1. Determining the Best
Heat Recovery System to
Maximize Boiler Efficiency

2. Overview
• Background
• Reduce facility operating costs
• Waste heat recovery
• Boiler Economizer
• Benefits
• Flow types
• Applications
• Designs
• Blowdown Heat Recovery
• Benefits
• Types
• Flash tank economizer
• Blowdown heat recovery unit
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3. Thermodynamic Laws
First Law
• The energy of the universe is
constant – it can be neither
created or destroyed, but only
transferred and transformed
• Heat and mechanical energy can
be converted one to another
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4. Thermodynamic Laws
Second Law
• Heat travels from hot to cold
THIS IS TODAY’S TOPIC
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5. Steam – Basic Concepts
Temperature-Heat Diagram For 1 lb
Of Water At Atmospheric Pressure (14.7 psia)
1 lb steam
at 212O F
Sensible Latent Sensible Latent
212º F
200º F
1 lb water
at 212O F
100º F
32º F 1 lb water
at 32O F
0º F
16 144 180 970
Btu per pound of water
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6. Design Considerations For Heat Transfer
Heat Transfer Coefficient
• Proportionality between heat flux/unit area and Delta T
LMTD
• Logarithmic average Delta T between hot and cold streams at each end of Hx
Reynolds #
• Ratio of inertia forces to viscous forces: <2000 = Laminar, >4000 = Turbulent
U Factor
• Rate of heat transfer over an area
Fouling Factor
• Depends on the foulent and temperature
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7. Heat Transfer
Typical Boiler Tube
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8. Heat Transfer
Cleaver-Brooks Advanced
Heat Transfer Tube
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9. Economizer Flow Types – Parallel
• Fluids flow in the same direction
• Hot side inlet temperature is in contact with the
cold side inlet temperature
• Delta T narrows
• Used in applications where minimum temperature is adequate
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10. Economizer Flow Types – Counter
• Fluids flow opposite direction and enter heat exchanger
at opposite ends
• Most efficient when comparing heat transfer rate per unit surface area
• Average T (difference in temperature) between the two fluids
over the length of the heat exchanger is maximized
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11. Economizer Flow Types – Cross
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12. Economizer Flow Types – Cross
• Fluids flow perpendicular to each other
• Can be most practical design for high
mass flows and space restrictions
• Still maintains high Delta T throughout the
exchanger
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13. HT Coefficient And High Reynolds
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14. Boiler Economizers
14

15. Economizer Design
Non-Condensing
• Most common design
• Recover sensible heat only
• Single stage
• Cross or counter flow
• Applications
• Steam boiler feedwater preheat
• Proportional feedwater control
• On/off feedwater control
• Process water (higher temperatures)
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16. Economizer Design
Condensing
• Recover sensible and latent
• Saturation temperature
• Natural gas ~135ºF
• Single stage
• Condense on natural gas only
• Applications – water temperature
less than ~120ºF
• Cold make-up water
• Process flow (lower temperature)
• Hot water return
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17. Economizer Design
2 Stage Condensing
• 2 Stages
• 1st stage – sensible heat recovery
• Standard (non-condensing)
economizer
• 2nd stage – sensible and latent heat
• Condensing economizer
• Condense on natural gas only
• Applications
• 1st stage
• Preheat boiler feedwater
(MFWV)
• On/off feedwater control
• 2nd stage
• Any cool liquid stream (50-120ºF)
• Make-up water
• Process water
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18. Economizer Applications
Steam Boiler
Feedwater Make-up water Process Water
Preheat
• Run feedwater from
a boiler feed system
or deaerator through
the economizer and
into the boiler
• Modulating boiler
feedwater control
• On/off feedwater
control
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19. Economizer Applications:
Steam Boiler Feedwater Preheat With MFWV
Non-condensing
Outlet
Inlet
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20. Economizer Applications:
On/Off Feedwater Control
Non-condensing
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21. Economizer Applications:
Make-Up Water
Condensing or Non-condensing
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22. Economizer Applications:
Process Flow
Condensing or Non-condensing
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23. Economizer Benefits
Increase Boiler Room
Efficiency By 3-8%
• Dependent on product design and application
Reduce NOx Emissions
Save Operating Costs
• Average total payback: 12-18 months
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24. Blowdown Heat Recovery Types:
Blowdown Heat Recovery Unit
• Proportional blowdown heat recovery system
• Atmospheric feed or deaerator
• Automatically controls TDS
• Single or multiple boilers
• Blowdown is cooled before discharge (code compliance)
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25. Blowdown Heat Recovery Types:
Flash Tank Economizer
• Recover blowdown energy
(latent and sensible heat)
• Use flash steam for LP use
• Blowdown is cooled before
discharge (per code)
• Used for single or multiple boilers
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26. Calculating Flash Steam %
HF1 – HF2 / HFG2 = % Flash
HF1: Sensible BTUs in
HF2: Sensible BTUs out
HFG2: Latent BTUs out
For example, a boiler operating at
200 psig with discharge @ 5 psig:
362 – 196 = 166
166 / 961 = 17% Flash
BTUs SAVED
Water and sewer charges SAVED
Chemicals SAVED
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27. Integration
• Reclaim heat loss
and reduce facility
operating costs
• Economizer
• BDHR
• TDS control
• Include O2 trim
• HTDB
• Parallel positioning
• VSD
• Integrated lead lag
• DA control
• Draft control
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28. Integrating The System
Deaerator
Bypass Damper
Water
Treatment
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29. Questions?