chap11-heat exchangers传热传质

CHAPTER 11 Heat ExchangersIntroductionHeat exchangerThe device used to implement heat exchange between two fluids that are at different temperatures and separated by a solid wall.ApplicationsSpace heating and air-conditioning, power production, waste heat recovery, and chemical processing.Objectives(1) To introduce performance parameters for assessing the efficacy of a heat exchanger (2) To develop methodologies for designing a heat exchanger or for predicting the performance of an existing exchanger operating under prescribed conditions.11.1 Heat Exchanger TypesHeat exchangers are typically classified according to flow arrangement and type of construction. concentric tube Parallel-flow The hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end. Counterflow The fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends.Alternatively, the fluids may move in cross flow (perpendicular to each other), as shown by the finned and unfinned tubular heat exchangers of figure 11.2.11.1 Heat Exchanger TypesAnother common configuration is the shell-and-tube heat exchanger. Specific forms differ according to the number of shell-and-tube passes, and the simplest form, which involves single tube and shell passes, is shown in figure 11.3 . 11.1 Heat Exchanger TypesBaffles are usually installed to increase the convection coefficient of the shell-side fluid by inducing turbulence and a cross-flow velocity component. 11.1 Heat Exchanger TypesA special and important class of heat exchangers is used to achieve a very large( 700m2/m3) heat transfer surface area per unit volume. Termed compact heat exchangers, these devices have dense arrays of finned tubes or plates and are typically used when at least one of the fluids is a gas, and is hence characterized by a small convection coefficient. 11.1 Heat Exchanger Types11.2 The overall Heat Transfer CoefficientFor cylindrical walls Surface fouling The subsequent deposition of a film or scale on the surface can greatly increase the resistance to heat transfer between the fluids. This effect can be treated by introducing an additional thermal resistance, termed the fouling factor, Rf. Its value depends on the operating temperature, fluid velocity, and length of service of the heat exchanger.Fin effect Fins are often added to surface exposed to either or both fluids and that, by increasing the surface area, they reduce the resistance to convection heat transfer.Accordingly, with inclusion of surface fouling and fin effects, the overall heat transfer coefficient may be expressed as11.2 The overall Heat Transfer Coefficientwhere c and h refer to the cold and hot fluids, Rw is the conduction resistance, respectively. Note that calculation of the UA product does not require designation of the hot or cold side (UcAc=UhAh). However, calculation of an overall coefficient depends on whether it is based on the cold or hot side surface area, since Uc Uh if Ac Ah . Although representative fouling factors are listed in Table 11.1, the factor is variable during heat exchanger operation (increasing from zero for a clean surface , as deposits accumulate on the surface).11.2 The overall Heat Transfer CoefficientFluid Rf/ (m2.K/W)Seawater and treated boiler feedwater (below 50 )0.0001Seawater and treated boiler feedwater (above 50 )0.0002River water (below 50 ) 0.0002-0.001Refrigerating liquids 0.0002TABLE 11.1 Representative fouling factors11.2 The overall Heat Transfer CoefficientThe quantity o in Equation 11.1 is termed the overall surface efficiency or temperature effectiveness of a finned surface. For the hot or cold surface without fouling, the heat transfer rate iswhere Tb is the base surface temperature and A is the total (fin plus exposed base) surface area. The following expression was derived:where Af is the entire fin surface area and f is the efficiency of a single fin.11.2 The overall Heat Transfer CoefficientIf a straight or pin fin of length L (Figture 3.16 ) is used and an adiabatic tip is assumed, where m=(2h/kt)1/2 and t is the fin thickness. For several common fin shapes, the efficiency may be obtained from Table 3.5.Equation 11.2 corresponds to negligible fouling. However, if fouling is significant, the convection coefficient in Equation 11.2 must be replaced by a partial heat transfer coefficient of the form, Up=h/(1+hR”f). 