Soln_Fall2008_HW6 - SOLUTIONS HW6 1 In a steam power plant...

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SOLUTIONS- HW6 1. In a steam power plant 2 MW is added in the boiler, 1.2 MW is taken out in the condenser and the pump work is 0.1 MW. Find the plant thermal efficiency. If everything could be reversed find the coefficient of performance as a refrigerator. Solution: CV. Total plant: Energy Eq.: Q . H + W . P,in = W . T + Q . L W . T = 2 + 0.1 – 1.2 = 0.9 MW η TH = (W . T – W . P,in )/ Q . H = (900– 100)/2000 = 0.4 β = Q . L ( W . T – W . P,in ) = 1200/(900 – 100) = 1.5 2. R-410a at 50°C, x = 0.2 flowing at 3 kg/s is brought to saturated vapor in a constant- pressure heat exchanger. The energy is supplied by a heat pump with a coefficient of performance of β′ = 3. Find the required power to drive the heat pump. Solution: C.V. Heat exchanger m . 1 = m . 2 ; m . 1h1 + Q . H = m . 1h2 Given coefficient of performance β′ = Q . H / W . = 3 Table B.4.1: h 1 = h f + x 1 h fg = 143.65 + 0.2 × 135.73 = 170.836 kJ/kg, h 2 = h g = 279.58 kJ/kg Energy equation for line 1-2: Q . H = m . R-12 (h 2 - h 1 ) = 326.232 kW W . = Q . H / β′ = 326.232/3 = 108.743 kW
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4 A certain solar-energy collector produces a maximum temperature of 50°C. The energy is used in a cyclic heat engine that operates in a 20°C environment. What is the
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