CHAP. 4: APPLICATION OF THERMODYNAMICS [CONTENTS] 121
This relation may be written in the form
H 2 −H 1 =Q+Wsh, (4.35)which may be considered the formulation of the first law of thermodynamics for heat engines
with a steady flow of the working medium [compare with relation (3.3)].
In the event that no heat is exchanged with the surroundings (the engine works adiabati-
cally), the shaft work equals the loss of enthalpy in the medium passing through the engine.
The shaft workWsh[see4.1.4] may be understood as volume work corrected for work needed
to drive the working medium in and out of the engine.
Note:It is important to realize the difference between the Carnot heat engine [see4.3.1]
and a heat engine with a steady flow of substance. In the Carnot engine, the working
medium is part of the engine, and this engine exchanges only heat and work with its
surroundings. In a heat engine with a steady flow of substance, the working medium
passes through the engine. A steam turbine or a petrol engine are typical examples of the
latter.Example
Water steam of temperaturet 1 = 500◦C and pressurep 1 = 10 MPa flows into a steam turbine
converting shaft work into electric energy. For this temperature and pressure, the following values
of molar enthalpy and molar volume can be found in tables:Hm1= 60705 J/mol,Vm1= 0.6039
dm^3 /mol. After leaving the steam turbine, the steam has a temperaturet 2 = 160◦C and pressure
p 2 = 0.5 MPa (Hm2= 49800 J/mol,Vm2= 7.056 dm^3 /mol). What amount of electric energy
can be obtained from one mole of steam provided that the turbine operates adiabatically? How
much is accounted for by volume work? Why cannot volume work be fully converted into electric
energy?