Q.3 As shown a steam turbine at steady state is controlled by throttling the ste
ID: 2995753 • Letter: Q
Question
Q.3 As shown a steam turbine at steady state is controlled by throttling the steam to a lower pressure before it enters the turbine. Before throttling, the pressure and temperature are, respectively, 1.4 M P a and . After throttling, the pressure is 1 M Pa. At the turbine exit, the steam is at 10 kPa and a quality of 90%. Heat transfer with the surroundings and all kinetic and potential energy effects can be ignored. Determine the enthalpy at and . Determine the enthalpy leaving the throttle at . Determine the power per mass flow rate developed by the turbine (in kJ/kg). Q.4 A de-superheater works by injecting liquid water at with a temperature T2 = and mixing it with a superheated water vapor stream entering at of 10 kg/s with properties 300 kPa and the exiting water at is saturated vapor. This type of device is isobaric: p1 = p2 = p3. Assume the device is adiabatic and neglect kinetic and potential energy changes: htot = h. a) What are the enthalpies at and ? b) Combining expressions for mass and energy conservation, what is the mass flow rate of liquid water? c) What is the exiting mass flow rate of sat, vapor?Explanation / Answer
a)
From steam properties at T1 = 200 deg c and P1 = 300 kPa we get h1 = 2870 kJ/kg
From steam properties at T2 = 20 deg C and P2= 300 kPa (subcooled. liquid)we get h2 = 84.2 kJ/kg
From steam properties at P3 = 300 kPa and x3 = 1 we get h3 = 2720 kJ/kg
b)
m1*h1 + m2*h2 = m3*h3
m1 + m2 = m3
m1 = 10 kg/s
10*2870 + m2*84.2 = (10 + m2)*2720
m2 = 0.569 kg/s
c)
m3 = m1 + m2
= 10 + 0.569
= 10.569 kg/s
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