化工四大化学(专业)政法计算(课堂讲义).ppt

* 2. Shell-and-tube heat exchanger Fig.15.1 Counterflow shell-and-tube heat exchanger A, baffles; B, tubes; C, guide rods;D, D, tube sheets; E, spacer tubes. 4-09 4-10 * It consists of A—a bundle of parallel tubes(管束) B1,B2—tube sheets(管板，花板) C—shell(外壳) D1,D2—channels E1,E2 — channels covers (封头) F—vapor inlet G—condensate outlet H—cold-liquid inlet J—warm-liquid outlet K—non-condensed gas vent FIGURE 11.1 Single-pass tubular condenser * FIGURE 11.2 Temperature-length curves for condenser. * 11.2 Energy-Balances 1. Introduction 2. Enthalpy (焓) balances in heat exchangers 3. Enthalpy (焓) balances in total condensers * 1. Introduction Quantitative attack on heat-transfer problems is based on Equations of energy balances Equations of rates of heat transfer * 2. Enthalpy (焓) balances in heat exchangers Assumptions 1. steady-state heat transfer 2. neglecting the heat to or from the ambient(heat loss is zero). * For the warm stream through the heat exchanger, (11.3) For the cold fluid through the heat exchanger, (11.4) where --mass flow rates of cold fluid and warm fluid. Hca, Hha--enthalpy per unit mass of entering cold fluid and entering warm fluid. Hcb, Hhb--enthalpy per unit mass of leaving cold fluid and leaving hot fluid. qc,qh--rates of heat addition to cold fluid and warm fluid. * The heat lost by the warm fluid is gained by the cold fluid, (11.5) If only sensible heat is transferred, specific heat is constant or use mean value at average temp. (11.6) the overall enthalpy balance * 3. Enthalpy (焓) balances in total condensers If the vapor enters the condenser as saturated vapor (no superheat), and the condensate leaves at condensing temperature(no further cooled). (11.7) If the condensate leaves at a temp. Thb that is less than Th, the condensing temperature of the vapor (11.8) λ latent heat of vaporization of vapor * 1. Monolayer flat-walled one-dimensional steady-state conduction F