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8/12/2019 Heat Exchangers & Waste Heat Recovery
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HEAT EXCHANGERS&
WASTE HEAT RECOVERYBY
R. SAHAYFOUNDER & CEO
eQube
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Waste
Introduction
Type of waste heat recovery
Assessment of waste heat recovery
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Introduction
Dumped heat that can still be reused
Value (quality) more important thanquantity
Waste heat recovery saves fuel
What is Waste Heat?
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Introduction
Source and Quality Table: Waste heat source and qualityS. No Source of Waste Heat Quality of Waste Heat
1 Heat in fl ue gases The higher the temperature, the greater thepotential value for heat r ecovery
2 Heat in vapour streams As above but when con densed, lat ent heat
also r ecoverable 3 Convective & radiant heat lo st
from exterior of equipment Low grade if collected may be used forspace heating or air preheats
4 Heat losses in cool ing w ater Low grade useful gains if heat is exchangedwith incoming fresh w ater
5 Heat losses in pro vidingchill ed water or in thedispo sal of chilled water
1.High grade if it can be utili zed to reducedemand for refrigeration 2.Low gr ade if refrigeration unit used as aform o f Heat pump
6 Heat stored in prod uctsleaving the process
Quality depends up on temperature
7 Heat in gaseous & liq uideffluents leaving process
Poor if heavily contaminated & thus requiri ngalloy heat exchanger
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IntroductionHigh Temperature Heat Recovery
Table: Typical waste heat temperature at high temperaturerange from various sources
Types of Devices Temperature ( 0C) Nickel refining f urnace 1370 1650
Alu mini um ref in in g furnace 650 760
Zinc refin ing furnace 760 1100
Copper refinin g furnace 760 815
Steel heating fu rnace 925 1050
Copper reverberatory fu rnace 900 1100
Open h earth furnace 650 700
Cement kiln (Dry process) 620 730
Glass melting f urnace 1000 1550
Hydrogen plants 650 1000
Solid waste incinerators 650 1000
Fume incinerators 650 1450
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Introduction
Medium Temperature HeatRecovery Table: Typical waste heat temperature at medium temperature rangefrom various sources
Types of Devices Temperature ( 0C) Steam boiler exhaust 230 480
Gas tu rbine exhaust 370 540
Recipro cating engin e exhaust 315 600
Recipro cating engine exhaust (turbocharged)
230 370
Heat treatment furnace 425 650
Drying & baking ovens 230 600
Catalytic crackers 425 650
Anneal in g fu rn ace co ol in g syst ems 425 650
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Introduction
Low Temperature Heat Recovery Source Temperature 0C
Process steam condensate 55-88
Cooling water from: Furnace doors 32-55
Bearings 32-88
Welding machines 32-88
Injection molding machines 32-88
Anneal in g fu rnaces 66-230
Forming dies 27-88
Air c ompressor s 27-50
Pumps 27-88
Internal combust ion engines 66-120
Air c ondi tion in g and refr ig erat ion con densers 3243
Liquid still condensers 32-88
Drying, baking and curin g ovens 93-230
Hot processed liqui ds 32-232
Hot processed solid s 93-232 T a
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Type of Waste Heat Recovery
Commercial Waste Heat Recovery
Recuperators Heat exchange betweenflue gases and the air
through metallic/ceramicwalls
Ducts/tubes carrycombustion air forpreheating
Waste heat stream onother side
Inlet air f romatmosphere
Outsideducting
Tune plate
Preheatedair
Centre tube plate
Exhaust gasfrom process
Figure 1 : Waste heat recoveryusing recuperator, Source: SEAV
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Type of Waste Heat Recovery
Commercial WasteHeat Recovery Metallic radiationrecuperators
F i g u r e 2
. M e t a l l i c R a d i a t i o n R e c u p e r a t o r
( H a r d t e c
h G r o u p
)
Simplest recuperator
Two metal tubes
Less fuel is burned per
furnace load Heat transfer mosly by
radiation
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Type of Waste Heat RecoveryCommercial Waste Heat Recovery
Convectiverecuperators
Figure 3. Convective Recuperator
Hot gas throughparallel small diametertubes
Tubes can be baffledto allow gas to passover them again
Baffling increases heatexchange but moreexpensive exchangeris needed
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Type of Waste Heat Recovery
Commercial Waste Heat Recovery Radiation/convectivehybrid recuperators
Figure 4. Hybrid Recuperator
Combinations of radiation &convection
More effective heat transfer
More expensive but less
bulky than simple metallicradiation recuperators
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Type of Waste Heat Recovery
Commercial Waste Heat Recovery
Ceramic recuperators Less temperature limitations:
Operation on gas side up to 1550 C
Operation on preheated air side to 815 C
New designs
Last two years
Air preheat temperatures
