Chapter 7 | 413opened, and one-half of the total mass is withdrawn from the
tank in the liquid form. Heat is transferred to water from a
source at 200°C so that the temperature in the tank remains
constant. Determine (a) the amount of heat transfer and
(b) the total entropy generation for this process.
7–147E An iron block of unknown mass at 185°F is
dropped into an insulated tank that contains 0.8 ft^3 of water at
70°F. At the same time, a paddle wheel driven by a 200-W
motor is activated to stir the water. Thermal equilibrium is
established after 10 min with a final temperature of 75°F.
Determine (a) the mass of the iron block and (b) the entropy
generated during this process.
7–148E Air enters a compressor at ambient conditions of
15 psia and 60°F with a low velocity and exits at 150 psia,
620°F, and 350 ft/s. The compressor is cooled by the ambient
air at 60°F at a rate of 1500 Btu/min. The power input to the
compressor is 400 hp. Determine (a) the mass flow rate of air
and (b) the rate of entropy generation.
7–149 Steam enters an adiabatic nozzle at 4 MPa and
450°C with a velocity of 70 m/s and exits at 3 MPa and 320
m/s. If the nozzle has an inlet area of 7 cm^2 , determine
(a) the exit temperature and (b) the rate of entropy generation
for this process. Answers:(a) 422.3°C, (b) 0.0361 kW/K
Special Topic: Reducing the Cost of Compressed Air
7–150 Compressed air is one of the key utilities in manufac-
turing facilities, and the total installed power of compressed-
air systems in the United States is estimated to be about 20
million horsepower. Assuming the compressors to operate at
full load during one-third of the time on average and the aver-
age motor efficiency to be 85 percent, determine how much
energy and money will be saved per year if the energy con-
sumed by compressors is reduced by 5 percent as a result of
implementing some conservation measures. Take the unit cost
of electricity to be $0.07/kWh.
7–151 The energy used to compress air in the United States
is estimated to exceed one-half quadrillion (0.5 1015 ) kJ
per year. It is also estimated that 10 to 40 percent of the com-
pressed air is lost through leaks. Assuming, on average, 20
percent of the compressed air is lost through air leaks and the
unit cost of electricity is $0.07/kWh, determine the amount
and cost of electricity wasted per year due to air leaks.
7–152 The compressed-air requirements of a plant at sea
level are being met by a 125-hp compressor that takes in air
at the local atmospheric pressure of 101.3 kPa and the aver-
age temperature of 15°C and compresses it to 900 kPa. An
investigation of the compressed-air system and the equipment
using the compressed air reveals that compressing the air to
750 kPa is sufficient for this plant. The compressor operates
3500 h/yr at 75 percent of the rated load and is driven by an
electric motor that has an efficiency of 88 percent. Taking the
price of electricity to be $0.085/kWh, determine the amount
of energy and money saved as a result of reducing the pres-
sure of the compressed air.
7–153 A 150-hp compressor in an industrial facility is
housed inside the production area where the average tempera-
ture during operating hours is 25°C. The average temperature
outdoors during the same hours is 10°C. The compressor
operates 4500 h/yr at 85 percent of rated load and is driven
by an electric motor that has an efficiency of 90 percent. Tak-
ing the price of electricity to be $0.07/kWh, determine the
amount of energy and money saved as a result of drawing
outside air to the compressor instead of using the inside air.
7–154 The compressed-air requirements of a plant are being
met by a 100-hp screw compressor that runs at full load dur-
ing 40 percent of the time and idles the rest of the time during
operating hours. The compressor consumes 35 percent of the
rated power when idling and 90 percent of the power when
compressing air. The annual operating hours of the facility are
3800 h, and the unit cost of electricity is $0.075/kWh.
It is determined that the compressed-air requirements of
the facility during 60 percent of the time can be met by a 25-
hp reciprocating compressor that consumes 95 percent of the
rated power when compressing air and no power when not
compressing air. It is estimated that the 25-hp compressor
runs 85 percent of the time. The efficiencies of the motors of
the large and the small compressors at or near full load are
0.90 and 0.88, respectively. The efficiency of the large motor
at 35 percent load is 0.82. Determine the amount of energy
and money saved as a result of switching to the 25-hp com-
pressor during 60 percent of the time.
7–155 The compressed-air requirements of a plant are
being met by a 125-hp screw compressor. The facility stops
production for one hour every day, including weekends, for
lunch break, but the compressor is kept operating. The com-
pressor consumes 35 percent of the rated power when idling,
and the unit cost of electricity is $0.09/kWh. Determine the
amount of energy and money saved per year as a result of
turning the compressor off during lunch break. Take the effi-
ciency of the motor at part load to be 84 percent.
7–156 The compressed-air requirements of a plant are met
by a 150-hp compressor equipped with an intercooler, an
aftercooler, and a refrigerated dryer. The plant operates 4800
h/yr, but the compressor is estimated to be compressing air
during only one-third of the operating hours, that is, 1600
hours a year. The compressor is either idling or is shut off the
rest of the time. Temperature measurements and calculations
indicate that 40 percent of the energy input to the compressor
is removed from the compressed air as heat in the aftercooler.
The COP of the refrigeration unit is 3.5, and the cost of elec-
tricity is $0.06/kWh. Determine the amount of the energy and
money saved per year as a result of cooling the compressed
air before it enters the refrigerated dryer.
7–157 The 1800-rpm, 150-hp motor of a compressor is
burned out and is to be replaced by either a standard motor
that has a full-load efficiency of 93.0 percent and costs $9031
or a high-efficiency motor that has an efficiency of 96.2 per-
cent and costs $10,942. The compressor operates 4368 h/yr at