It is common that the final condition is atmospheric pressure 0 bar gauge (1 bar absolute) and temperature 100 oC. But closed condensate receivers with some pressure elevated condensate temperature are also used.
Condensate is produced inside an heat exchanger with steam pressure 5 bar gauge (6 bar absolute) on the heating surfaces. The condensate at 5 bar gauge (6 bar absolute) contains 670.9 kJ/kg of heat energy - the enthalpy - at saturation temperature 159 oC.
The pressure in the condensate is reduced to atmospheric pressure - 0 bar gauge (1 bar absolute) when passing through the steam trap. The maximum heat energy - enthalpy - in the water (condensate) at atmospheric pressure and 100 oC is 419.0 kJ/kg.
Evaporation energy of water at the final condition at atmospheric pressure is 2257.9 kJ/kg.
The flash steam generated can then be calculated:
w = ((670.9 kJ/kg)- (419.0 kJ/kg)) / (2257.9 kJ/kg)
= 0.11 (kg flash steam / kg condensate)
= 11 %
The volume of condensate as flash steam is much larger than the volume of condensate as condensate.
The specific volume of condensate - or water - at 100 oC (212 oF) is 0.00104 m3/kg (0.0167 ft3/lb). The specific volume of steam at atmospheric pressure is 1.67 m3/kg (26.8 ft3/lb).
By using the values from the example above we can calculate
- the volume of the condensate Vc
Vc = (1 - 0.11) (0.00104 m3/kg)
= 0.00093 m3/kg
- the volume of the flash steam Vf
Vf = 0.11 (1.67 m3/kg)
= 0.1837 m3/kg
This is a flash steam to condensate liquid rate of
(0.1837 m3/kg) / (0.00093 m3/kg)
Flash steam can be used to heat consumers with temperature demand lower than 100 oC (212 oF) - like HVAC systems, hot water service systems and similar.
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