Regeneration of Steam

Steam Regeneration 

It is defined as, "The heating of feed water during its way to the steam boiler by extracting the steam (i.e. tapping from hot portion of cycle) from various points of turbine".

• Regenerator OR Feed Water Heater  ↹  device used for the purpose of regeneration.
• Around 5-8 FWH in practical powerplant (on must be Open type).

There are two types of Feed Water Heater:

1. Open Type Feed Water Heater
2. Closed Type Feed Water Heater

Regenerative Rankine Cycle with Two Open Type FWH

A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. 

• Reduces plant operating costs 
• Helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.

Closed Type Feed Water Heater

Closed feedwater heaters are typically shell and tube heat exchangers where the feedwater passes throughout the tubes and is heated by turbine extraction steam. 

• No physical mixing of steam and water (closed system).
• Extracted steam (from Turbine) transfers energy to FW and condensed.
• Condensation results in filling of container with water hinders (heat transfer) and lock incoming steam. To remove condensate, either:
• Move it to high pressure region using pumps
• Release it to condenser

• With respect to Efficiency, 
• Cascaded Backward (Energy decrease to Drain) < Pumped Forward (Energy increase due to Pump).
• Dead State  ↹  State at which system is in thermodynamic equilibrium with the environment.
• Exergy or Availability  ↹  useful work potential of the system at the specified state. We need exergy destruction to be minimum.
• Temperature Terminal Difference (TTD)  ↹  Difference of temperature between the bled steam entering FWH and the exit temperature of sub-cooled water in pipes.

Regenerative Rankine Cycle with Closed Type Feed Water Heater with Drains Cascaded Backwards

Closed type FWH's are mostly used for both before and after deaerator heater (has desuperheater drain cooling sections in higher pressure stages).

• Steam is generated from boiler, hit turbine blades extracted from 2 and 3.
• Extracted heat transfer its energy contents to water running through the pipe channels of (e.g. Shell and Tube Heat Exchanger).

Regenerative Rankine Cycle with Closed Type Feed Water Heater with Drains Pumped Forward

• One closed FWH with drain pumped forward  ↹  used as lowest pressure heater to pump all accumulating drains back into FW line.

Ques: Why we take actual entropy of a point let say '2a' towards increasing trend?

Ans: According to Law of Increase of Entropy, "the entropy of the system either remains constant or it increases (in adiabatic process)".

• For reversible process  ↹  entropy of system remains constant.

Ques: What is the effect of heat source fluid type?

Ans: Specific capacity of water = 4.2 KJ / kg . ℃ and air = 1.005 KJ / kg . ℃. Which means it is difficult to change the temperature of water.

Ques: Is superheating of fluid beneficial?

Ans: Consider 2 scenarios:

• In case of liquid (Higher Cp) i.e. less temperature difference  ↹  No superheating (a-1).
• In case of air (lower Cp) i.e. larger temperature difference  ↹  superheating is beneficial (a-1).
• Gives higher efficiencies (more heat is transferred)
• Lesser irreversibilities due to less temp. difference.

Ques: What are the parts of Feed Water Heater?

Ans: There are three parts, described below:

• De-superheating Zone  ↹  Which convert superheated steam to saturated vapor.
• Condensing Zone  ↹  Which convert Sat. vapor into Saturated Liquid.
• Sub-cooling Zone  ↹  If Further heat is required (Saturated liquid to subcooled liquid).

Note:

• For Steam generation  ↹  Boiler
• 50 MW Industrial Gas Turbine

References:

• Material from Class Lectures + Book named Power plant Engineering by P.K Nag (4th Edition) + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).