Читать книгу Introduction to Desalination - Fuad Nesf Alasfour - Страница 32

1 v1 = vf @ 20 kPa = 0.001 017 m3/kgEnergy balance (first law):Since Δs = 0 (s2 = s1) at state 3 h3 = 3423.1 kJ/kg and s3 = 6.8826 kJ/(kg K) again the isentropic expansion turbine (s3 = s4) givesh4 = 2264.7 kJ/kg with quality x4 = 0.8552.Energy balance in turbine:and cycle Wnet = Wt − WpthenThe rate of heat added in the boiler (due to fuel burning)

2 Rate of rejected heat:

3 

4 The thermal availability (exergy) for control volume system defined aswhere kinetic and potential exergies are neglected and ho and so are measured at environmental state (298 K).Exergy at state:One can notice that Ψ3 is the highest (explain).

5 Exergy destruction (irreversibility) for steady flow process type is (isentropic process which is adiabatic and reversible type)where qin = h3 − h2 = 3165.6 kJ/kg and where qout = h4 − h1 = 2016.1 kJ/kg.Total amount of exergy destruction (irreversibility):Note that 40% of energy added (4818 kW) is destructed (wasted) due to irreversible processes, the reasons behind irreversibilities in any thermal system are combustion, heat transfer, mixing, friction, and non‐quasi equilibrium processes.

6 The second law efficiency can be defined as (Carnot efficiency) then

Extra activity:

Student can perform the following:

1 Explain the exergy value of fluid stream at each state.

2 Explain the sources of irreversibilities in boiler. How can we reduce them?

3 Calculate exergetic efficiency using the following three forms: (called rational) Compare and discuss.

4 Resolve example assuming that seawater is at 30 °C (summer season). Explain results, and compare.

5 Find the required fuel flow rate during summer using fuel oil and natural gas, and calculate the amount of emitted emissions: CO2 and SO2.