Heat Transfer Su Yongkang School of Mechanical Engineering # 1 HEAT TRANSFER CHAPTER 11 Heat Exchangers

Heat Transfer Su Yongkang School of Mechanical Engineering # 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

Heat Transfer Su Yongkang School of Mechanical Engineering # 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

Heat Transfer Su Yongkang School of Mechanical Engineering # 4 Heat Exchanger Types Example:

Heat Transfer Su Yongkang School of Mechanical Engineering # 5 Heat Exchanger Types (Cont’d) Shell and Tube: Shell and Tube: (common in chemical process industry)

Heat Transfer Su Yongkang School of Mechanical Engineering # 6 Heat Exchanger Types (Cont’d) Shell and Tube:

Heat Transfer Su Yongkang School of Mechanical Engineering # 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

Heat Transfer Su Yongkang School of Mechanical Engineering # 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

Heat Transfer Su Yongkang School of Mechanical Engineering # 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 how these were calculated

Heat Transfer Su Yongkang School of Mechanical Engineering # 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

Heat Transfer Su Yongkang School of Mechanical Engineering # 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

Heat Transfer Su Yongkang School of Mechanical Engineering # 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

Heat Transfer Su Yongkang School of Mechanical Engineering # 13 Analysis of parallel flow heat transfer (Cont’d) From the overall energy balance, total heat transfer: thus: Combining For parallel flow: Eq

Heat Transfer Su Yongkang School of Mechanical Engineering # 14 Analysis of parallel flow heat transfer (Cont’d) Temperature profile for parallel flow: InOut Fig 11.7

Heat Transfer Su Yongkang School of Mechanical Engineering # 15 Analysis of counter flow heat transfer For counter flow: Temperature profile for counter flow: InOut Fig 11.8

Heat Transfer Su Yongkang School of Mechanical Engineering # 16 Typical use of the LMTD method: Given: Need to cool a certain mass flow rate of fluid A from T A,i to T A,o using the fluid B at T B,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)

Heat Transfer Su Yongkang School of Mechanical Engineering # 17 Special cases For a condensing vapor For an evaporating liquid What if C h = C c in a counter-flow HX? x T In Out x T x T

Heat Transfer Su Yongkang School of Mechanical Engineering # 18 Multipass and cross-flow heat exchangers The equations are the same. Counter-flow conditions To find F, please refer to the figures

Heat Transfer Su Yongkang School of Mechanical Engineering # 19 Typical Example E11.1 ( textbook, pp619)

Heat Transfer Su Yongkang School of Mechanical Engineering # 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 ParallelCounterflow