Condensate Pipes - Flash Steam Generated

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 (h_il - h_fl ) / h_fe                             (1)

where

w_% = ratio of flash steam generated (%)

h_il = initial liquid enthalpy (kJ/kg)

h_fl = final liquid enthalpy (kJ/kg)

h_fe = enthalpy of evaporation (kJ/kg)

The condensate load can be calculated as

w_c = w_s w_% / 100                                          (2)

where

w_c = condensate load (kg/h)

w_s = 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 :

h_il = 781 kJ/kg, initial liquid enthalpy at 11 bar abs

h_fl = 418 kJ/kg, final liquid enthalpy at 1 bar abs

h_fe = 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

w_c = (200 kg/h) (16.1 %) / 100

= 32 kg/h