1)1 bar abs = 0 bar gauge = 100 kPa abs = atmospheric pressure
Vacuum steam is the general term used for saturated steam at temperatures below 100°C.
Example - Boiling Water at 100 oC, 0 bar(100 kPa) Atmospheric Pressure
At atmospheric pressure (0 bar g, absolute 1 bar ) water boils at 100 oC and 417.51 kJ of energy is required to heat 1 kg of water from 0 oC to evaporating temperature 100 oC.
Therefore the specific enthalpy of water at 0 bar g (absolute 1 bar ) and 100 oC is 417.51 kJ/kg.
Another 2257.92 kJ of energy is required to evaporate 1 kg of water at 100 oC into 1 kg of steam at 100 oC. Therefore at 0 bar g (absolute 1 bar) the specific enthalpy of vaporation is 2257.19 kJ/kg.
The total specific enthalpy for steam at 0 bar gauge is:
hs = (417.51 kJ/kg) + (2257.92 kJ/kg)
= 2675.43 kJ/kg
Example - Boiling Water at 170 oC and 7 bar (700 kPa) Atmospheric Pressure
Steam at atmospheric pressure is of a limited practical use because it cannot be conveyed by its own pressure along a steam pipe to the points of use. In a steam distribution system the pressure is always more than 0 bar gauge.
At 7 bar g (absolute 8 bar) the saturation temperature of water is 170.42 oC. More heat energy is required to raise its temperature to saturation point at 7 bar g than needed when the water is at atmospheric pressure. According the table 720.94 kJ is required to raise 1 kg of water from 0 oC to saturation temperature 170 oC.
The heat energy (enthalpy of evaporation) needed at 7 bar g to vaporize the water to steam is actually less than required at atmospheric pressure. The specific enthalpy of vaporization decreases with steam pressure. The evaporation heat is 2046.53 kJ/kg at 7 bar g.
Note! The specific volume of steam decreases with increased pressure - and the amount of heat energy distributed by the same volume increase. With higher pressure - more energy can be transferred in a steam distribution system.
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