"Role of Feed Water Heaters in Regenerative Rankine Cycles"

🔥 What is a Feed Water Heater?

A Feed Water Heater is a heat exchanger that raises the temperature of boiler feedwater using extracted steam from the turbine. It is a key component in regenerative cycles, commonly found in thermal power plants, particularly those using the Rankine Cycle for power generation.

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⚙️ Why Preheat the Feed water?

1. Improves Efficiency:
   Preheating reduces the temperature difference between water and boiler, requiring less fuel energy to convert water into steam.

2. Thermodynamic Benefits:
   Increases cycle efficiency by reducing irreversibilities and improving the average temperature of heat addition (as per the Second Law of Thermodynamics).

3. Equipment Protection:
   Reduces thermal shock and corrosion inside the boiler by stabilizing water temperatures and removing dissolved gases.

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🔄 Working Principle (with Rankine Cycle Integration)

In a Regenerative Rankine Cycle, a portion of steam is extracted from between turbine stages and routed to feedwater heaters:

Steam Extraction:

• Some steam is bled from intermediate turbine stages.
• This steam has moderate enthalpy (neither full pressure nor completely expanded).

Heat Exchange:

• This extracted steam flows into feedwater heaters, transferring its thermal energy to incoming cold feedwater.
• The condensed steam (called drain) is collected and returned to the cycle (e.g., via a drain cooler or back to the condenser).

Pressurization & Reheating:

• The preheated feedwater is then pumped into the boiler, already at a higher temperature, thus reducing required fuel input.

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🧪 Thermodynamic Cycle Context

Regenerative Feed Heating in Rankine Cycle:

• Enthalpy of feedwater increases before entering the boiler.
• Overall specific steam consumption decreases.
• The net work output of the turbine slightly drops (due to extraction), but the fuel savings and thermal efficiency gain outweigh this.

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🏗️ Types of Feed Water Heaters

1. Open Feedwater Heater (Direct-Contact Type)

Feature	Details
Operation	Direct mixing of steam and feedwater
Structure	Tank with inlet for feedwater and extraction steam
Output	Saturated water at intermediate pressure
Advantage	Simple, high heat transfer
Limitation	Requires condensate and feedwater at same pressure

2. Closed Feedwater Heater (Indirect or Surface Type)

Feature	Details
Operation	Steam and feedwater remain separate
Structure	Shell-and-tube heat exchanger
Output	Water is preheated, steam condenses separately
Advantage	More flexible in pressure differences
Limitation	Slightly less efficient than open type

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📍 Placement in Power Plants

Typical Configuration:

• Low-Pressure Heaters: Between the condenser and the feed pump.
• High-Pressure Heaters: After the feed pump but before the boiler.
• Deaerator (open type): Often placed between LP and HP heaters to remove O₂/CO₂ gases.

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📉 Example Efficiency Gain

• A typical regenerative Rankine cycle with 6–8 feedwater heaters can improve efficiency by 5–10%.
• Each added heater offers diminishing returns, so there is a practical limit to how many are used.

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🛠️ Components in a Closed FWH:

• Tubes (for feedwater)
• Shell (for extraction steam)
• Drain Cooler (to recover heat from condensed steam)
• Vent and Drain Lines
• Level Controls and Safety Valves