HEAT TRANSFER & HEAT EXCHANGERS

Report
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:
•
•
•
•
•
•
•
•
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
 =  ×  × ∆
•  =   ℎ  ()
•  =  ℎ  
•  =   ℎ 
• ∆ =    
Log Mean Temperature Difference (LMTD)

Used to determine the temperature driving force for heat transfer in flow
systems, most notably heat exchangers.
∆1 − ∆2
∆ =
∆1
ln(
)
∆2
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

 =  , − ,
Where:
 = 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 0
ℎ 


1
+  0 +
 
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 )
. 

v.D.

C p .
k
Nu  a.Re b .Pr c
 . 
 =
 . 

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)

=
 × ()
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..

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