3.3 Superheaters, Reheaters, Economizers & Air Heaters

3.3 Superheaters, Reheaters, Economizers & Air Heaters

Core Trade Concept: Modern high-pressure boilers do not stop at generating saturated steam. To achieve high thermodynamic efficiency and protect steam turbines from moisture droplet erosion, boilers incorporate heat recovery systems: superheaters and reheaters (which add sensible heat to steam), economizers (which capture residual flue gas heat to preheat incoming feedwater), and air heaters (which preheat combustion air).

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1. Thermodynamic Steam Cycle & Heat Transfer Principles

Converting subcooled feedwater into high-energy superheated steam occurs across three distinct thermodynamic heat absorption phases:

                  TEMPERATURE - ENTHALPY HEAT ADDITION DIAGRAM
                  
  Temp (^F) ^
            |                                 / [ SUPERHEATING ] (Superheated Steam)
            |                                /  Adds Sensible Heat (h_sup)
      T_sup +-------------------------------+ - - - - - - - - - - - - - - - -
            |             [ VAPORIZATION ]  |
      T_sat +-------------+=================+ (Dry Saturated Vapor, x = 1.0)
            |  [ SENSIBLE | Latent Heat (h_fg)
            |    HEATING  |
            |    OF WATER |
            |    (h_f)    |
            +-------------+-------------------------------------------------->
                                                               Enthalpy (Btu/lb)

1. Sensible Heat of Liquid ($h_f$): Heat absorbed to raise water temperature from its subcooled inlet state up to the saturation temperature ($T_{sat}$) corresponding to boiler operating pressure. No steam is generated during this stage ($1\text{ Btu}$ raises $1\text{ lb}$ of water by $1^\circ\text{F}$).
2. Latent Heat of Vaporization ($h_{fg}$): The massive quantity of heat required to change water from liquid at saturation temperature into dry saturated steam at the exact same temperature and pressure. As pressure increases, latent heat decreases until reaching zero at the critical point ($3{,}206.2\text{ psia}$, $705.1^\circ\text{F}$).
3. Superheat ($h_{sup}$): Sensible heat added to dry saturated steam after it leaves the steam drum, raising its temperature above $T_{sat}$. Superheated steam contains no liquid moisture.

Saturation Temperature vs. Pressure Relationships

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2. Superheaters: Types, Geometry & Characteristics

Superheaters are banks of high-alloy steel tubes (such as ASME SA-213 T11, T22, or T91) positioned in the hot flue gas path. They are categorized by heat transfer mode and physical arrangement:

               SUPERHEATER LOAD TEMPERATURE CHARACTERISTICS
               
    Steam Temp (^F) ^
                    |       /  CONVECTIVE (Rises with load)
                    |      /------------------------------ COMBINED (Flat)
                    |     / 
                    |    /   \ 
                    |   /     \  RADIANT (Droops with load)
                    +--------------------------------------------->
                    0%                     50%                 100% Boiler Load

Radiant vs. Convective Superheaters

• Radiant Superheater: Located directly in the furnace wall or roof exposed to open flame radiation. At low boiler loads, flame temperature is high relative to steam flow, yielding high steam temperature. As load increases, steam flow increases faster than radiant heat transfer, causing steam temperature to decrease (droop) with increasing load.
• Convective Superheater: Located in the gas convection pass shielded from direct flame radiation. As boiler load increases, the mass flow rate and velocity of hot flue gases increase, dramatically increasing convection heat transfer. Thus, steam temperature rises with increasing load.
• Combined Superheater: By routing steam through a radiant superheater followed in series by a convective superheater, the opposing drooping and rising characteristics cancel out, maintaining a flat, stable superheat temperature across a wide load profile ($25%\text{ to }100%$ load).

Pendant vs. Horizontal Superheater Tube Banks

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3. Steam Temperature Control & Attemperators (Desuperheaters)

To prevent over-temperature failure of superheater tubes and turbine components, the final superheat temperature must be controlled precisely ($\pm 5^\circ\text{F}$). This is accomplished using attemperators (also called desuperheaters).

                  SPRAY-WATER ATTEMPERATOR CONSTRUCTION
                  
  Superheated Steam ===> +----------------------------------+ ===> Controlled Temp
                         |   [ Internal Thermal Sleeve ]    |      Superheated Steam
                         |  +----------------------------+  |
                         |  |    ~~~~~~~~~~~~~~~~~~~~~   |  |
  High-Purity Polish  --->--+-> | * * Spray Nozzle * * | |--+
  Feedwater Injection    |  |    ~~~~~~~~~~~~~~~~~~~~~   |  |
                         |  +----------------------------+  |
                         +----------------------------------+
                         (Thermal sleeve prevents cold water from
                          impinging on hot header shell wall)

1. Direct-Contact Spray Attemperators: High-pressure polished boiler feedwater is injected through a spray nozzle / atomizing venturi directly into the superheated steam flow between the primary and secondary superheaters. The fine mist flashes instantly into steam, absorbing heat and lowering the bulk steam temperature.
   • Boilermaker Quality Standard: An internal thermal sleeve (protective liner) must be installed inside the attemperator pipe. The thermal sleeve prevents cold spray water droplets from directly striking the hot outer pressure header, preventing catastrophic cyclic thermal shock and fatigue cracking of the pressure boundary.
   • Water Purity: Spray water must be high-purity polished condensate or deaerated feedwater with zero solid chemicals (no sodium phosphate), as any non-volatile solids would carry straight into the steam turbine.
2. Surface Attemperators (Indirect): Superheated steam passes through a heat exchanger bundle submerged inside the boiler steam drum or mud drum, rejecting excess heat indirectly to boiler water without mixing fluid streams.

