传热学上海交通大学Heat Echangers 4.pptVIP

HEAT TRANSFER Heat Exchangers, NTU-? Method Where we’ve been …… Analysis of heat exchangers using log mean temperature difference (LMTD) Where we’re going: Computation of heat exchanger performance compared to the theoretical maximum possible for the flow conditions and HX type and size. Recall earlier discussion For a condensing vapor For an evaporating liquid What if Ch = Cc in a counterflow HX? Heat exchanger effectiveness Maximum possible heat transfer rate for any given inlet temperatures and flow rates occurs in a infinitely long counterflow HX Heat exchanger effectiveness (Cont’d) Define: Heat exchanger effectiveness, ? Actual heat transfer, q, can be determined from simple energy balance Thus: If the heat exchanger effectiveness were known, then the actual heat transferred could be found from: Number of Transfer Units Define: Number of Transfer Units, NTU NTU depends on both the heat exchanger design (UA) and the operating conditions (Cmin). Define: Capacity Ratio, Cr Effectiveness is a function of capacity ratio and the NTU Relationships between ?, NTU and Cr can be computed. Tables 11.3 and 11.4 and Figures 11.14 – 11.19 in textbook NTU - ? Tables Summary of Solution Method Typical scenario for using ?-NTU method: Given: Find: Solution method: Determine UA for this heat exchanger Find Find Calculate  Determine ? from tabulated formulas or plots Compute actual heat transfer Find outlet temperatures from Calculation Methodology Performance calculation: Given: Find: Solution method: NTU Design problems: Given: Find: Solution method: LMTD Example #1 Situation: Light lubricating oil (cp=2090 J/kg-K) is cooled with water in a small heat exchanger. Oil flow = 0.5 kg/s, inlet T = 375 K Water flow = 0.2 kg/s, inlet T = 280 K Part 1: If desired outlet temperature of the oil is 350 K, and you know the estimated overall heat transfer coefficient, U = 250 W/m2-K, from manufacturer’s d