11.2 The overall Heat Transfer CoefficientPartial coefficient for the hot and cold sides are then Up,h=hh/(1+hhR”f,h) and Up,c=hc/(1+hcR”f,c), respectively. Equation 11.3 may still be used to evaluate 0 for the hot and/or cold side, but Up must be used in lieu of h to evaluate the corresponding fin efficiency. Moreover, it is readily shown that the second and fourth terms on the right-hand side of Equation 11.1 may be deleted, if the convection coefficients in the first and fifth terms are replaced by Up,c and Up,h, respectively. 11.2 The overall Heat Transfer CoefficientTable 11.2 Representative Values of the Overall Heater Transfer CoefficientFluid Combination U (W/m2.K)Water to water 850-1700Steam condenser (water in tubes) 1000-6000Ammonia condenser (water in tubes) 800-1400Finned tube heat exchanger (water in tubes, air in cross flow)25-5011.2 The overall Heat Transfer CoefficientFor the unfinned, tubular heat exchangers of Figures 11.1 to 11.4, Equation 11.1 reduces towhere subscripts i and o refer to inner and outer tube surfaces (Ai= DiL, Ao= DoL), which may be exposed to either the hot or the cold fluid.11.2 The overall Heat Transfer Coefficient11.3 Heat Exchanger Analysis: Use of theLog Mean Temperature Differencen To design or to predict the performance of a heat exchanger, it is essential to relate the total heat transfer rate to quantities such as the inlet and outlet fluid temperatures, the overall heat transfer coefficient, and the total surface area for heat transfer. 11.3 Heat Exchanger Analysis: Use of theLog Mean Temperature Differencen Assumptionsn (1)There is negligible heat transfer between the exchanger and its surroundings,n (2)There is negligible potential and kinetic energy changes.Application of the steady flow energy equation, Equation 1.11e, gives n n where i is the fluid enthalpy. The subscripts h and c refer to the hot and cold fluids, whereas i and o designate the fluid inlet and outlet conditions. 11.3 Heat Exchanger Analysis: Use of theLog Mean Temperature Differencen If the fluids are not undergoing a phase change and constant specific heats are assumed, these expressions reduce ton where the temperatures appearing in the expressions refer to the mean fluid temperatures at the designated locations. Note that Equations 11.6 and 11.7 are independent of the flow arrangement and heat exchanger type. 11.3 Heat Exchanger Analysis: Use of theLog Mean Temperature Differencen Another useful expression may be obtained by relating the total heat transfer rate q to the temperature difference T between the hot and cold fluids, wheren However , since T varies with position in the heat exchanger, it is necessary to work with a rate equation of the formn where Tm is an appropriate mean temperature difference. Equation 11.9 may be used with Equations 11.6 and 11.7 to perform a heat exchanger analysis. Before this can be done, however, the specific form of Tm must be established. 11.3.1 The Parallel-Flow Heat Exchangern 11.3.1 The Parallel-Flow Heat Exchangern It is important to note that, for such an exchanger, the outlet temperature of the cold fluid never exceeds that of the hot fluid. Th,i= Th,1 , Th,o= Th,2, Tc,i= Tc,1 , Tc,o= Tc,2.n The form of Tm may be determined by applying an energy balance to differential elements in the hot and cold fluids. 11.3.1 The Parallel-Flow Heat Exchangern Assumptions:n 1. The heat exchanger is insulated from its surroundings, in which case the only heat exchange is between the hot and cold fluids.n 2. Axial conduction along the tubes is negligible.n 3. Potential and kinetic energy changes are negligible.n 4. The fluid specific heats are constant.n 5. The overall heat transfer coefficient is constant. The specific heats and the overall heat transfer coefficient may change because of variations of temperature and flow conditions. However, in many applications such variations are not significant, and it is reasonable to work with average values of cp,c, cp,h, and U for the heat exchanger.Applying an energy balance to each of the differential elements of Figure 11.7, it follows thatwhere Ch and Cc are the hot and cold fluid heat capacity rates, respective