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Heat Exchanger Design – Part II

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Heat Exchanger Design – Part II ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Heat Exchanger Design – Part II Sameer Sameer Khandekar 1 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India In this lecture… Part I ◉ Types of Heat Exchangers ◉ The Overall Heat Transfer Coefficient ◉ Fouling factor Part II ◉ The Log Mean Temperature Difference Method ◉ Counter-Flow Heat Exchangers ◉ Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor ◉ The Effectiveness–NTU Method ◉ Selection of Heat Exchangers Sameer Sameer Khandekar 2 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 1 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Analysis of heat exchangers An engineer often finds himself or herself in a position: 1. to select a heat exchanger (Sizing) that will achieve a specified temperature change in a fluid stream of known mass flow rate - the log mean temperature difference (or LMTD) method. 2. to predict the outlet temperatures of the hot and cold fluid streams in a specified heat exchanger (Rating)- the effectiveness–NTU method. Sameer Sameer Khandekar 3 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Design/Analysis of a heat exchanger ◉ To design, or to predict the performance of a heat exchanger, it is essential to relate the total heat transfer rate to quantities of interest such as inlet and outlet fluid temperatures, overall heat transfer coefficient, and the total surface area of the heat exchanger. ◉ The LMTD method, described herein, is a simple approach when the fluid inlet temperatures are known and the outlet temperatures are either specified or readily determined from the energy balance expressions. ◉ It is also used for determining the size of a heat exchanger to realize prescribed outlet temperatures when the mass flow rates and the inlet and outlet temperatures of the hot and cold fluids are specified. Sameer Sameer Khandekar 4 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 2 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India The Lograrithmic Mean 1 Temperature The effective representation of temperature difference Sameer Sameer Khandekar 5 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Heat capacity rate (sensible cooling/heating) ◉ The rate of heat transfer in heat exchanger (HE is insulated): DTm an appropriate mean (average) temperature difference between the two fluids Two fluid streams that have the same capacity rates experience the same temperature change in a well-insulated heat exchanger. Sameer Sameer Khandekar 6 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 3 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Heat capacity rate (phase-change) ◉ The heat capacity rate of a fluid during a phase- change process must approach infinity since the temperature change is practically zero. 푄̇ = 푚̇ℎ where, 푚̇ is the rate of evaporation or condensation of the fluid and hfg is the enthalpy of vaporization of the fluid at the specified temperature or pressure. Variation of fluid temperatures in a Hx when one of the fluids condenses or boils. Sameer Sameer Khandekar 7 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Log-mean temperature difference Derivation of LMTD for: Parallel flow heat exchanger Variation of the fluid temperatures in a parallel-flow double-pipe heat exchanger. Sameer Sameer Khandekar 8 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 4 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Log-mean temperature difference 푇 − 푇 푇 − 푇 ∆ , , , , 푙푛 = −푈퐴 − ∆ 푄̇ 푄̇ Log Mean Temperature Difference (LMTD) Variation of the fluid temperatures in a parallel-flow double-pipe heat exchanger. Sameer Sameer Khandekar 9 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India LMTD: Counter-flow heat exchangers ◉ In contrast to parallel flow Hx, this configuration provides for heat transfer between hotter portions of the two fluids at one end and colder portions at the other. ◉ In the limiting case, the cold fluid will be heated to the inlet temperature of the hot fluid. ◉ However, the outlet temperature of the cold fluid can never exceed the inlet temperature of the hot fluid. The variation of the fluid temperatures in a counter-flow double pipe heat exchanger Sameer Sameer Khandekar 10 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 5 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Counter flow heat exchangers ◉ The change in temperature difference ΔT = Th -Tc along the length of the Hx, is in nowhere as large as for the inlet region of a parallel flow Hx. ◉ Note, that the outlet temperature of the cold fluid may now exceed outlet temperature of the hot fluid. ◉ We can show that the definition of LMTD will be identical as the parallel flow case, however ∆푇 = 푇, − 푇, ◉ ∆푇 = 푇, − 푇, The variation of the fluid temperatures in a counter-flow double pipe heat exchanger Sameer Sameer Khandekar 11 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Counter flow heat exchangers ◉ For specified inlet and outlet temperatures, ΔTlm a counter-flow heat exchanger is always greater than that for a parallel-flow heat exchanger. ◉ That is, ΔTlm, CF > ΔTlm, PF, and thus a smaller surface area (and thus a smaller Hx) is needed to achieve a specified heat transfer rate in a counter- flow Hx. ◉ When the heat capacity rates of the two fluids are equal The variation of the fluid temperatures in a counter-flow double pipe heat exchanger Sameer Sameer Khandekar 12 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 6 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India LMTD: Multi and cross flow Hx ◉ Use of a correction factor ◉ F correction factor depends on the geometry of the heat exchanger and the inlet and outlet temperatures of the hot and cold fluid streams. Sameer Sameer Khandekar 13 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Multi and cross flow heat exchangers ◉ F for common cross-flow and shell-and- tube heat exchanger configurations is given in the figure versus two temperature ratios P and R defined as ◉ 1 and 2 inlet and outlet ◉ T and t shell- and tube-side temperatures F = 1 for a condenser or boiler Sameer Sameer Khandekar 14 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 7 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Correction factor charts for common Shell-Tube Hx Sameer Sameer Khandekar 15 ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur
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