Flash steam is produced when high pressure condensate is discharged to a lower pressure. The word ‘flash’ describes the way it is formed. At atmospheric pressure, water boils at 100 Deg. C. Inside steam piping or any pressurized vessel, steam is generally utilised at a pressure above the atmospheric pressure.
When this steam loses its heat either by transferring it to the product being heated or by radiation loss to the environment, condensate is formed. This condensate which is formed, is also at same pressure and temperature as that of steam.
When this pressurised condensate is exposed to atmospheric pressure, it has energy more than it can contain at atmospheric pressure. This excess energy is used to convert a portion of this condensate into steam. This phenomenon is called as flashing and the steam so generated is referred to as flash steam.
Eg. Consider a process where steam is used at a pressure of 6 bar. At this pressure, the enthalpy of steam would be 659.33 Kcal/Kg. After losing its latent heat to the product being heated, condensate will be formed at same pressure with an enthalpy of 160.53 Kcal/Kg.
Let us assume, after the trap, this condensate is discharged to a pressure of 1 bar. As per the steam tables, water in liquid form can contain 99.76 Kcal/Kg of enthalpy. As it can be seen, there is 60.77 Kcal/Kg of excess energy. This energy will convert a part of the liquid condensate into vapour.
Calculating quantity of flash steam formed
The standard formula to calculate quantity of flash steam generated is as follows-
Hf1= Sensible heat present in the condensate at higher pressure.
Hf2= Sensible heat present in the condensate at the lower pressure.
Hfg2= Latent heat of steam formation at lower pressure.