2300_les21-1 - VI. Entropy L. Adiabatic or Isentropic...

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1 lesson 21 VI. Entropy L. Adiabatic or Isentropic Efficiencies of Steady Flow Devices 1. Turbines ( η T = 80-90%) s a T w w work turbine Isentropic work turbine Actual = = η Recall () 2 1 2 2 2 1 2 1 in z z g 2 2 h h w 0 + + + = V V Isentropic work, w s , is calculated assuming same inlet condition and same outlet pressure as actual turbine. s 2 1 a 2 1 s a T h h h h w w = (7-61) (7-60) lesson 21 VI. Entropy 2. Compressors and Pumps ( η C = 75-85%) a s C w w work compressor Actual work compressor Isentropic = = Recall 2 1 2 2 2 1 2 1 in z z g 2 2 h h w 0 + + + = V V Isentropic work, w s , is calculated assuming same inlet condition and same outlet pressure as actual compressor. 1 a 2 1 s 2 a s C h h h h w w = (7-63)
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2 lesson 21 VI. Entropy 3. Compressor Example (Ex. 7-15, p. 374 of text) ? W in = ± 1 2 0 Q in = ± Air P 1 = 100 kPa T 1 = 12 C = 285 K P 2 = 800 kPa s kg 2 . 0 m = ± Given an adiabatic efficiency, η C = 0.80, find required power input. lesson 21 VI. Entropy 3. Compressor Example (cont.) Approach: calculate the isentropic work, w s , and then find the actual work, w a , using the adiabatic efficiency. Do this (1)
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2300_les21-1 - VI. Entropy L. Adiabatic or Isentropic...

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