Question

Steam at 15 bar, 540 °C, 60 m/s enters an insulated turbine operating at steady state and exits at1.5 bar, 89.4 m/s. The work developed per kg of steam flowing is claimed to be (a) 606 kJ/kg, (b)765.9 kJ/kg. Can either claim be correct? Explain. textbook

Answer 1

Answer:

Both can be correct is steam exits as superheated steam.

Explanation:

The model for the turbine is created by using the First Law of Thermodynamics:

$w_{out} = h_{in}-h_{out} + \frac{1}{2}\cdot (v_{in}^{2}-v_{out}^{2})$

Specific enthalpies for both states are presented below:

State 1 (Superheated Steam)

$h = 3562.10\,\frac{kJ}{kg}$

State 2 (Saturated Vapor)

$h = 2693.10\,\frac{kJ}{kg}$

The specific work of turbine is:

$w_{out} = 3562.10\,\frac{kJ}{kg} - 2693.10\,\frac{kJ}{kg}+\frac{1}{2}\cdot [(60\,\frac{m}{s} )^{2}-(89.4\,\frac{m}{s} )^{2}] \cdot (\frac{1\,kJ}{1000\,J} )$

$w_{out} = 866.803\,\frac{kJ}{kg}$

Both can be correct is steam exits as superheated steam.