HEAT TRANSFER & HEAT EXCHANGERS
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Transcript HEAT TRANSFER & HEAT EXCHANGERS
HEAT TRANSFER
&
HEAT EXCHANGERS
CHBE 446 – Group5
Stephan 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)
THERMAL
Region III: Solid –
Cold Liquid
Convection
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.
NEWTON’S LAW OF
CCOLING
dqx hc .Tow Tc .dA
Th
Ti,wall
To,wall
Tc
Region I : Hot LiquidSolid Convection
Q hot
Q cold
NEWTON’S LAW OF
CCOLING
dqx hh .Th Tiw .dA
Region II : Conduction
Across Copper Wall
FOURIER’S LAW
dT
dqx k.
dr
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:
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•
•
•
•
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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
Double tube – Single Pass Heat Exchanger
Counter-current flow
TEMPERATURE PROFILE
HEAT TRANSFER EQUATION IN HEAT
EXCHANGERS
𝑄 = 𝑈 × 𝐴 × ∆𝑇𝑙𝑚
• 𝑄 = 𝑅𝑎𝑡𝑒 𝑜𝑓 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 (𝑑𝑢𝑡𝑦)
• 𝑈 = 𝑂𝑣𝑒𝑟𝑎𝑙𝑙 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
• 𝐴 = 𝐴𝑟𝑒𝑎 𝑓𝑜𝑟 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟
• ∆𝑇𝑙𝑚 = 𝐿𝑜𝑔 𝑀𝑒𝑎𝑛 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒
Log Mean Temperature Difference (LMTD)
Used to determine the temperature driving force for heat transfer in flow
systems, most notably heat exchangers.
∆𝑇1 − ∆𝑇2
∆𝑇𝑙𝑚 =
∆𝑇1
ln(
)
∆𝑇2
CO-CURRENT CONFIGURATION
COUNTER CURRENT CONFIGURATION
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.
dE
ˆ
ˆ
.h
m
m
.
h
in
out Q ws e generated
dt
out
in
𝑸 = 𝒎 𝒉𝒇𝒍𝒖𝒊𝒅,𝒊𝒏 − 𝒉𝒇𝒍𝒖𝒊𝒅,𝒐𝒖𝒕
Where:
𝑚 = flow rate
Hfluid = Fluid enthalpy (temperature dependent)
ASSUMPTIONS
-
Steady State
No phase changes
Negligible heat loss
Constant overall heat transfer
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=
1
1 𝐷0
ℎ𝑖 𝐷𝑖
𝑥
𝐷
1
+ 𝑤 0 +
𝐾𝑚 𝐷𝑖
ℎ0
Xw: wall thickness
Km: thermal conductivity of wall
hi, ho: individual convective heat transfer coef
coefficients in & out of tube
Di, Do: Inner & outer diameter
DIMENSIONLESS ANALYSIS TO CHARACTERIZE H.E
Nu f (Re, Pr, L / D, i / o )
𝒉. 𝐷
𝐾
v.D.
C p .
k
Nu a.Re b .Pr c
𝑪𝒐𝒏𝒗𝒆𝒄𝒕𝒊𝒗𝒆 𝑯. 𝑻
𝑵𝒖 =
𝑪𝒐𝒏𝒅𝒖𝒄𝒕𝒊𝒗𝒆 𝑯. 𝑻
h = convective H.T coef
K = conductive H.T coef
µ = dynamic viscosity
ρ = density
Cp = heat capacity
ν = mean velocity
D & 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)
𝑸𝒉
𝑨=
𝑼 × (𝑳𝑴𝑻𝑫)
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-SweetTreat.pdf
• www.authorstream.com/Presentation/baher-174192-heat-exchangers-ent..