Rankine cycle based power plants

Replies

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  Rohan_sK

  The Steam cant be condensed in the turbine blades. This is because the condensation will lead to formation of water droplets at high pressure and tempreature in the turbine. These droplets will corrode the blades which reduces their operational life and this is not desired economically.
  The corrosion also causes the blade surfaces to roughen spoiling the finish and the blade profile. This results in the disturbnace of the flow causing seperation of the flow, due to which the steam does not expand ON the blade surface completely as it would do otherwise. This leads to a pressure loss and reduction in the possible maximum use of steam enthalpy. This reduces the Work output and thus the efficiency which is not desired at all.
  On the other hand the efficiency can be increased by the following ways :
  
  1. By Superheating the steam to a higher tempreature in the superheated region at the same saturation pressure ie isobarically in the boiler. When the superheated steam enters the turbine it expands more thus producing greater work output and efficiency.
  
  2. By Regenerative feed cycle, whereby the steam is expanded in the high pressure turbine to a lower tempreature but to a saturated state. At this point some amount of the steam is extracted and this is called BLEED OFF steam. The rest of the steam is sent to a mini boiler and again REHEATED to a superheat degree. This re-superheated steam is further exapanded in a Low pressure turbine placed further inline. This causes more expansion of the steam extracting greater work from the steam. The efficiency of the cycle increases by this REHEAT procedure.
  As for the REGENERATIVE FEED, the Bleed off steam is mixed with the condensate before it enters the boiler after the pumping (Note that the steam is condensed after exhaust from the turbines, and the tempreature and pressure of the bleed steam is greater than that of the condensate even after pumping). This HEATS the condensate and raises its tempreature before entering the boiler, thus decreasing the external heat required in the boiler. This increases the efficiency of the cycle.
  The above process is actually called COMBINED REGENERATIVE FEED AND REHEAT CYCLE.
  The bleed steam is also utilised to heat the feed water in the pump externally without actual mixing of the fluids again resulting in teh increase in efficiency.
  The latent heat of the condensate cant be directly used in the cycle as condensation tempreature ids the lowest in the cycle. However we can use it to heat some secondary system externally.
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  gohm

  good info!
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  sagar3013

  Thanks a lot rohan for the valuable insight....
  
  "Droplets corroding the blades" yeah i completely agree with u... they do corrode and disturbe the system balance... also causing abnormal vibrations..... { hope u've come across stellite turbine blades --- which have higher resistance to corrosion and erosion}
  
  >== the case i have put forward deals with the end stages... where ther is no expansion and only condensation takes place..==<
  
  SUperheating -- yeah when we increase the energy content (enthalpy) in input steam, it is obvious we lose proportionately less through condenser (550 kcal/kg) -- hence the increase in efficiency.
  
  Regeneration and reheat --- if i remember we see these in the mega power plants and nuclear plants right..by regenerative feed -- i think u meant the steam extraction for deaeration, lp heating and HP heating of boiler feed water... they truly add to efficiency... the entire heat is utilized without the same being sent to the condenser to lose that 550 kcal/kg through the cooling tower....
  
  
  Well the latent heat, as u say, is at the lowest temperature in cycle. It may not be used in the power cycle. Big challenge..
  
  CHeers
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  Ernesha

  Inside a condensing steam turbine - the steam expands below the atmospheric pressure and then "condenses" while heating the cooling water in a condenser. ​

  After the steam exits the outlet of the condensing turbine, the steam's pressure is so low, it is no longer available for providing power for industrial applications.
  
  Condensing steam turbines could be used in industrial power plants as condensing tails connected to back-pressure turbines. In cases of low demand for process steam the steam surplus is run through the condensing tail to generate more power.
  
  A condensing steam turbine does not differ much from a back-pressure turbine in respect of its' overall dimensions, steam values (with the exception of outlet pressure), delivery time and price.
  
  The steam condensing equipment requires some additional "#-Link-Snipped-#" investments plus the availability of cooling water. ​

  A 1 MW condensing steam turbine plant needs about 0.1 m3/s of cooling water. ​

  What are #-Link-Snipped-#? ​

  #-Link-Snipped-# are a type of steam turbine that is used in connection with industrial processes where there is a need for low or medium pressure steam.
  
  The high pressure steam enters the #-Link-Snipped-# and while the steam expands - part of its thermal energy is converted into mechanical energy. The mechanical energy is used to run an electric generator or mechanical equipment, such as pumps, fans, compressors etc.
  
  The outlet steam leaves the #-Link-Snipped-# at "overpressure" and then the steam returns to the plant for #-Link-Snipped-# application such as heating or drying purposes.
  ​