Performance Analysis of Power Plant Condensers P M V Subbarao Professor Mechanical Engineering Department I I T Delhi A Device Which makes Power Plant A True Cycle.. A Device Which set the limit on minimum cycle pressure….. T-S Diagram : Rankine Cycle with FWHs. pturbine ,exhaust phood pcond,in pcond,exit ? TTD ? TCW ,out TCW ,in Two-Pass Surface Condenser T-S Diagram : Rankine Cycle with FWHs. pturbine ,exhaust phood TTD ? pcond psat .steam ? TCW ,out TCW ,in Thermal Processes Occurring in Condensers • The condenser never receives pure seam from the turbine. • A mixture of steam and non-condensable gases (Air-steam mixture) enters the condenser. • The ratio of the quantity of gas that enters the condenser to the quantity of steam is called the relative air content. m air m c,s •The value of , depends on type, capacity, load and design dimensions of the condenser plant. Variation of Steam-air Mixture Parameters mc , s m air pc ,in p pi ,steam Tsat pc ,in p pi ,steam p pi ,air pc ,exit p pe,steam p pe,air p pe,steam exit p pe,air Tc,exit Te,steam • Using Dalon’s Law: pc ps pa Gas laws: pa va ma RaTa & ps vs ms RsTs Volumes and temperatures are same. ma RaTa ms RsTs va vs pa ps pa Ra m a 0.622 ps Rs m s pc ps 1 0.622 • At the entry to condenser the relative content of air is very low and partial pressure of steam is almost equal to condenser pressure. • As air-steam mixture moves in the condenser, steam is condensed and the relative content of air increases. • Accordingly, the partial pressure of steam drops down. • The total pressure in the bottom portion of condenser is lower than that of the top portion. • The pressure drop from inlet to exit of condenser is called steam exhaust resistance of a condenser. pc pc ,in pc ,exit • The partial pressure of air at the bottom of the condenser cannot be neglected. pc ,exit p pe,steam p pe,air pc,in p pe,steam p pe,air pc • The temperature of steam is a function of condenser pressure. • As the air-steam mixture moves through the condenser and the steam is condensed, its temperature decreases owing to decreasing partial pressure of saturated steam. • This is due to increase in relative content of air in the mixture. • The pressure also decreases due to resistance to flow of steam. • The zone of intensive condensation. • The zone of cooling of air-steam mixture. Variation of Steam Partial Pressure & Saturation Temperature 70 60 50 Saturation Temperature, 0C Tcw,out 40 30 Tcw,in 20 Steam Partial Pressure, kPa 10 0 1 2 3 4 5 6 Inlet exit Condenser with Air Removal Pump : A practicable Concept TCW ,in Tamb TCW ,exit pcondenser ma psat 1 0.622 ms Combined Condensation & Air Pumping Action: m a : Slowly decreases & m s : Continuosly decreases to zero. Can be controlled to a finite value. Air Pumping Action Vs Condensing Action !!! Effect of Air Leakage on Condenser Pressure Inlet Condenser Pressure, mm of Hg Air pump controlling the back pressure Condenser controlling the back pressure Air pump controlling the back pressure Condenser controlling the back pressure 10 30 40 50 20 Cooling water Inlet Temperature Power Loss Due to Excess Back Pressure What is Techno-economically Viable value of TTD ? Condensate Depression • The temperature of condensate is always a few degrees lower than the coincident condensing steam temperature. • Subcooling of condensate is undesirable on two accouts: • It lowers the thermodynamic efficiency of the power cycle. • It enhances the propensity of the condensate to reabsorb non-condensibles.
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