9/2003 heat transfer review heat transfer review what is required to size a heat exchanger what is...
TRANSCRIPT
9/2003
www.alfalaval.us
• Heat transfer reviewHeat transfer review
• What is required to size a heat exchangerWhat is required to size a heat exchanger
• Compact heat transfer solutionsCompact heat transfer solutions– Plate Heat ExchangerPlate Heat Exchanger
– Spiral Heat ExchangerSpiral Heat Exchanger
– Welded Plate Heat ExchangersWelded Plate Heat Exchangers
– Specialty Plate Heat ExchangersSpecialty Plate Heat Exchangers
• Installation PhotosInstallation Photos
• Design of Heat Exchanger SolutionsDesign of Heat Exchanger Solutions
• Order Flow ProcessOrder Flow Process
Agenda
9/2003
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Heat TransferHeat TransferBasicsBasics
9/2003
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• RadiationRadiation– Electromagnetic wavesElectromagnetic waves
– When it reaches a body it has 3 options:When it reaches a body it has 3 options:
Reflected
Absorbed
Transmitted
• ConvectionConvection
– Energy is transferred by the motion and Energy is transferred by the motion and intermixing of small mass elementsintermixing of small mass elements
– Natural convection caused by density difference Natural convection caused by density difference
– Forced convection is man-made (ex., pump)Forced convection is man-made (ex., pump)
• ConductionConduction
– Molecular or atomic vibrationsMolecular or atomic vibrations
– No material transportNo material transport
Three ways to transfer heatThree ways to transfer heat
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• LaminarLaminar– Conduction: low heat transfer rateConduction: low heat transfer rate
• TurbulentTurbulent– Convection: high heat transfer rateConvection: high heat transfer rate
– Film layer at the wallFilm layer at the wall - - ConductionConduction- - Low heat transferLow heat transfer
Flow PrinciplesFlow Principles
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• Flows and temperatures for both sides
• Fluid properties including: density, specific heat, thermal conductivity, and viscosity for at least two points.
• For condensers and evaporators data such as a condensing curve, boiling point elevation, and/or other parameters may be required.
• Process conditions and limitations such as system pressure, potential for fouling or plugging, pressure drop limitations etc.
• The supplier may be able to use their experience to assist in determining proper values from above.
Data needed to design a heat exchanger
9/2003
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9/2003
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• WhereWhere
Q = heat transferred (Kbtu/hr)Q = heat transferred (Kbtu/hr)
m = mass flow rate (hot fluid) (lb/hr)m = mass flow rate (hot fluid) (lb/hr)
C = specific heat (hot fluid) (Btu/lb,F)C = specific heat (hot fluid) (Btu/lb,F)
T = hot fluid entering temperature (F)T = hot fluid entering temperature (F)
T = hot fluid leaving temperature (F)T = hot fluid leaving temperature (F)
HHII
HHOO
pp
HHII HHOO
Q = m CQ = m Cpp (T - T ) (T - T )
Heat Transfer
9/2003
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TTHHII
TTHHOO
TT CCOO
TTCCII
Tem
per
atu
reT
emp
erat
ure
Length of ChannelLength of Channel
HHII HHOO CCOO CCIIQ = m CQ = m Cpp (T - T ) = m C (T - T ) = m Cpp (T - T ) (T - T )
Thermodynamics at work!!
9/2003
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• WhereWhere
Q = heat transferredQ = heat transferred
U = overall heat transfer coefficientU = overall heat transfer coefficient
A = heat transfer surface areaA = heat transfer surface area
LMTD = log mean temperature differenceLMTD = log mean temperature difference
Q = U A (LMTD)Q = U A (LMTD)
Heat TransferHeat Transfer
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(T -T ) - (T - T )(T -T ) - (T - T )HHII CCOO HHOO CCII
(T -T )(T -T )HHII CCOO
(T -T )(T -T )HHOO CCII
lnln
LMTD =LMTD =
TTHHII
TTHHOO
TTCCOO
TTCCII
Tem
per
atu
reT
emp
erat
ure
Length of ChannelLength of Channel
Log Mean Temperature Difference
9/2003
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Q = mCp(T – T ) = U A (LMTD) = U A (LMTD)HHII HHOO
Determining Heat Transfer Area
m C (T - T )m C (T - T )
U (LMTD)U (LMTD)
HHIIHHOOPP
A =A =
Therefore
Determining Proper “U” value is the key!!
9/2003
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Items That Effect “U” value and Fouling Tendency
• Channel Geometry (turbulence)
• Fluid velocity and wall shear
• Fluid Properties (particularly viscosity)
*Viscosity also has a major impact on the pressure drop that will be seen in the heat exchanger
TubeTube SpiralSpiral PlatePlate
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Hot Fluid Fouling Layers Cold Fluid
Metal
Wall
Film Boundary Layers
TemperatureTemperature
TemperatureTemperature
TemperatureTemperature
Q = U A (LMTD)Q = U A (LMTD)
Heat Flow
11UU
11hh
+ Rf+ Rf11hh
ttkk
++++==and
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Nu Pr B Rey n
NuDh
vD
Re
PrCp
Nusselt NumberNusselt Number
Prandtl NumberPrandtl Number
Reynolds NumberReynolds Number
h
Heat Transfer Basics – Three Wise MenHeat Transfer Basics – Three Wise Men
9/2003
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TTHHII
TTHHOO
TTCCOO
TTCCII
Tem
per
atu
reT
emp
erat
ure
Length of ChannelLength of Channel
Lower LMTD and no temperature cross means
less efficiency
Cocurrent FlowCocurrent Flow