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Type of Waste Heat Recovery
Regenerator
Figure 5. Regenerator
Large capacities
Glass and steel meltingfurnaces
Time between the reversalsimportant to reduce costs
Heat transfer in oldregenerators reduced by
Dust & slagging on surfacesheat losses from the walls
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Type of Waste Heat Recovery
Heat Wheels
Figure 6. Heat Wheel
Porous disk rotatingbetween two side-by-side ducts
Low to medium
temperature wasteheat recovery systems
Heat transferefficiency up to 85 %
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Type of Waste Heat Recovery
Heat Pipe
Figure 7. Heat Pipe
Transfer up to 100 timesmore thermal energy thancopper
Three elements: -sealed container- capillary wick
structure -working fluid
Works with evaporationand condensation
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Type of Waste Heat Recovery
Heat Pipe Performance and advantage Lightweight and compact
No need for mechanical maintenance, input power,cooling water and lubrication systems
Lowers the fan horsepower requirement andincreases the overall thermal efficiency of the system
Can operate at 315
C with 60% to 80% heat recovery
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Type of Waste Heat Recovery
Heat Pipe Typical application
Process to space heating
- Transfers thermal energy from process exhaustfor building heating
Process to process- Transfers recovered waste thermal energy fromthe process to the incoming process air
HVAC applications- Cooling and heating by recovering thermal energy
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Type of Waste Heat Recovery
Economizer Shell and tube heat exchanger Used when the medium containing waste heat is a
liquid or a vapor that heats another liquid
Figure 9. Shell & Tube Heat Exchanger
Shell contains thetube bundle, andusually internal bafflesto direct the fluid
Vapor containedwithin the shell
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Type of Waste Heat Recovery
Plate Heat Exchanger
Figure 10. Plate Heat Exchanger
Parallel plates forming a thin flow pass
Avoids high cost of heat exchange surfaces
Corrugated platesto improve heattransfer
When directionsof hot and coldfluids areopposite, thearrangement iscounter current
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Type of Waste Heat Recovery
Plate Heat Exchanger Run around coil exchanger
Figure 11. Run Around Coil Exchanger
Heat transferfrom hot to colder
fluid via heattransfer fluid
One coil in hotstream
One coil in coldstream
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Type of Waste Heat RecoveryPlate Heat Exchanger
Waste heat boiler
Figure 12. Two-Pass Water Tube Waste Heat Recovery Boiler
Water tube boiler: hotexhaust gases passover parallel tubes withwater
Capacities: 25 m3 to30,000 m3 /min ofexhaust gas
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Type of Waste Heat RecoveryHeat Pump
Figure 13. Heat Pump Arrangement
The vapour compressioncycle
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Type of Waste Heat RecoveryHeat Pump
Developed as a space heating system
Can upgrade heat >2X the energy consumed by the device
Most promising when heating and cooling capabilities arecombined
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Type of Waste Heat Recovery
Heat Pump Thermo compressor Compress low-pressure steam by very high-pressuresteam and reuse as medium pressure steam
Nozzle for acceleration of HP steam to a high velocityfluid.
Figure: Thermo compressor
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Introduction
Type of waste heat recovery
Assessment of waste heat recovery
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Assessment of waste heat recovery
Quality:
Higher temperatures = Higher quality = Lower heatrecovery costs
Quantity:
The amount of recoverable heat can be calculated as:
Heat Losses
Q = heat content in kCalV = the flow rate of the substance in m3/hr = density of the flue gas in kg/m3Cp = the specific heat of the substance inkCal/kg oC
T = the temperature difference in oCCp (Specific heat of flue gas) = 0.24 kCal/kg/oC
Q = V x x Cp x T
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Heat Saving Calculation Example Saving money by recovering heat from hot waste water:
Q = m x Cp x T x
Discharge of the waste water is 10000 kg/hr at 75 C
Preheat 10000 kg/hr of cold inlet water of 20 C
A heat recovery factor of 58%
An operation of 5000 hours per year
The annual heat saving (Q) is:
Assessment of waste heat recovery
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Heat Saving Calculation Example m = 1000 kg/hr = 10000 x 5000 kg/yr = 50000000 kg/yearCp = 1 kCal/kg C
T = (75 20) C = 55 C
= Heat Recovery Factor = 58% or 0.58
GCV of Oil = 10,200 kCal/kg
Equivalent Oil Savings = 159500000 / 10200 = 156372 LCost of Oil = INR 60/L
Monetary Savings = INR 9382320/Annum
Q = 50000000 x 1 x 55 x 0.58= 1595000000 kCal/year
Assessment of waste heat recovery
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