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# 7327959 - Engineering units(SI for pressures& Pe Pi is...

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Efficiency for a centrifugal compressor for any polytropic process is given by: Polytropic efficiency, = = - ηpol wpolwact PiPevdPhe hi where, wpol is the polytropic work of compression And, wact is the actual work of compression. For any polytropic compression process following = Pvn C = = - - - wpol PiPevdP nn 1PiviPePin 1 n 1 Now, For an ideal gas, Polytropic efficiency can be directly calculated using the equation: = - - ηpol nn 1γ 1γ ....................................................................................... [1] Where, n is the index of compression which can be calculated using = Pivin Peven For this, we need to know the pressure, P and specific volume, v at the inlet and outlet of the compressor. The pressure and temperature at inlet and outlet can be measured and using the thermodynamic property table for propane, we can obtain the value of specific volumes.
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Unformatted text preview: Engineering units (SI) for pressures & Pe Pi is , Pascal Pa for specific volumes, & ve vi is / m3 kg ** Subscripts ‘e’ for exit and ‘i’ for inlet. And, γ is the specific heat ratio which is a constant for ideal gas given by = γ CpoCv0 From the thermodynamic property table for ideal gas, we can directly obtain the values of Cpo and Cvo for Propane. Then, CpoCvo will give us value of γ Engineering units (SI) for specific heat at constant pressure, Cpo is kJkgK specific heat at constant volume, Cvo is kJkgK specific enthalpy at exit, he is / kJ kg specific enthalpy at inlet, hi is / kJ kg Using equation [1] we can obtain the polytropic efficiency for this case....
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