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

Chapter 5 | 269

constant by an electric resistance heater placed in the tank.

Determine the electrical work done during this process.

5–140 A vertical piston–cylinder device initially contains

0.2 m^3 of air at 20°C. The mass of the piston is such that it

maintains a constant pressure of 300 kPa inside. Now a valve

connected to the cylinder is opened, and air is allowed to

escape until the volume inside the cylinder is decreased by

one-half. Heat transfer takes place during the process so that

the temperature of the air in the cylinder remains constant.

Determine (a) the amount of air that has left the cylinder and

(b) the amount of heat transfer. Answers:(a) 0.357 kg, (b) 0

5–141 A balloon initially contains 65 m^3 of helium gas at

atmospheric conditions of 100 kPa and 22°C. The balloon is

connected by a valve to a large reservoir that supplies helium

gas at 150 kPa and 25°C. Now the valve is opened, and helium

is allowed to enter the balloon until pressure equilibrium with

the helium at the supply line is reached. The material of the

balloon is such that its volume increases linearly with pressure.

If no heat transfer takes place during this process, determine

the final temperature in the balloon. Answer:334 K

5–136 A 0.2-m^3 rigid tank equipped with a pressure regula-
tor contains steam at 2 MPa and 300°C. The steam in the
tank is now heated. The regulator keeps the steam pressure
constant by letting out some steam, but the temperature
inside rises. Determine the amount of heat transferred when
the steam temperature reaches 500°C.

5–137 A 4-L pressure cooker has an operating pressure of
175 kPa. Initially, one-half of the volume is filled with liquid
and the other half with vapor. If it is desired that the pressure
cooker not run out of liquid water for 1 h, determine the
highest rate of heat transfer allowed.

5–142 An insulated vertical piston–cylinder device initially

contains 0.8 m^3 of refrigerant-134a at 1.2 MPa and 120°C. A

linear spring at this point applies full force to the piston. A

valve connected to the cylinder is now opened, and refrigerant

5–138 An insulated 0.08-m^3 tank contains helium at 2 MPa
and 80°C. A valve is now opened, allowing some helium to
escape. The valve is closed when one-half of the initial mass
has escaped. Determine the final temperature and pressure in
the tank. Answers:225 K, 637 kPa

5–139E An insulated 60-ft^3 rigid tank contains air at 75
psia and 120°F. A valve connected to the tank is now opened,
and air is allowed to escape until the pressure inside drops to
30 psia. The air temperature during this process is maintained

Q·in

V = 4 L

(P = 175 kPa)

FIGURE P5–137

We,in

AIR

V = 60 ft^3

P = 75 psia

T = 120 °F

FIGURE P5–139E

T 1 = 22°C

Pi = 150 kPa

Ti = 25°C

He

P 1 = 100 kPa

FIGURE P5–141

R-134a

0.8 m^3

1.2 MPa

120 °C

FIGURE P5–142