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HEAT TRANSFER

CHAPTER 11 Heat Exchangers. HEAT TRANSFER. Heat Exchangers, LMTD Method. Where we’ve been …… So far have focused on detailed heat transfer analysis of specific conditions, such as external heat transfer coefficient Where we’re going:

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HEAT TRANSFER

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  1. CHAPTER 11 Heat Exchangers HEAT TRANSFER # 1

  2. Heat Exchangers, LMTD Method Where we’ve been …… • So far have focused on detailed heat transfer analysis of specific conditions, such as external heat transfer coefficient Where we’re going: • Investigate methods for larger system level analysis that combine all these modes of heat transfer in heat exchangers # 2

  3. Heat Exchangers, LMTD Method KEY POINTS THIS LECTURE • Types of heat exchangers, advantages and disadvantages • Overall heat transfer coefficient, concept of fouling factor • Log mean temperature difference • Application of LMTD to heat exchanger analysis • Text book sections: §11.1 – 11.3 # 3

  4. Heat Exchanger Types Example: # 4

  5. Heat Exchanger Types (Cont’d) Shell and Tube: (common in chemical process industry) Shell and Tube: Shell and Tube: Shell and Tube: # 5

  6. Heat Exchanger Types (Cont’d) • Shell and Tube: # 6

  7. Heat Exchanger Types (Cont’d) Plate and Frame • Series of plates with flow channels embossed in them. • The two fluids are guided through alternating rows of the plates • Advantages: __________________________ • Application pictured: Electrocoat paint in automotive assembly plant # 7

  8. Heat Exchanger Types (Cont’d) Plate and Fin • Dense array of plates that guide alternating channels of fluids (typically air) • Series of fins connect the plates and greatly increase the heat transfer area • Advantage: very large heat transfer surface area per unit volume . • One common application: Aircraft environmental control systems # 8

  9. Overall heat transfer coefficient for HX • Recall from earlier the overall thermal resistance concept: • Types of resistances involved with heat exchangers (covered in previous sessions) • Cold side internal convection • Cold side fouling factor • Conduction through wall • Hot side external convection (smooth wall or may involve fins) • Hot side fouling factor • Review howthese werecalculated # 9

  10. Analysis of heat transfer • Total heat transfer rate is found through energy balance, regardless of the HX type or flow path For hot fluid:Energy balance: • Define For the cold fluid: (Note: no minus sign “-” in this equation, since heat flow in) q # 10

  11. Analysis of heat transfer (Cont’d) • Energy balance gives: • For the entire flow length • A convenient way to compute the heat transfer is from the mean temperature difference between the hot and cold fluids • Next: Evaluation of Tm different for parallel and counter flow # 11

  12. Analysis of parallel flow heat transfer Parallel flow heat exchanger • At any location along the heat exchanger Where: • So: • Integrating from the inlet to the outlet dq Eq. 11.13 # 12

  13. Analysis of parallel flow heat transfer (Cont’d) • From the overall energy balance, total heat transfer: thus: • Combining • For parallel flow: Eq. 11.14 # 13

  14. Analysis of parallel flow heat transfer (Cont’d) • Temperature profile for parallel flow: In Out Fig 11.7 # 14

  15. In Out Analysis of counter flow heat transfer • For counter flow: • Temperature profile for counter flow: Fig 11.8 # 15

  16. Typical use of the LMTD method: Given: • Need to cool a certain mass flow rate of fluid Afrom TA,i to TA,o using the fluid B at TB,i Find: • Design / size the heat exchanger Solution Method: • Use the overall energy balance to find • Select the heat exchanger type (based on the other project needs, available resources, size and weight considerations, etc., etc.) • Select tube diameters and types of heat transfer surfaces (fins, no fins, etc.) • Use to determine the needed heat exchanger heat transfer area ( length) # 16

  17. Special cases • For a condensing vapor • For an evaporating liquid • What if Ch= Cc in a counter-flow HX? T x In Out T x In Out T x In Out # 17

  18. Multipass and cross-flow heat exchangers • The equations are the same. Counter-flow conditions To find F, please refer to the figures 11.10-13. # 18

  19. Typical Example E11.1 ( textbook, pp619) # 19

  20. Heat Exchangers, LMTD Method KEY POINTS THIS LECTURE • Various types of heat exchangers that are commonly used in industry and product designs. Understanding of when to consider using each type. • Defined the fluid heat capacity: • Log mean temperature difference introduced again • Temperature distribution parallel vs. counterflow Parallel Counterflow # 20

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