Modern Engineering Thermodynamics

Problems 201
32.* A commercial slide projector contains a 500. W lightbulb. The
bulb is to be air cooled. Determine the steady flow mass flow rate
of air required if it enters the projector at 0.101 MPa and 22.0°C,
and leaves the projector at 0.101 MPa and 50.0°C (neglect
changes in kinetic and potential energies). Assume the air is an
ideal gas.
33. Saturated liquid water at 70.0°F enters an aergonic device at a
rate of 1.00 lbm/s. Heat is transferred to the water so that it
exits the device as superheated steam at 100. psia and 600.°F.
Determine the steady state heat transfer rate (ignore kinetic and
potential energy effects).
34. Liquid water (ρ = 62.4 lbm/ft 3 ) enters one end of a 6.00 ft long,
1.00 in. diameter pipe with a uniform velocity. The entering
pressure and velocity are 20.0 psia and 1.00 ft/s, respectively.
Heat is added to the water as it flows down the pipe such that it
exits the pipe as a saturated vapor at 14.7 psia. Determine the
exit velocity.
35. Saturated liquid mercury at 100. psia enters an electrically heated
1.00 inch diameter horizontal pipe at a rate of 10.0 lbm/s with a
negligibly small velocity. What is the steady flow heat transfer in
Btu/s if the mercury exits the pipe at 80.0 psia as a saturated vapor
with a velocity of 500. ft/s?
36.* A straight, horizontal, constant-diameter pipe contains an
internal electrical heating coil (a resistance heater), and the
outside of the pipe is insulated. Water enters the pipe as a
saturated liquid at 0.500 kg/s and 0.200 MPa. How much
electrical power must be dissipated in the electrical heater
(in kilowatts) to produce saturated vapor at 1.00 MPa at the
outlet?
37. An engineer wants to make a steady state, steady flow steamcleaning
jet by wrapping an electric heater around a water pipe.
Water enters the pipe at 2.00 lbm/min as a slightly compressed
liquid at 50.0°F and exits the pipe as a jet of saturated vapor at
14.7 psia. If the electric heater is plugged into a standard 110. V
ac outlet, how much effective current does it draw? Ignore any
kinetic or potential energy effects.
38.* The proposed solar collector installation shown in Figure 6.26
has a frontal area (exposed to the sun) of 50.0 m 2 and
combines a thermoelectric generator with the air heating system
of a building. The thermoelectric generator produces dc power
FIGURE 6.26
Problem 38.
Q solar
Glass
cover
plate
Thermoelectric
generator
Electrical
power out
Air out
Air in
Building
air duct
with an efficiency of 5.00%. Solar radiation provides a net heat
transfer rate to the absorber plate of 1000. W/m 2 . The incoming
air is at 18.0°C, and to obtain “good” operating efficiency, the
exit air temperature must be maintained at 30.0°C. (a) How
many watts of dc power are produced by the generator, and
(b) what is the required air flow rate?
39. Determine the final temperature and the power required to
compress 10.0 ft 3 /s of air from 14.7 psia and 80.0°F to a state
where its specific volume is 2.84 ft 3 /lbm in a steady state, steady
flow process where pv 1.4 = constant. Assume ideal gas behavior.
40.* Find the power delivered by an adiabatic, isenthalpic turbine in
which the mass flow rate is 2.00 kg/s and the flow enters at
1667 m/s and leaves at 404 m/s.
41. Liquid nitrogen can be made by a simple adiabatic expansion
process through a turbine, in which 10.0 lbm/h N 2 enters the
turbine at 500. R and 2000. psia and leaves the turbine at 1.00
atm as a liquid-vapor mixture. If the turbine produces work at a
rate of 1500. Btu/h, what is the liquid nitrogen mass flow rate at
the exit of the turbine? Neglect kinetic and potential energy
effects.
42.* Determine the power required to compress a gas at a rate of
3.00 kg/s in a steady flow process from 0.100 MPa, 25.0°C to
0.200 MPa, 60.0°C. The specific enthalpy of the gas increases by
34.8 kJ/kg as it passes through the compressor, and the heat loss
rate from the compressor is 16.0 kJ/s. Neglect any changes in flow
stream kinetic and potential energies.
43.* Calculate the power required to compress air in a steady state,
steady flow process with no change in elevation at a rate of
2.00 kg/s from 0.101 MPa, 40.0°C, 10.0 m/s to 0.300 MPa,
50.0°C, at 125 m/s. During this process, the enthalpy of the air
increases by 40.15 kJ/kg, while 8.00 kJ/s of heat is lost to the
environment.
44. Mercury enters the steady flow, steady state, adiabatic turbine of
a starship warp drive system as a saturated vapor at 300. psia
and exits the turbine with a quality of 75.0% at 1.00 psia.
Determine
a. The mass flow rate of mercury required to produce 100. hp
of turbine output power.
b. The inlet flow area if the inlet velocity is 1.00 ft/s.
45.* A simple air conditioner can be made by isothermally
compressing air at atmospheric conditions of 0.101 MPa
and 20.0°C to 0.700 MPa then adiabatically expanding it
through a turbine back to its initial pressure. Determine the
turbine outlet temperature if the turbine produces 750. W of
power at an air flow rate of 0.100 kg/s. Assume ideal gas
behavior.
46. The water pump on the engine of an automobile has a mass
flow rate of 8.30 lbm/s. The water enters at 0.00 psig with a
velocity of 1.00 ft/s and leaves at 10.0 psig with a velocity of
10.0 ft/s with no change in height or temperature. Assuming
that the water is an incompressible liquid with a density of
62.4 lbm/ft 3 and the pump is adiabatic, determine the power
(in horsepower) required to drive the pump.
47. A 20.0 hp aircraft engine is used to supply air at a rate of 0.982
lbm/s to support the ground effect vehicle shown in Figure 6.27.
The vehicle has a support area of 50.0 ft 2 . Estimate the
maximum weight that this system can lift. Assume that the
environmental temperature and pressure are 80.0°F and
14.7 psia, respectively, and the process path is pv k = constant
(where k = c p /c v ).