A 1m^3 rigid tank initially contains 5*kg of steam at 150degC. A relief valve on

Question

A 1m^3 rigid tank initially contains 5*kg of steam at 150degC. A relief valve on top of the tank is opened to allow steam to slowly escape the tank. Heat is added to the water in the tank to maintain the temperature at 150degC. How much heat has been added when only saturated vapor remains in the tank?

Explanation / Answer

first check the initial state of the steam by calculating the specific volume v1 = V/m = 1/5 = 0.2 m^3/kg From steam tables At 150C, vf = 0.001091 m^3/kg , vg = 0.3948 m^3/kg Hence v1 lies in this zone and the quality is x = (0.2-0.001091)/(0.3948-0.001091) = 0.5052 => 50.52% is the initial quality ie the 0.5052*5 = 2.526 kg of steam is already saturated and the water that is to be heated upto saturated steam state 5-2.526 = 2.474 kg Hence energy required for this process is 2.474*h_fg = 2.474*2113.8 = 5229.54 kJ

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