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HEAT TRANSFER&
HEAT EXCHANGERS
CHBE 446 – Group5Stephan Donfack
Benjamin Harbor
Nguyen Huynh
Cyndi Mbaguim
AGENDA Concept and Mechanism Heat Transfer Equations Design Material Selection Conclusion
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
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)
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.
PROJECT FLOWSHEET
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
TYPES of HEAT EXCHANGERS
•Double-pipe•Shell and tube •Plate and frame•Spiral•Pipe coil
CONFIGURATIONS IN HEAT EXCHANGERS
Co-current flow Counter-current flow
Double tube – Single Pass Heat Exchanger
TEMPERATURE PROFILE
HEAT TRANSFER EQUATION IN HEAT EXCHANGERS
•
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.
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.
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
)/,/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
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…
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
HEAT EXCHANGERS IN GAS SWEETENING
Simplified schematic of gas sweetening process
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)
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
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.
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
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.
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..