Homework 7 Due
Friday May 2, 2008
1.
Consider a steam power plant that operates between the pressure limits of 10 MPa and 20 kPa. Steam enters the pump as a saturated liquid
and leaves the turbine as saturated vapor.
Determine the ratio of the work delivered by the turbine to the work
consumed by the pump. Assume the entire
cycle to be reversible and the heat losses from the pump and the turbine to be
negligible. (C&B) (wturbine/wpump)
= 207
2.
A well-insulated, thin-walled, double-pipe, counter-flow heat exchanger is to
be used to cool oil (Cp = 2.20 kJ/kg °C) from 150 °C to 40 °C at a rate of
2 kg/s by water (Cp = 4.18 kJ/kg °C) that enters at a rate of 1.5 °C/s. Determine
(a) the rate of heat transfer and (b) the rate of entropy generation in the
heat exchanger. (C&B)
(Q-dot)oil = 484 kW, T4 = 372 K, (S-dot)gen
= 0.132 kJ/K s
3.
Steam expands in a turbine steadily at a rate of 25,000 kg/h, entering at 8 MPa and 450 °C and leaving at 50 kPa as
saturated vapor. If the power generated
by the turbine is 4 MW, determine the rate of entropy generation for this
process. Assumme
the surrounding medium is at 25 °C. (C&B) (S-dot)gen = 8.38 kJ/K s
4.
Steam enters an adiabatic nozzle at 3 MPa and 400 °C with a
velocity of 70 m/s and exits at 2 MPa and 320 m/s. If the nozzle has an inlet area of 7 cm2,
determine (a) the exit temperature and (b) the rate of entropy generation for
this process. (C&B)
T2 = 370.4 °C , (S-dot)gen = 0.0517 kJ/K s
"(C&A)"
indicates a problem taken from Y. A. çengel and M. A. Boles, "Thermodynamics – An
Engineering Approach," fifth edition.