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HEAT TRANSFER&
HEAT EXCHANGERS
CHBE 446 – Group5Stephan Donfack
Benjamin Harbor
Nguyen Huynh
Cyndi Mbaguim
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AGENDA Concept and Mechanism Heat Transfer Equations Design Material Selection Conclusion
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CONCEPT Definition
• Discipline of thermal engineering that involves the generation, use, conversion, and exchange of thermal energy and heat between physical systems.
• The driving force of heat transfer is as result of temperature gradient between two regions.
• During heat transfer, thermal energy always moves in the same direction:
• HOT COLD
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Mechanism for Heat Transfer
Three types of energy transfer:
- Conduction: Transfer of heat within a substance by molecular interaction.
- Convection: During macroscopic flow, energy associated with fluid is carried to another region of space.
- Radiation: Heat transferred through wave energy (electromagnetic waves)
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Q hot Q cold
Th Ti,wall
To,wall
Tc
Region I : Hot Liquid-Solid Convection
NEWTON’S LAW OF CCOLING
dqx hh . Th Tiw .dA Region II : Conduction Across Copper Wall
FOURIER’S LAW
dqx k.dT
dr
Region III: Solid – Cold Liquid Convection
NEWTON’S LAW OF CCOLING
dqx hc . Tow Tc .dA
THERMAL
BOUNDARY LAYER
Energy moves from hot fluid to a surface by convection, through the wall by conduction, and then by convection from the surface to the cold fluid.
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PROJECT FLOWSHEET
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HEAT EXCHANGERS in INDUSTRY
• Commonly used throughout the chemical process industries as a means of heating and cooling process in product streams.
• Common industry utilization:• Space heating• Refrigeration• Air conditioning• Power plants• Petrochemical plants• Petroleum refineries• Natural gas processing • Sewage treatment
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TYPES of HEAT EXCHANGERS
•Double-pipe•Shell and tube •Plate and frame•Spiral•Pipe coil
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CONFIGURATIONS IN HEAT EXCHANGERS
Co-current flow Counter-current flow
Double tube – Single Pass Heat Exchanger
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TEMPERATURE PROFILE
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HEAT TRANSFER EQUATION IN HEAT EXCHANGERS
•
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Log Mean Temperature Difference (LMTD)
CO-CURRENT CONFIGURATION COUNTER CURRENT CONFIGURATION
Used to determine the temperature driving force for heat transfer in flow systems, most notably heat exchangers.
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Heat Duty (Q)
• Amount of heat needed to transfer from a hot side to the cold side over a unit time.
• Derived from energy balance.
Where:
= flow rate
Hfluid = Fluid enthalpy (temperature dependent)
ASSUMPTIONS
- Steady State- No phase changes- Negligible heat loss- Constant overall heat transfer
generatedsin out
outin ewQhmhmdt
dE
ˆ.ˆ.
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Overall Heat Transfer Coef (U)
• The overall HT coefficient is used to analyze heat exchangers.
• It contains the effect of hot and cold side convection, conduction as well as fouling and fins.
U
Xw: wall thickness
Km: thermal conductivity of wall
hi, ho: individual convective heat transfer coef
coefficients in & out of tubeDi, Do: Inner & outer diameter
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)/,/Pr,(Re, oiDLfNu
DIMENSIONLESS ANALYSIS TO CHARACTERIZE H.E
..Dv
k
C p .
Nu a.Reb .Prc
𝑵𝒖=𝑪𝒐𝒏𝒗𝒆𝒄𝒕𝒊𝒗𝒆 𝑯 .𝑻𝑪𝒐𝒏𝒅𝒖𝒄𝒕𝒊𝒗𝒆𝑯 .𝑻
𝒉 .𝐷𝐾
h = convective H.T coefK = conductive H.T coefµ = dynamic viscosityρ = densityCp = heat capacity
ν = mean velocityD & L = Length scale parameters
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ESTIMATED U
Overall Heat Transfer Coefficient can
be estimated for different fluids as well
as the type of heat exchanger system
involved (Shell & Tube).
Frequently used sources:
o Perry’s Handbook
o ChemE Design Textbook
o Aspen Tech Software…
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Area (Sizing)
Sizing a Heat Exchanger Equipment (by area calculation):
Costing (Base Cost Installation Cost) Approximating number of pipes needed in the heat
exchanger• Shell diameter and tubes pitch
Performance
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HEAT EXCHANGERS IN GAS SWEETENING
Simplified schematic of gas sweetening process
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HEAT EXCHANGER DESIGN
• The main heat exchanger called rich/lean amine interchanger.
It requires:Good heat recovery the thermal length of heat exchanger is a
key feature.
To minimize the fouling tendencies: high pressure drop (above 70 kPa) to keep shear stress high (50Pa)
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GASKET MATERIAL SELECTION
• Normal ethylene propylene diene monomer (EPDM): used in amine systems due to its inherent resistance to H2S and CO2.
• Disadvantage: suffers degradation from hydrocarbons or other fluids on an increasing severity based on the degree of the non-polar nature of the fluid
Plate with EPDM gasket
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CONT’d
• EPDM XH is a combination of EPDM and other rubber
resins creating an extra hard EPDM rubber, developed for
applications with hydrocarbon exposure.
• Other rubber materials: Aflas gaskets can be used for amine
duties, but not longer lifetime and increase capital investment
and replacement cost.
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SHELL & PLATE HEAT EXCHANGER
• Using a shell and plate heat exchanger as a reboiler allows a small
temperature difference between the hot and cold sides-> prevent amine
from overheated and degradation
• A shell and plate heat exchanger followed by a separator vessel is
recommended for condenser.
A typical shell and plate heat exchanger
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CONCLUSION
• Select the fit for purpose heat exchanger will improve the performance
of the amine plant, reduce investment costs and overall costs of
ownership.
• Selecting the right gasket plate will increase the efficiency while
maintenance costs and intervals can be reduced.
• Shell and plate heat exchangers are more commonly used than shell
and tube heat exchangers.
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REFERENCE
• Middleman, Stanley. An Introduction to Mass and Heat Transfer, Principles of Analysis and Design.Wiley, Dec 1997.
• McCabe, Smith, and Harriott. Unit Operations of Chemical Engineering
• http://www.tranter.com/literature/markets/hydrocarbon-processing/Hydrocarbon-Eng-A-Sweet-Treat.pdf
• www.authorstream.com/Presentation/baher-174192-heat-exchangers-ent..