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4. Reheaters

In utility power plants, steam expands through the high-pressure (HP) turbine, dropping in pressure and temperature. To prevent moisture from condensing in the subsequent intermediate-pressure (IP) and low-pressure (LP) turbine stages, this steam is returned to the boiler through reheaters.

• Cold Reheat: Exhaust steam from the HP turbine (typically $500\text{--}650\text{ psig}$, $550\text{--}650^\circ\text{F}$) returns to the boiler reheater inlet header.
• Hot Reheat: The reheater heats the steam back up to $1{,}000\text{--}1{,}050^\circ\text{F}$ at relatively low pressure before discharging it to the IP turbine.
• Operating Rule (Pressure Drop): Because reheat steam is at lower pressure, reheater tubes have larger diameters and thinner walls than superheaters. Reheater circuits must be designed with extremely low pressure drop (typically $< 5\text{--}8%$ of inlet pressure); excessive pressure drop destroys the thermodynamic heat-rate gains of the Rankine cycle.
• Startup Protection: During initial boiler startup before the turbine rolls, no steam flows through the reheater. Burner firing rates must be strictly limited until turbine bypass systems establish cooling steam flow through reheater tube banks.

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5. Economizers & Air Heaters

                      BOILER FLUE GAS HEAT RECOVERY TRAIN
                      
    Furnace Flue Gas ===> [ SUPERHEATER / REHEATER ] (High Temp Heat Recovery)
                                      |
                                      v
                          [ ECONOMIZER TUBE BANK ]   (Preheats Feedwater)
                                      |
                                      v
                          [ AIR PREHEATER (APH) ]    (Preheats Combustion Air)
                                      |
                                      v
                         To Baghouse / FGD / Stack   (Flue Gas at ~280-320^F)

Economizers (Feedwater Preheating)

An economizer is a bank of serpentine tubes located in the low-temperature convective gas pass. It captures waste heat from the flue gas exiting the superheater zone to preheat boiler feedwater before it enters the steam drum.

• Efficiency Rule of Thumb: Every $10^\circ\text{F to }11^\circ\text{F}$ increase in feedwater temperature produced by the economizer increases overall boiler thermal efficiency by approximately $1%$ (correspondingly, every $40^\circ\text{F}$ drop in flue gas stack temperature increases efficiency by $1%$).
• Non-Steaming vs. Steaming Economizers:
  • Non-Steaming Economizer: The standard industrial design. Sized so exit water temperature remains $30^\circ\text{F to }50^\circ\text{F}$ below saturation temperature ($T_{sat}$). This prevents steam bubble formation, eliminating severe water hammer and vapor locking in economizer tubes.
  • Steaming Economizer: Used in select utility boilers where $5\text{--}20%$ of water is intentionally vaporized. Requires upward water flow and no downward hairpin bends where vapor locks could form.

Air Preheaters (APH)

Air preheaters recover remaining heat from flue gas leaving the economizer to preheat incoming combustion air to $400\text{--}600^\circ\text{F}$, dramatically improving combustion stability and furnace efficiency.

1. Tubular Recuperative Air Heaters: Stationary tube banks where hot flue gas passes through the inside of vertical tubes while ambient combustion air blows across the outside in multiple cross-flow passes. Zero cross-leakage between streams.
2. Regenerative Rotary Air Heaters (Ljungstrom / Rothemuhle): A slowly rotating ($1\text{--}3\text{ RPM}$) cylindrical wheel packed with corrugated sheet metal "baskets." The baskets absorb heat while rotating through the hot flue gas sector, then rotate into the incoming cold air sector to release the heat into the combustion air stream.
   • Radial and Axial Seals: Flexible alloy "feather" seals minimize air-to-gas leakage across the high-to-low pressure sectors.
   • Soot Blowers: Steam soot blowers must operate regularly to remove fly ash deposits from narrow basket corrugations, preventing basket plugging and catastrophic air heater fires.

Cold-End Acid Dew Point Corrosion

When fuels contain sulfur (coal, heavy fuel oil), sulfur burns to sulfur dioxide ($SO_2$), a small fraction ($1\text{--}5%$) of which oxidizes into sulfur trioxide ($SO_3$). In the flue gas, $SO_3$ combines with moisture ($H_2O$) to produce sulfuric acid vapor ($H_2SO_4$):

$\text{SO}_3 + \text{H}_2\text{O} \longrightarrow \text{H}_2\text{SO}_4$

• If economizer tube metal or air heater cold-end basket temperatures drop below the acid dew point (typically $240^\circ\text{F to }300^\circ\text{F}$ depending on fuel sulfur content), concentrated sulfuric acid condenses directly onto the metal, causing rapid perforation, tube thinning, and structural collapse.
• Safeguards: Deaerator temperature must maintain economizer inlet water above $250^\circ\text{F}$, and steam coil air preheaters (SCAPH) are installed ahead of regenerative air heaters to preheat cold winter ambient air before it contacts the rotating baskets.