Minimum Utility

Minimum Utility - PINCH AND MINIMUM UTILITY USAGE...

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PINCH AND MINIMUM UTILITY USAGE
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TEMPERATURE-ENTHALPY (T-H) DIAGRAMS • Assume one heat exchanger. These are alternative representations T H,in T C,out T C,in T H, out Q T H, out T C,out T H,in T C,in Q Slopes are the inverse of F*Cp. Recall that Q=F Cp T T C,in T C,out T H,in T H, out Q T H
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T-H DIAGRAMS • Assume one heat exchanger and a heater T H,in T C,out T C,in T H, out T C,in T C,out T H,in T H, out Q Q T H, out T C,out T H,in T C,in Q Q H Q H Q H H T H
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T-H DIAGRAMS • Assume one heat exchanger and a cooler T H,in T C,out T C,in T H, out T C,in T C,out T H,in T H, out Q Q T H, out T C,out T H,in T C,in Q Q C Q C Q C C T H
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T-H DIAGRAMS • Two hot-one cold stream T H1,in T C,out T C,in T H2,out T C,in T C,out T H2,in T H2, out Q 1 Q 1 T H1,out T C,out T H1,in T C,in Q 1 Q 2 Q 2 T H2,out T H2,in T H1,in T H1,out Q 2 T H2,in T H2,out Notice the vertical arrangement of heat transfer T H
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• How are streams subject to phase change represented? Liquid Slope change Phase change Single component Multicomponent T T H H
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ANSWERS • How can one represent a stream that has a variable Cp? H T T H
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T-H DIAGRAMS • Composite Curve Remark: By constructing the composite curve we loose information on the vertical arrangement of heat transfer between streams Obtained by lumping all the heat from different streams that are at the same interval of temperature. T T H H
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T-H DIAGRAMS • Moving composite curves horizontally T H1,in T C,out T C,in T H1,out Q 1 Q 2 T H2,out T H2,in Smallest T Smallest T Cooling Heating T H1,in T C,out T C,in T H1,out Q 2 T H2,out T H2,in Q H Q 1 Q C T T H H
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T-H DIAGRAMS Moving the cold composite stream to the right Increases heating and cooling BY THE SAME AMOUNT Increases the smallest T Decreases the area needed A=Q/(U* T ) Cooling Heating T H1,in T C,out T C,in T H1,out Q 2 T H2,out T H2,in Q H Q 1 Q C Smallest T Notice that for this simple example the smallest T takes place in the end of the cold stream T H
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T-H DIAGRAMS Cooling
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Minimum Utility - PINCH AND MINIMUM UTILITY USAGE...

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