Flash Steam Generation - Fundamental Physics

If the pressure of condensate - the saturated water at the boiling point at the actual pressure - is reduced, the heat energy in the water is reduced to a level appropriate to the final pressure. The relation pressure and the boiling temperature can be found in steam tables.

The energy - or enthalpy - made available when pressure is reduced, will evaporate a part of the water, producing flash steam.

Only a part of the condensate evaporates as flash steam. How much depends on the enthalpy in the condensate at the initial and the final pressures.

The amount of flash steam produced during the pressure reduction can be calculated:

w = (h_il - h_fl) / h_fe                              (1)

where

w = ratio of flash steam generated (kg flash steam / kg condensate)

h_il = initial condensate enthalpy (kJ/kg, Btu/lb)

h_fl = final condensate enthalpy (kJ/kg, Btu/lb)

h_fe = enthalpy of evaporation of condensate at final condition (kJ/kg, Btu/lb)

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.  

Example - Flash Steam Generation

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.

• Steam table

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 %

• Flash Steam Generation - Imperial Units
• Energy Loss through Flash Steam Generation

Volume of Flash Steam

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).

Example - Volume of Flash Steam

By using the values from the example above we can calculate

- the volume of the condensate V_c

V_c = (1 - 0.11) (0.00104 m³/kg)

    = 0.00093 m³/kg

- the volume of the flash steam V_f

V_f = 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)

  = 198

Recovery of Flash Steam

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.