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..