Flash Steam Formula - Metric


The percentage of flash steam (by weight) that will be formed when discharging hot condensate to a lower pressure is easily determined by using this formula. The sensible heat (heat of the saturated liquid) of the condensate at the lower pressure is subtracted from the sensible heat at the higher pressure. This number is then divided by the latent heat of the steam at the lower pressure and multiplied by 100 to get a percentage.

 

SH
=
Sensible heat in the condensate at higher pressure before discharge
SL
=
Sensible heat in the condensate at the lower pressure to which discharge takes place
H
=
Latent heat in the steam at the lower pressure to which the condensate has been discharged

 

Here is an example (to see this page using imperial values, click here). If hot condensate at 7 bar(g) is discharged to atmospheric pressure, the percentage of flash steam is determined as follows. (It may be helpful to have a copy of the steam tables available for reference.)

Subtract the sensible heat of the condensate at the lower pressure (417.5 kJ/kg) from the sensible heat of the condensate at the higher pressure (721.1 kJ/kg) and the answer is 303.6 kJ/kg. This figure is divided by the latent heat at the lower pressure (2258 kJ/kg) to yield .134 which, when multiplied by 100, provides an answer of 13.4 percent.