Though contaminated condensate cannot be returned to the boiler feed tank, it is possible to recover the flash steam by having the appropriate equipments in place.
In certain processes, condensate often gets contaminated because either it is used in multi-media jackets, fear of leakage in heat exchanger coils, or it comes in contact with the product. In such circumstances the hot condensate gets drained.
Flash steam is generated when the condensate is moved from a high to a lower pressure. In order to make up for the difference in sensible heat, the excess heat in condensate is used to boil a part of the condensate back to steam. This constitutes flash steam.
In contaminated condensate the water content gets boiled to steam, thereby separating it from the contamination. Thus this flash steam is free of contamination and can be recovered. Though valid for most cases of contamination, careful evaluation maybe necessary if the condensate is contaminated with alcohols or other lighter fluids.
1 Kg of condensate at 4 bar g with a saturation temp of 152oC contains 152 kcal of heat. The condensate is discharged via a steam trap to atmosphere. But water at atmospheric pressure can only attain 99oC i.e. 99 kcal of heat. This means there is now an excess heat of 53oC or 53kcal.
As per laws of thermodynamics, energy can neither be created nor be destroyed. This excess heat is used in boiling a portion of condensate into flash steam, thereby maintaining the energy balance.
1 kg of steam at atmospheric pressure has a total energy content of 638kcal. So 53kcal will boil 120gms of the 1kg of condensate into flash steam.