Download Influence of supercritical ORC parameters on plate heat

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
µU
 Classical heat transfer correlation of Dittus-Boelter for Nu cannot be used 
Jackson correlation for supercritical fluid parameters!
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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.