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
Example 1.power plant condenser Condensate >Q→0 Steam 0.1 bar 2二 0.1 bar Known:steam vapor 45°C x=0.95 no pressure change Determine: > and Cooling Cooling m m water water 20°C 35C Assumptions: Control volume for part(a) ·SS 2 ·ofCV/Surroundings→0 Condensate Steam Wcy =0 △PE,△KE→0 Energy transfer to cooling water At states 2,3,4 (liquids) Control volume for part(b) h≈h(T) 上游充通大 April 3,2019 6 SHANGHAI JIAO TONG UNIVERSITY
April 3, 2019 6 Example 1. power plant condenser Known: steam vapor no pressure change Determine: • and Assumptions: • SS • of CV/Surroundings0 • • ∆PE, ∆KE 0 • At states 2, 3, 4 (liquids) h ≈ hf (T) 3 1 m m CV 1 Q m QCV m1 CV W 0 Q0
Solution Mass balance:Separated channels/fluids m1=i2 m3=m4 0.1 bar 45.8C 2 Overall condenser: 4 3 Energy rate balance (SS): U -+a(a++)(++ underlined terms Condensate Steam drop out 0.1 bar 0.I bar 45C x=0.95 m3_h1-h2 > 0=i1(h-h)+i3(h3-h4)→ Cooling Cooling rt hs-h3 water water 20°C 35C 上游充通大 April 3,2019 7 SHANGHAI JIAO TONG UNIVERSITY
April 3, 2019 7 Solution Mass balance: Separated channels/fluids Overall condenser: Energy rate balance (SS): underlined terms drop out
Solution cont. State 1:two-phase h=h+x(hs-h) 0.1 bar 0.95 P1=0.1bar Sat.Table A-5 45.8℃ 21 =191.83+0.95(2584.7-191.83)=2465.1kJ/kg U States 2,3,4:compressed/saturated liquid h2≈h-(T2)=188.45kJ/kg h3≈h(T3)andh4≈hr(T4) 0 m3 h1-h2 2465.1-188.45 m 36.3 h4-h3 62.7 上游充通大学 April 3,2019 8 SHANGHAI JLAO TONG UNIVERSITY
April 3, 2019 8 Solution cont. State 1: two-phase 1 ( ) f g f h h x h h + - 0.95 p1=0.1bar + Sat. Table A-5 States 2, 3, 4: compressed/saturated liquid
Solution cont. Only condensing stream: Energy rate balance (SS): 0.1 bar V 45.8C 2 21 Mass balance (SS):m=riz underlined terms 0 drop out Cey ri(h2 -h) 2 Condensate Steam =h,-h=18845-2465.1=Q276.7k/e Energy transfer to cooling water my N energy is transferred from the condensing steam to the cooling water 上游充通大 April 3,2019 9 SHANGHAI JIAO TONG UNIVERSITY
April 3, 2019 9 Solution cont. Only condensing stream: Energy rate balance (SS): underlined terms drop out energy is transferred from the condensing steam to the cooling water + Mass balance (SS):
2.Throttling devices Examples: Flow restricting devices that cause a pressure drop without a work output ·Used to (a)An adjustable valve control flow (e.g.faucet) provide temperature drop (refrigeration,air conditioning) measure flow rates (b)A porous plug (c)A capillary tbe An orifice is also a common throttling device 上游充通大学 April 3,2019 10 SHANGHAI JLAO TONG UNIVERSITY
April 3, 2019 10 2. Throttling devices Examples: An orifice is also a common throttling device • Flow restricting devices that cause a pressure drop without a work output • Used to – control flow (e.g. faucet) – provide temperature drop (refrigeration, air conditioning) – measure flow rates
