BENG3 THERMAL & FLUIDS ENGINEERING (THER305)
TUTORIAL EXAMPLES in STEAM PLANT
1.(a) An ideal steam plant uses
the Rankine cycle. The steam generator
pressure is 2 MPa and the condenser pressure is 0.004 MPa. Neglecting feed pump work,determine the
thermal efficiency and the specific work transfer from the cycle.What is the
dryness fraction of the steam at turbine exit?
[Ans: 33.2%; 889 kJ/kg; 0.735]
(b) To improve the performance of the steam
plant above, the steam generator pressure is increased to 4 MPa. Recalculate the efficiency, specific work and
dryness fraction. [Ans:
36.4%; 973 kJ/kg; 0.701]
(c) To improve performance further and to avoid
the low dryness fraction, the steam is superheated at steam generator exit to a
temperature of 700C (at 4 MPa). Recalculate the efficiency, specific work and dryness
fraction. [Ans:
42.5%; 1608 kJ/kg; 0.894]
2. A steam plant is to be
designed using the ideal Rankine cycle with superheat. Steam enters the turbine through a pipe 600mm
in diameter and expands to 0.005 MPa. In
order to produce 12 MW a mass flow rate of 32000 kg/h is to be used and the
dryness fraction at turbine exit must not be less than 0.9. Determine the steam generator pressure,
temperature, the cycle efficiency and the velocity of the steam in the inlet
pipe, assuming feed pump work is negligible and changes in PE and KE in the
plant are negligible. [Ans: 2.3 MPa; 583C; 38.2%; 5.3 m/s]
3. Steam enters a two stage
adiabatic turbine at 4 MPa and 350C. The first stage has an
isentropic efficiency 0.84 and the second stage 0.78. After expansion in the first stage until the
steam is approximately dry saturated (do not make repetitive trial and error
solutions) the steam expands in the second stage to a condenser pressure of
0.0035 MPa. Calculate the specific work
and the cycle efficiency, assuming saturated liquid condensate. [Ans: 913 kJ/kg; 30.6%]
4. A small steam
turbo-generator produces 4 MWe. The
efficiency of the plant expressed as electrical output/steam plant output is
0.80. The steam generator conditions are
3 MPa, 500C and after expansion with isentropic efficiency 0.9 to 0.2 MPa the
steam is reheated to 500C. There is a further expansion
with isentropic efficiency 0.9 to the condenser pressure of 3 kPa. Determine the steam mass flow rate, the cycle
efficiency, and the state of the steam at exit from the LP turbine. [Ans: 200 kg/min., 37.4%, ~40K superheated]
5. In an ideal regenerative steam cycle steam
is supplied at 40 bar dry saturated and expands to 0.07 bar. Steam is bled at 3.6 bar and mixed
adiabatically (at equal pressure) with the plant condensate. The resultant mixture at the saturation
temperature is pumped to the boiler.
Calculate the cycle efficiency and the mass of steam bled per unit mass
flow through the boiler. [Ans: 37.2%;0.192]
6. A steam plant uses two turbines with
reheating between them. The isentropic
efficiency of both turbines is 0.8. The
steam leaves the steam generator at 6 MPa, 550oC and expands to
0.5MPa. Reheat at this pressure raises the temperature to 500C before expansion in the second turbine to 0.004 MPa. The steam flow rate is 4.8 kg/s. Determine the power output and thermal
efficiency of the plant. [Ans: 6.69 MW; 35.6%]