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

Chapter 4 | 185

Analysis We take the contents of the tank as the system(Fig. 4–30). This is
a closed systemsince no mass crosses the system boundary during the
process. We observe that there is shaft work done on the system.

(a) The amount of paddle-wheel work done on the system is

Under the stated assumptions and observations, the energy balance on the
system can be expressed as

Net energy transfer Change in internal, kinetic,

by heat, work, and mass potential, etc., energies

As we pointed out earlier, the ideal-gas specific heats of monatomic gases
(helium being one of them) are constant. The cvvalue of helium is deter-
mined from Table A–2Eato be cv0.753 Btu/lbm · °F. Substituting this
and other known quantities into the above equation, we obtain

(b) The final pressure is determined from the ideal-gas relation

where V 1 and V 2 are identical and cancel out. Then the final pressure
becomes

Discussion Note that the pressure in the ideal-gas relation is always the
absolute pressure.

P 2 52.1 psia

50 psia

180  4602 R



P 2

1 102.5 4602 R

P 1 V 1

T 1



P 2 V 2

T 2

T 2 102.5°F

25.45 Btu 1 1.5 lbm 21 0.753 Btu>lbm#°F 21 T 2
80°F 2

Wsh,in¢Um 1 u 2 
u 12 mcv,avg 1 T 2 
T 12

Ein
Eout¬

¬

¢Esystem

WshW

#

sh¢t^1 0.02 hp^21 0.5 h2a

2545 Btu>h

1 hp

b25.45 Btu

He

1

P, psia

P 2 2

m = 1.5 lbm

50

T 1 = 80°F

P 1 = 50 psia

V 2 = V 1

Wsh

V

FIGURE 4 –30

Schematic and P-Vdiagram for

Example 4 –8.

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