Thermodynamics Notes21

Thermodynamics Notes21 - ME 311 FALL 2007 CHAPTER 8...

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ME 311 THERMODYNAMICS S. Masutani FALL 2007 CHAPTER 8 (continued) The ideal Rankine cycle consists of internally reversible processes. We determined earlier that ) = = b a b a rev rev TdS Q TdS Q . . int . . int δ Or, in terms of a unit mass flowing through the system: = = = b a rev b a b a rev Tds m Q Tds m Tds m Q . . int . . int & & & & & Graphically, T s in T 3 2’ 1’ out T m Q in & & m Q out & & 4 ) ( 3 ' 2 . . int s s T m Q out rev out = & & ) ( 4 ' 1 . . int s s T m Q in rev in = & & where ) ( 4 ' 1 ' 1 4 s s Tds T in Note that the net work out in net Q Q W & & & = 1
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ME 311 THERMODYNAMICS S. Masutani FALL 2007 and ) ( ) ( ) ( 4 ' 1 3 ' 2 4 ' 1 s s T s s T s s T m Q m W in out in in net = = & & & & η But for the ideal cycle, ' 1 ' 2 s s = and ) ( ) ( 4 ' 1 3 ' 2 4 3 s s s s s s = = So in out in out in T T T T T = = 1 What does this imply? Like the Carnot cycle, energy should be added at as high a temperature as possible and extracted at as low a temperature as possible. We will use this model to evaluate possible improvements to actual Rankine cycle power systems. We could configure a Rankine cycle as 3 river or reservoir turbine boiler pump t W & 1 2 4 Discharge p > p amb 2
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ME 311 THERMODYNAMICS S. Masutani FALL 2007 In this case, T s 100 ° C 3 2 1 amb T 4 p amb 1 bar p 0.03 bar closed loop pumping process T 2 increases with in T Since the turbine exit pressure, p 4 , must be greater than p amb for flow to occur, out T (the average temperature at which heat is rejected) > . By installing a condenser and closing the steam loop, the cooling water resource at T can be exploited more fully; p ambient T ambient T 4 can be reduced to about 0.03 bar (the saturation pressure at ) and ambient T out T will be only very slightly greater than ; i.e., ambient T out T . ambient T In addition, for the same T 3 , in T can be raised slightly (see dotted line above). For an ideal cycle: in out T T = 1 η Through this example, we demonstrate that, by adding a sealed condenser we are able to reduce out T and increase cycle efficiency. The reduction of out T is accomplished by reducing the condensation pressure below atmospheric. Since the cooling fluid in most practical systems is taken from a river or is recycled (cooled) in a cooling tower, ) min out T ( ambient T out T must be greater than the temperature of the cooling liquid if heat transfer is to occur).
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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 Notes21 - ME 311 FALL 2007 CHAPTER 8...

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