Presentation on theme: "Heat Exchangers: Design Considerations"— Presentation transcript:
1 Heat Exchangers: Design Considerations Chapter 11Sections 11.1 through 11.3
2 TypesHeat Exchanger TypesHeat exchangers are ubiquitous to energy conversion and utilization. They involve heat exchange between two fluids separated by a solid and encompass a wide range of flow configurations.Concentric-Tube Heat ExchangersParallel FlowCounterflowSimplest configuration.Superior performance associated with counter flow.
3 Unfinned-One Fluid Mixed Types (cont.)Cross-flow Heat ExchangersFinned-Both FluidsUnmixedUnfinned-One Fluid Mixedthe Other UnmixedFor cross-flow over the tubes, fluid motion, and hence mixing, in the transversedirection (y) is prevented for the finned tubes, but occurs for the unfinned condition.Heat exchanger performance is influenced by mixing.
4 Shell-and-Tube Heat Exchangers Types (cont.)Shell-and-Tube Heat ExchangersOne Shell Pass and One Tube PassBaffles are used to establish a cross-flow and to induce turbulent mixing of theshell-side fluid, both of which enhance convection.The number of tube and shell passes may be varied, e.g.:One Shell Pass,Two Tube PassesTwo Shell Passes,Four Tube Passes
5 Compact Heat Exchangers Types (cont.)Compact Heat ExchangersWidely used to achieve large heat rates per unit volume, particularly whenone or both fluids is a gas.Characterized by large heat transfer surface areas per unit volume, smallflow passages, and laminar flow.(a) Fin-tube (flat tubes, continuous plate fins)(b) Fin-tube (circular tubes, continuous plate fins)(c) Fin-tube (circular tubes, circular fins)(d) Plate-fin (single pass)(e) Plate-fin (multipass)
6 Overall Heat Transfer Coefficient Overall CoefficientOverall Heat Transfer CoefficientAn essential requirement for heat exchanger design or performance calculations.Contributing factors include convection and conduction associated with thetwo fluids and the intermediate solid, as well as the potential use of fins on bothsides and the effects of time-dependent surface fouling.With subscripts c and h used to designate the hot and cold fluids, respectively,the most general expression for the overall coefficient is:
7 Assuming an adiabatic tip, the fin efficiency is Overall CoefficientAssuming an adiabatic tip, the fin efficiency is
8 A Methodology for Heat Exchanger Design Calculations LMTD MethodA Methodology for Heat ExchangerDesign Calculations- The Log Mean Temperature Difference (LMTD) Method -A form of Newton’s Law of Cooling may be applied to heat exchangers byusing a log-mean value of the temperature difference between the two fluids:Evaluation of depends on the heat exchanger type.Counter-Flow Heat Exchanger:
9 Parallel-Flow Heat Exchanger: LMTD Method (cont.)Parallel-Flow Heat Exchanger:Note that Tc,o can not exceed Th,o for a PF HX, but can do so for a CF HX.For equivalent values of UA and inlet temperatures,Shell-and-Tube and Cross-Flow Heat Exchangers:
10 Overall Energy Balance Application to the hot (h) and cold (c) fluids:Assume negligible heat transfer between the exchanger and its surroundingsand negligible potential and kinetic energy changes for each fluid.Assuming no l/v phase change and constant specific heats,
11 Special Operating Conditions Special ConditionsSpecial Operating ConditionsCase (a): Ch>>Cc or h is a condensing vaporNegligible or no change inCase (b): Cc>>Ch or c is an evaporating liquidNegligible or no change inCase (c): Ch=Cc.
12 Problem: Overall Heat Transfer Coefficient Problem 11.5: Determination of heat transfer per unit length for heat recoverydevice involving hot flue gases and water.
13 Problem: Overall Heat Transfer Coefficient (cont.)
14 Problem: Overall Heat Transfer Coefficient (cont.)
15 Problem: Overall Heat Transfer Coefficient (cont.)
16 Problem: Overall Heat Transfer Coefficient (cont.)
17 Problem: Ocean Thermal Energy Conversion Problem 11.47: Design of a two-pass, shell-and-tube heat exchanger to supplyvapor for the turbine of an ocean thermal energy conversionsystem based on a standard (Rankine) power cycle. The powercycle is to generate 2 MWe at an efficiency of 3%. Oceanwater enters the tubes of the exchanger at 300K, and its desiredoutlet temperature is 292K. The working fluid of the powercycle is evaporated in the tubes of the exchanger at itsphase change temperature of 290K, and the overall heat transfercoefficient is known.
18 Problem: Ocean Thermal Energy Conversion (cont)
19 Problem: Ocean Thermal Energy Conversion (cont)