Properties of Saturated Steam - Pressure in Bar

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Example - Boiling Water at 100 ^oC, 0 bar Atmospheric Pressure

At atmospheric pressure (0 bar g, absolute 1 bar ), water boils at 100 ^oC, and 417.51 kJ of energy are required to heat 1 kg of water from 0 ^oC to its evaporating temperature of 100 ^oC.

Therefore the specific enthalpy of water at 0 bar g (absolute 1 bar ) and 100 ^oC is 417.51 kJ/kg, as shown in the table.

Another 2257.92 kJ of energy are 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 evaporation is 2257.19 kJ/kg, as shown in the table.

Total specific enthalpy for steam:

h_s = (417.51 kJ/kg) + (2257.92 kJ/kg)

    = 2675.43 kJ/kg

Example - Boiling Water at 170 ^oC, 7 bar Atmospheric Pressure

Steam at atmospheric pressure is of a limited practical use because it cannot be conveyed under its own pressure along a steam pipe to the point of use.

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 would be needed if the water were at atmospheric pressure. The table gives a value of 720.94 kJ to raise 1 kg of water from 0 ^oC to its saturation temperature of 170 ^oC.

The heat energy (enthalpy of evaporation) needed by the water at 7 bar g to change it into steam is actually less than the heat energy required at atmospheric pressure. This is because the specific enthalpy of evaporation decreases as the steam pressure increases. The evaporation heat is 2046.53 kJ/kg according the table.

• Note! Because the specific volume also decreases with increasing pressure, the amount of heat energy transferred in the same volume actually increases with steam pressure.

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