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Overview of Heat Exchangers: Types, Analysis Methods, and Example Problem

Heat exchangers play a crucial role in transferring heat between fluids efficiently. This article covers the types of heat exchangers including parallel flow, counterflow, and various configurations like tube and shell. It delves into heat transfer rates, overall heat transfer coefficient, temperatures, and the log mean temperature difference. Additionally, the effectiveness-NTU method for analyzing heat exchangers is discussed, along with an example problem involving a finned-tube, cross-flow heat exchanger.

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Overview of Heat Exchangers: Types, Analysis Methods, and Example Problem

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  1. ME 345 Heat Transfer (HTx) Professor: Dr. Dan Cordon (AKA Dr. Dan)

  2. TYPES OF HEAT EXCHANGERS Parallel Flow (wrong) Counterflow Cross Flow, finned (unmixed) Cross Flow, unfinned (external fluid mixed)

  3. TYPES OF HEAT EXCHANGERS Tube and Shell (one shell pass, two tube passes) Tube and Shell (single pass each) Tube and Shell (two shell passes, four tube passes)

  4. COMPACT HEAT EXCHANGERS Usually for gas-gas or gas-liquid when h is small

  5. RATE of HEAT TRANSFER The rate of heat transfer associated with either stream in a heat exchanger in which incompressible fluid or ideal gas with constant specific heats flows is Hot Side Cold Side

  6. OVERALL HEAT TRANSFER COEFFICIENT If you’re modeling a heat exchanger where the thermal conductivity of the material between the two flows is very high, you can neglect the thermal resistance in the equation above. If you’re modeling a heat exchanger that is clean (no scaling) you can neglect the fouling resistance terms as well.

  7. TEMPERATURES IN HEAT EXCHANGERS Parallel Flow (wrong) Counterflow

  8. LOG MEAN TEMPERATURE DIFFERENCE

  9. ANALYZING HEAT EXCHANGERS If all but one temperature is known, the following system of equations can usually predict the heat rate, unknown temperature, overall heat transfer coefficient, and surface area required. Hot Side Cold Side Parallel Flow Counterflow

  10. ANALYZING HEAT EXCHANGERS Effectiveness-NTU Method This system of equations can be used to simulate heat exchangers: Where Effectiveness Number of Transfer Units (dimensionless) Effectiveness is a function of NTU and the ratio of Cr= Cmin/Cmax

  11. EFFECTIVENESS RELATIONSHIPS

  12. EFFECTIVENESS RELATIONSHIPS

  13. NTU RELATIONSHIPS Sometimes it is more convenient to find NTU as a function of Effectiveness and the ratio of Cr= Cmin/Cmax

  14. NTU RELATIONSHIPS

  15. EFFECTIVENESS, NTU, and CrRELATIONSHIPS

  16. EFFECTIVENESS, NTU, and CrRELATIONSHIPS

  17. EFFECTIVENESS, NTU, and CrRELATIONSHIPS

  18. EXAMPLE PROBLEM 1 A finned-tube, cross-flow heat exchanger is to use the exhaust of a gas turbine to heat pressurized water. Laboratory measurements are performed on a prototype version of the exchanger, which has a surface area of 8 m2, to determine the overall heat transfer coefficient as a function of operating conditions. Measurements made under particular conditions were m_doth= 1.5 kg/s, Th, i= 325 ⁰C , m_dotc= 0.5 kg/s, and Tc, i= 25 ⁰C , and Tc, 0= 150 ⁰C . Calculate the overall heat transfer coefficient of the exchanger

  19. EXAMPLE PROBLEM 1

  20. EXAMPLE PROBLEM 1 Cr= 0.751 and ε = 0.444 NTU ~ 0.75 from figure on the left Alternatively, equation 11.32: NTU = 0.79 From: We get:

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