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How much work can be obtained from an adiabatic, continuous-flow turbine, if steam at 60 bar and 500^oC is used and the outlet stream is at 1 bar and 400^oC?(a) -144 kJ/kg(b) -155 kJ/kg(c) -166 kJ/kg(d) -177 kJ/kgI had been asked this question in an international level competition.This interesting question is from Energy Balance Numericals Without Reactions topic in portion Energy Balance of Bioprocess Engineering

Answer»

The correct choice is (a) -144 kJ/KG

The explanation is: BASIS: 1 kg steam

For superheated steam:

Inlet: (60 bar, 500°C) h1 = 3422 kJ/kg

Outlet: (1 bar, 400°C) h2 = 3278 kJ/kg

min = mout = 1 kg

DECV + Sni(Ek + Ep + H)i = Q + WS (General form of the energy balance)

Now we make the following simplifying assumptions for this turbine:

▪ Adiabatic: Q = 0

▪ The inlet and outlet lines are not tremendously different in height: Ep = 0

▪ There is little difference in the velocity of the FLUID in the inlet and outlet lines: Ek = 0

▪ There are no accumulation terms (steady-state operation with steady flow): DECV = 0

The simplified form of the energy balance is therefore:

Hout – Hin = Ws = mouthout – minhin = m(hout – hin)

Substituting in the values for the specific enthalpies gives:

Ws = m (hout – hin) or Ws/m = (3278 – 3422) kJ/kg = -144 kJ/kg.



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