh/kg
(1 hour = 3600 seconds, 1 kW = 1 kJ/s)
Steam at atmospheric pressure is of limited practical use since it can not be conveyed by its own pressure along a steam pipe to the points of consumption.
At 7 bar gauge (absolute 800 kN/m2) - the saturation temperature of water is 170 oC. More heat energy is required to raise the temperature to the saturation point at 7 bar gauge than needed for water at atmospheric pressure. From the table a value of 720.9 kJ is needed to raise 1 kg of water from 0 oC to the saturation temperature 170 oC.
The heat energy (enthalpy of evaporation) needed at 7 bar gauge to evaporate the water to steam is actually less than the heat energy required at atmospheric pressure. The specific enthalpy of evaporation decrease with steam pressure increase. The evaporation heat is 2047 kJ/kg according the table.
Note! Because the specific volume of steam decreases with increasing pressure, the amount of heat energy transferred in the same volume actually increases with steam pressure. In other words the same pipe may transfer more energy with high pressure steam than with low pressure steam.
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