上岸充通大学 SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lecture 43-44 Cengel_Chapter 11 Refrigeration Cycles Spring,5/9/2019 Prof.,Dr.Yonghua HUANG 强 M是。 目e http://cc.sjtu.edu.cn/G2S/site/thermo.html 1日G
Engineering Thermodynamics I Lecture 43-44 Spring, 5/9/2019 Prof., Dr. Yonghua HUANG Cengel_ Chapter 11 Refrigeration Cycles http://cc.sjtu.edu.cn/G2S/site/thermo.html
Carnot Refrigeration Cycle Warm region at TH Practical? Uuuut Condenser TH Turbine Compressor Tc Evaporator m b a Cold region at Tc Oin Coefficient of Performance B area 1-a-b-4-1 Tc(Sa -Sp) inlm area1-2-3-4-1 (TH-Tc)(sa-sp) 阝max= Te W/m-Wi/m TH-Tc Represents the maximum theoretical coefficient of performance of any refrigeration cycle operating between regions at Tc and TH 上游充通大 May9,2019 2 HANGHAI JIAO TONG UNIVERSITY
May 9, 2019 2 Carnot Refrigeration Cycle Practical? Coefficient of Performance β Represents the maximum theoretical coefficient of performance of any refrigeration cycle operating between regions at TC and TH . ?
Departures from the Carnot Cycle Limited△T. 1'→2' Compression liquid-vapor mixture.Wet compression. Most compressors handle vapor only.Dry compression. (Expansion)Turbine Condenser temperature,T Throttling valve Temperature of warm region,TH Temperature of cold Te region,Tc savings in W initial and Evaporator temperature,Tc maintenance costs. b a S 上游充通大 May9,2019 3 SHANGHAI JIAO TONG UNIVERSITY
May 9, 2019 3 Departures from the Carnot Cycle • Limited ∆T. • Compression liquid–vapor mixture. Wet compression. Most compressors handle vapor only. Dry compression. savings in initial and maintenance costs. (Expansion) Turbine Throttling valve W 0 1’2’
Vapor-compression refrigeration cycle Evaporator: =h1-h4 m Qin-refrigeration capacity.(kW). Alternative unit:ton of refrigeration,211 k]/min. W ●Compressor h2 -hy n Condenser ●Condenser Expansion Avalve Compressor Evaporator M Throttling process Saturated or superheated vapor 上游通大学 May9,2019 4 SHANGHAI JIAO TONG UNIVERSITY
May 9, 2019 4 Vapor-compression refrigeration cycle
Working substance:p-h diagram 4000 3000 R134a 2000 cond4055℃ 1000 ressure 900 800 00 700 ratio 600 500 v阳105C ×2.0-3.0 evap 400 小年年想想年年想中期想期用” 300 200 100 30 Enthalpy [kJ/kg] 40 50 x-0.10 020 0.30 0.40 0.50 0.60 0.70 0.80 0.90 40 20 =020 0.40 0.60 0.80 100 120 140 160 180 200 220 300 150-250W △h≈120~150k/kg 240260280 △h≈25~35k/kg evap.r m.Ah = im△h comp.e ≈65% 3.5-6.0kg/hr (COP. 50-80W comp,e 上泽文通大学 3.0-5.0 May9,2019 5 SHANGHAI JIAO TONG UNIVERSITY
May 9, 2019 5 comp,e evap,r COP W Q MSRS 150-250 W Enthalpy [kJ/kg] Pressure ratio 2.0-3.0 h 120~150 kJ/kg Qevap,r m r h Tevap: 10-25 C Tcond: 40-55 C h 25~35 kJ/kg r comp,e o,is m h W 65% 3.5-6.0 kg/hr 50-80 W 3.0-5.0 R134a Working substance: p-h diagram
Cycle Analysis cond Condenser 3 2 Throttling Compressor Valve 4 1 Evaporator T-s diagram? p-h diagram? subcooling TA △Tsc 2 P Tcond -3--- --TH Pcond Tevap ·T Pevap superheat::△Tsh h S 上游充通大 May9,2019 6 SHANGHAI JIAO TONG UNIVERSITY
May 9, 2019 6 Cycle Analysis P h 3 2 4 1 pcond pevap s T 2 1 3 4 Tcond Tevap TH TL subcooling Tsc superheat: Tsh Compressor Throttling Valve Condenser 1 3 2 4 Evaporator W Qcond Qevap T-s diagram? p-h diagram?
Cycle Analysis Typical Assumptions specified evaporating(Tevap)and condensing (Tcond) temperatures specified superheat entering the compressor specified subcooling leaving the condenser ● constant pressure throughout heat exchangers negligible kinetic and potential energy changes for all components ● adiabatic throttling valve specified isentropic efficiency for compressor adiabatic compressor 上游充通大 May9,2019 7 SHANGHAI JLAO TONG UNIVERSITY
May 9, 2019 7 Cycle Analysis Typical Assumptions • specified evaporating (Tevap) and condensing (Tcond) temperatures • specified superheat entering the compressor • specified subcooling leaving the condenser • constant pressure throughout heat exchangers • negligible kinetic and potential energy changes for all components • adiabatic throttling valve • specified isentropic efficiency for compressor • adiabatic compressor
Cycle Analysis Compressor Inlet State P1=Pevap h1=h(Tevap+ATsh Pevap Compressor Outlet State P2 =Pcond T◆ isentropic 32-p specific work Tcond h2 =h,+has h) ns Tevap isentropic efficiency S h2s=h(p2,S2=S1) 图 上游充通大 May9,2019 8 SHANGHAI JLAO TONG UNIVERSITY
May 9, 2019 8 Cycle Analysis Compressor Inlet State p1 = pevap h1 = h (Tevap+Tsh , pevap) Compressor Outlet State p2 = pcond h2s = h (p2 , s2 = s1 ) isentropic specific work 2s 1 2 1 s isentropic efficiency (h h ) h h T s 2 Tcond Tevap p2 1 2s
Cycle Analysis Condenser Outlet State P 3 P3=Pcond 444444444444444 h3=h (Tcond+ATse,Pcond) negative Pevap Evaporator Inlet State P4 Pevap h h4=h3 上游充通大学 May9,2019 9 SHANGHAI JLAO TONG UNIVERSITY
May 9, 2019 9 Cycle Analysis Condenser Outlet State p3 = pcond h3 = h (Tcond+Tsc , pcond) Evaporator Inlet State p4 = pevap h4 = h3 P h 3 4 Pcond negative Pevap
Cycle Analysis Coefficient of Performance P refrigerating effect 3 2 Pcond COP h-ha hh Pevap 4 specific work h Volumetric Capacity refrigerating effect indicator of the compressor h1-h4 displacement requirement qv compressor inlet specific volume 上游通大学 May9,2019 10 SHANGHAI JLAO TONG UNIVERSITY
May 9, 2019 10 Cycle Analysis Coefficient of Performance Volumetric Capacity P h 3 2 4 1 pcond pevap q = v refrigerating effect compressor inlet specific volume h h v 1 4 1 refrigerating effect 1 4 2 1 specific work COP = h h h h indicator of the compressor displacement requirement