Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

steam. Neglecting any heat losses from the outer surfaces of
the heater, determine (a) the ratio of the mass flow rates of
the extracted steam and the feedwater heater and (b) the
reversible work for this process per unit mass of the feed-
water. Assume the surroundings to be at 25°C.
Answers:(a) 0.247, (b) 63.5 kJ/kg

8–122 Reconsider Prob. 8–121. Using EES (or other)
software, investigate the effect of the state of
the steam at the inlet of the feedwater heater on the ratio of
mass flow rates and the reversible power. Assume the entropy
of the extracted steam is constant at the value for 1 MPa,
200°C and decrease the extracted steam pressure from 1 MPa
to 100 kPa. Plot both the ratio of the mass flow rates of the
extracted steam and the feedwater heater and the reversible
work for this process per unit mass of feedwater as functions
of the extraction pressure.

8–123 In order to cool 1 ton of water at 20°C in an insulated
tank, a person pours 80 kg of ice at 5°C into the water.
Determine (a) the final equilibrium temperature in the tank
and (b) the exergy destroyed during this process. The melting
temperature and the heat of fusion of ice at atmospheric pres-
sure are 0°C and 333.7 kJ/kg, respectively. Take T 0
20°C.

8–124 Consider a 12-L evacuated rigid bottle that is sur-
rounded by the atmosphere at 100 kPa and 17°C. A valve at
the neck of the bottle is now opened and the atmospheric air
is allowed to flow into the bottle. The air trapped in the bottle
eventually reaches thermal equilibrium with the atmosphere
as a result of heat transfer through the wall of the bottle. The
valve remains open during the process so that the trapped air
also reaches mechanical equilibrium with the atmosphere.
Determine the net heat transfer through the wall of the bottle
and the exergy destroyed during this filling process.

482 | Thermodynamics

8–126 Two constant-pressure devices, each filled with 30 kg

of air, have temperatures of 900 K and 300 K. A heat engine

placed between the two devices extracts heat from the high-

temperature device, produces work, and rejects heat to the low-

temperature device. Determine the maximum work that can be

produced by the heat engine and the final temperatures of the

devices. Assume constant specific heats at room temperature.

8–127 A 4-L pressure cooker has an operating pressure of

175 kPa. Initially, one-half of the volume is filled with liquid

water and the other half by water vapor. The cooker is now

placed on top of a 750-W electrical heating unit that is kept

on for 20 min. Assuming the surroundings to be at 25°C and

100 kPa, determine (a) the amount of water that remained in

the cooker and (b) the exergy destruction associated with the

12 L

Evacuated

100 kPa

17 °C

FIGURE P8–124

8–125 Two constant-volume tanks, each filled with 30 kg
of air, have temperatures of 900 K and 300 K. A heat engine
placed between the two tanks extracts heat from the high-
temperature tank, produces work, and rejects heat to the low-
temperature tank. Determine the maximum work that can be
produced by the heat engine and the final temperatures of the
tanks. Assume constant specific heats at room temperature.

4 L

175 kPa

750 W

FIGURE P8–127

AIR

30 kg

300 K

HE W

QH

QL

AIR

30 kg

900 K

FIGURE P8–125