上游充通大学 SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lectures 20-21 Chapter 5 Mass and Energy Analysis of Control Volume Analysis Spring,4/3/2019 Prof.,Dr.Yonghua HUANG 强 几R是A http://cc.sjtu.edu.cn/G2S/site/thermo.html 1日
Engineering Thermodynamics I Lectures 20-21 Spring, 4/3/2019 Prof., Dr. Yonghua HUANG Chapter 5 Mass and Energy Analysis of Control Volume Analysis http://cc.sjtu.edu.cn/G2S/site/thermo.html
1.Heat exchanger Transfer energy between fluids at different temperature b)~(e) Recuperators: Channel separated (a) (b) Direct contact(mixing) Tube-within-a-tube counter flow feedwater heater (c) (d) Cross-flow (e)Shell-and-tube Tube-within-a-tube parallel flow condenser/evaporator 上游充通大 April 3,2019 2 SHANGHAI JIAO TONG UNIVERSITY
April 3, 2019 2 1. Heat exchanger Transfer energy between fluids at different temperature Direct contact (mixing) Tube-within-a-tube counter flow Tube-within-a-tube parallel flow Cross-flow : feedwater heater (b) ~ (e) Recuperators: Channel separated (e) Shell-and-tube : condenser/ evaporator
Work/heat transfer of a heat exchanger Only flow work (boundary)for the CV-> Wiev =0 High rate of heat transfer from stream to stream (internal) Low rate of heat transfer between CV and surrounding Common assumptions for heat exchangers SSSF:no changes with time -APE =0:small elevation change -negligible△KE terms Fluid B CV boundary Fluid B CV boundary Negligible heat transfer between external enclosure of heat exchanger shell and Fluid A Heat Fluid A surroundings (external surface area is small Heat compared to surface area that exchanges heat between the two fluids) (a)System:Entire heat (b)System:Fluid A (Ocy0) The pressure of each fluid does not change as it exchanger (Ocv =0) flows through the heat exchanger 上游充通大 April 3,2019 3 SHANGHAI JIAO TONG UNIVERSITY
April 3, 2019 3 Work/heat transfer of a heat exchanger Only flow work (boundary) for the CV High rate of heat transfer from stream to stream (internal) Low rate of heat transfer between CV and surrounding CV W 0 Common assumptions for heat exchangers – SSSF: no changes with time – ∆PE = 0: small elevation change – negligible ∆KE terms – Negligible heat transfer between external enclosure of heat exchanger shell and surroundings (external surface area is small compared to surface area that exchanges heat between the two fluids) – The pressure of each fluid does not change as it flows through the heat exchanger
Simplified Mass and Energy Balances Fluid A Fluid B mA,inlet =mA,exit =mA mB,inlet mB,exit =mB QA=mA(hAexit-hAinlet QB=mB(hB.exit-hB.inlet) Relationship Between Fluid Heat Transfer Rates QA=-QB Entire Heat Exchanger (both fluids) Fluid B CV boundary Fluid B CV boundary mA hA.exit mB hB.exit =mA hAinlet+mg hBinlet Fluid A Heat Fluid A or Heat mA(hA.exit-hAinlet)=mB(hB inlet-hB.exit) (a)System:Entire heat (b)System:Fluid A (Ocv0) exchanger (Ocy =0) 上游充通大 April 3,2019 4 SHANGHAI JIAO TONG UNIVERSITY
April 3, 2019 4 Simplified Mass and Energy Balances Fluid A mA,inlet mA,exit mA Fluid B mB ,inlet mB ,exit mB QA mA (hA,exit - hA,inlet) QB mB (hB ,exit - hB ,inlet ) Entire Heat Exchanger (both fluids) mA (hA,exit- hA,inlet) mB (hB ,inlet - hB ,exit ) mA hA,exit + mB hB,exit mA hA,inlet + mB hB,inlet or QA -QB Relationship Between Fluid Heat Transfer Rates
Applications:Air-Cooled Condenser for AC Air Flow Fins Tube refrigerant 3 2 Refrigerant Changes p T 3 ideally V 上游充通大 April 3,2019 5 SHANGHAI JIAO TONG UNIVERSITY
April 3, 2019 5 Applications: Air-Cooled Condenser for AC Refrigerant Changes ideally