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Schuster A, Karellas S, Leontaritis AD. Influence of supercritical ORC parameters on plate heat exchanger design, Applied Thermal Engineering 2012; 33-34: 70-76. Abstract and conclusion: Supercritical fluid parameters for lower exergy destruction with higher heat utilization systems. In current literature no work about determing the heat transfer mechanism under SC organic fluid state related to ORC applications Dimensioning HX with existing models for subcritical parameters can lead to inaccurate results and false conclusions. Investigation HX design and heat exchange coefficients important as it reflects to the process energetic and exergetic efficiency. As the 2 curves are closer together, the LMTD is smaller and so a lower HX thermal efficiency is expected HX area has to rise! o Study relative unknown heat transfer mechanisms around pcrit to improve HX surface and design algorithms. Aim paper: investigate influence of ORC parameters on the HX design basic HX design parameters in SC fluid parameters and convective coefficients + calculating HX surface for various fluid parameters. Working fluid: R134a, R227ea and R245fa Investigation HTC U o Influence of Tmax and pmax on HX design o Thermal properties fluid are strongly dependent on T in pseudo-critical temperature range U ≠ cte o Numerical approach by dividing HX into n elementary areas assuming equal Δℎ o Determination value of n calculation error (reference = 1000 points) Near pcrit for R134a: partitioning into 32 sections (3.24% error 52.6% 1 section) Near pcrit for R227ea: partitioning into 32 sections (2.35% error 50.67% 1 section) As pmax ↑ calculation error ↓ (32 8 sections for ± same error), due to a smoother variation of the thermo-physical properties under greater pressure around pseudo-Tcrit. Here 500 sections are used error 0.01% o In each section i i+1 LMTD method can be used o Pressure losses are neglected pSC = cte o cp of heat source is cte (cp≠f(T)) o (UA)i, i+1 can be calculated in each section o Near Tcrit at pcrit, the properties vary significantly pseudo-critical temperature instant phase-change Prandtl Pr, specific heat capacity cp, thermal conductivity λ, dynamic viscosity µU Classical heat transfer correlation of Dittus-Boelter for Nu cannot be used Jackson correlation for supercritical fluid parameters! o After calculation Ui Ai Atot can be calculated Pinch point difference 10°C Rectangular cross-section in HX (width = 100mm, distance between plates b = 2mm and plate thickness = 0.45mm) hydraulic diameter calculated Results o Influence of Tmax and pmax on U ± linear If pmax ↑ U↓ Low Tmax gradient U-p is larger influence of pmax is bigger For cte pmax and Tmax ↑ U↓ During superheating HTC between heat transfer fluid and vapour of working fluid is very low (superheating↑ U↓) o Influence of Tmax and pmax on A Influence of U Also to keep pinch point difference at 10°C, when pmax↑, higher HX efficiency is needed and so thus A has to ↑ o Influence of Tmax and pmax on HX efficiency NOT possible to use NTU-method to calculated HX efficiency, as in some parts of the heat transfer procedure, neither T nor cp are constant Subcritical: one is always constant o Sensible heat transfer cp = cte o Latent heat transfer T = cte Adapted definition for SC HX Observation: there is a pressure range for all fluids (from pcrit to 10-15 bar above pcrit) where HX efficiency is slightly dropping at pmax ↑ Future work to be done: o Investigate heat transfer mechanism in partial loads and transient procedures. o Verify with experiments and tests. o Techno-economic investigation of real-scale supercritical ORS applications.