GATE 2021 ME (ME2) – Question 57

Thermal Engineering Applications · Power Engineering: vapour and gas power cycles, regeneration and reheat · 2 marks · Numerical answer

Consider the open feed water heater (FWH) shown in the figure given below: Specific enthalpy of steam at location 2 is 2624 kJ/kg, specific enthalpy of water at location 5 is 226.7 kJ/kg and specific enthalpy of saturated water at location 6 is 708.6 kJ/kg. If the mass flow rate of water entering the open feed water heater (at location 5) is 100 kg/s then the mass flow rate of steam at location 2 will be _____ kg/s (round off to one decimal place).

Rankine-cycle diagram with an open feed water heater receiving extracted steam at 2 and pumped water at 5, and discharging at 6. See the attached source image.

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Correct answer: 25.07 to 25.33

Explanation

Mass balance gives outlet flow 100+m. The adiabatic mixing energy balance is $2624m+100(226.7)=(100+m)(708.6)$.

Thus $m=100(708.6-226.7)/(2624-708.6)=25.15924$ kg/s. **Answer: 25.2 kg/s**.