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J. Sarkar et al. / Energy Conversion and Management 46 (2005) 2053–2067 2057 where the heat gain in the evaporator refrigerant Qevr is given by XN i1⁄41 The cooling effect Qevw is given by Qevw 1⁄4m_evwcpwðTevi TevoÞ: ð3Þ The irreversibility or exergy loss in the evaporator is expressed as ðUAÞev;iðLMTDÞev;i; ð2Þ Qevo is the heat gain from the ambient. Qevr 1⁄4 Iev 1⁄4T0 TeviDPDP T0 m_refðs6 s5Þm_evwcpw lnT þIevr þIevw þQevo T 1 ; ð4Þ evo evw where the temperature related terms on the right hand side are due to the temperature difference and heat interaction with the ambient, respectively. The pressure related terms are due to the pres- sure drops in the refrigerant and water sides, respectively, and are given by IDP þIDP 1⁄4m_refDPevrþm_evwDPevw; ð5Þ evr evw qevr qevw DPevw is the pressure drop on the water side. The refrigerant side pressure drop DPevr (summation of pressure drops in all segments) is given by (using Lockhart and Martinelli equation) XNLf G2 DPevr 1⁄4 4 ev evrð1xÞ2 r /2l ; ð6Þ i1⁄41 devi2 ql i where the friction factor is expressed as fevr 1⁄4 0:0791Re0:25. l The two phase frictional pressure drop multiplier is evaluated from 7:2421=2 /l 1⁄4 1:376þX1:655 ; ð7Þ tt where Xtt is the Lockhart–Martinelli factor. 2.2. Compressor The exergy input to the compressor is given by Ein 1⁄4m_refðh2 h1Þ: ð8Þ The overall thermal efficiency for the semi-hermetic compressor has been calculated as [10] gis;c 1⁄4 0:26 þ 0:7952rp 0:2803r2p þ 0:0414r3p 0:0022r4p; ð9Þ with the compressor pressure ratio rp (=Pdis/Psuc) varying between 1.5 and 6.5. The irreversibility in the compressor is estimated from Icomp 1⁄4T0m_refðs2 s1Þ: ð10ÞPDF Image | Transcritical CO2 heat pump systems
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