Condensate Lines - Flash Steam
Flash steam produced in condensate pipe lines
In general condensate lines should be designed to handle start-up conditions with huge amounts of condensate generated from cold steam pipes and cold heating equipment. As a rule of thumb - the maximum condensate load at start up is often twice the maximum steam load during operation.
Sometimes, especially in high pressure steam systems with heavy load heat exchangers, generated flash steam may be the limitation of the condensate lines.
Flash steam generated can be calculated as
w% = 100 (hil - hfl) / hfe (1)
where
w% = ratio of flash steam generated (%)
hil = initial liquid enthalpy (kJ/kg)
hfl = final liquid enthalpy (kJ/kg)
hfe = enthalpy of evaporation (kJ/kg)
The condensate load can be calculated as
wc = ws w% / 100 (2)
where
wc = condensate load (kg/h)
ws = steam flow rate (kg/h)
Example - Generated Flash Steam
The gauge pressure in a heat exchanger before the steam trap is 10 bar. The condensate lines and condensate receiver are vented so the gauge pressure is equal to 0 bar (atmospheric).
From the steam properties table:
hil = 781 kJ/kg, initial liquid enthalpy at 11 bar abs
hfl = 418 kJ/kg, final liquid enthalpy at 1 bar abs
hfe = 2258 kJ/kg, enthalpy of evaporation at 1 bar abs
The flash steam generated can be calculated as
w% = 100 ((781 kJ/kg) - (418 kJ/kg)) / (2258 kJ/kg)
= 16.1 %
If the steam flow rate through the heat exchanger is 200 kg/h - the condensate load can be calculated as
wc = (200 kg/h) (16.1 %) / 100
= 32 kg/h