Thermodynamics Notes23

Thermodynamics Notes23 - ME 311 THERMODYNAMICS S. Masutani...

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Unformatted text preview: ME 311 THERMODYNAMICS S. Masutani FALL 2007 CHAPTER 9 Gas Power Cycles Chapter 8 focused on power systems in which the working fluid existed as either a liquid or a gas at different points in the cycle. The chief advantage of these vapor power systems lies in the fact that work required to pressurize a liquid (by the pump) << work performed during expansion of a gas (in a turbine) bwr is << 1. Early compressors were much less efficient than turbines; all-gas systems could not produce significant net power. Consider 1 pump or compressor W & Q & 2 2 m & 1 m & 1 Energy balance (SSSF) outflow = inflow Q h h m W Q h m W h m & & & & & & & = + + = ) ( 1 2 1 2 For an internally reversible device: = 2 1 Tds m Q & & (heat added) (note: if Q is taken to be heat lost from the system ) & = 2 1 Tds m Q & & The Gibbs equation yields = = = 2 1 1 2 2 1 2 1 ) ( vdp m h h m vdp dh m Q vdp dh Tds & & & & Substituting = + + = 2 1 2 1 1 2 2 1 ) ( ) ( vdp m vdp m h h m h h m W & & & & & ME 311 THERMODYNAMICS S. Masutani FALL 2007 A similar analysis will show that power produced by an internally reversible turbine is = = 1 2 2 1 vdp m vdp m W t & & & t W & Q & 2 2 m & 1 m & 1 For a power cycle of the type we have examined previously 3 2 p p (state 2 state 3) t W & p W & 4 3 2 1 4 1 p p (state 1 state 4) For ideal devices: = = 2 1 3 4 vdp vdp m W W W p t net & & & & If it were possible to substitute an average value of v in the expression for W , then t & ) )( ( ) ( 1 2 2 1 compressor or pump turbine compressor or pump turbine p t net v v p p dp v v W W m W = = & & & & Since, typically, v(liquid) << v(gas), it is advantageous, from a power production standpoint, to use a liquid pump and a gas turbine. However , vapor systems have several disadvantages: possible erosion in the turbine; cavitation in the pump; pressures are relatively high, which requires heavy cladding and piping power/weight ratio moderate to low poor choice for transportation applications; 2 ME 311 THERMODYNAMICS S. Masutani FALL 2007 long heat-up time (stationary power generation applications)...
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This note was uploaded on 05/19/2008 for the course ME 311 taught by Professor Masutani during the Fall '07 term at Hawaii.

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Thermodynamics Notes23 - ME 311 THERMODYNAMICS S. Masutani...

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