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Unformatted text preview: be 1.391.
Then the final temperature of air becomes T2 (295 K)(8)1.391 1 665.2 K This will give an average temperature value of 480.1 K, which is sufficiently
close to the assumed value of 450 K. Therefore, it is not necessary to repeat the
calculations by using the k value at this average temperature.
The result obtained by assuming constant specific heats for this case is in
error by about 0.4 percent, which is rather small. This is not surprising since
the temperature change of air is relatively small (only a few hundred degrees)
and the specific heats of air vary almost linearly with temperature in this temperature range. EXAMPLE 7–11 Isentropic Compression of an Ideal Gas Helium gas is compressed in an adiabatic compressor from an initial state of
14 psia and 50°F to a final temperature of 320°F in a reversible manner. Determine the exit pressure of helium. SOLUTION A sketch of the system and the Ts diagram for the process are
given in Fig. 7–40. We note that the process is reversible and adiabatic.
Assumptions At specified conditions, helium can be treated as an ideal
gas since it is at a high temperature r...
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 Spring '09

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