Determining the BestHeat Recovery System to Maximize Boiler Efficiency

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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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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Thermodynamic Laws

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Second Law• Heat travels from hot to cold

THIS IS TODAY’S TOPIC

212º F200º F

100º F

32º F

0º F16

Temperature-Heat Diagram For 1 lb

Btu per pound of water

Steam – Basic Concepts

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180

1 lb water at 212O F

970

1 lb steam at 212O F

144

1 lb water at 32O F

Sensible Latent Sensible Latent

Of Water At Atmospheric Pressure (14.7 psia)

Design Considerations For Heat Transfer

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• Proportionality between heat flux/unit area and Delta T

Heat Transfer Coefficient

• Logarithmic average Delta T between hot and cold streams at each end of Hx

LMTD

• Ratio of inertia forces to viscous forces: <2000 = Laminar, >4000 = Turbulent

Reynolds #

• Rate of heat transfer over an area

U Factor

• Depends on the foulent and temperature

Fouling Factor

Heat Transfer

Typical Boiler Tube

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Heat Transfer

Cleaver-Brooks Advanced Heat Transfer Tube

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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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Economizer Flow Types – Counter

• Fluids flow opposite direction and enter heat exchangerat 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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Economizer Flow Types – Cross

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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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HT Coefficient And High Reynolds

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Boiler Economizers

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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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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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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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Economizer Applications

Steam Boiler Feedwater

PreheatMake-up water Process Water

• 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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Economizer Applications:Steam Boiler Feedwater Preheat With MFWV

Inlet

Outlet

Non-condensing

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Economizer Applications:On/Off Feedwater Control

Non-condensing

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Economizer Applications:Make-Up Water

Condensing or Non-condensing

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Economizer Applications:Process Flow

Condensing or Non-condensing

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Economizer Benefits

•Dependent on product design and application

Increase Boiler Room Efficiency By 3-8%

Reduce NOx Emissions

•Average total payback: 12-18 months

Save Operating Costs

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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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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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Calculating Flash Steam %

For example, a boiler operating at 200 psig with discharge @ 5 psig:

362 – 196 = 166166 / 961 = 17% Flash

BTUs SAVEDWater and sewer charges SAVED

Chemicals SAVED

HF1 – HF2 / HFG2 = % Flash

HF1: Sensible BTUs inHF2: Sensible BTUs outHFG2: Latent BTUs out

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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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Integrating The System

Bypass DamperDeaerator

WaterTreatment

Questions?