Thermodynamics I

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Transcript Thermodynamics I

Ideal Gas Law
 Definition
 Pure substance
Compressibility
 p = ZRT
 Example
Compressibility
 Generalized chart
Ideal Gas Processes
 p/T relationships
 Polytropic processes: exponent
 Reversible process
Specific heat
 Energy to raise temperature of unit
mass one degree
 C = Q/ (mT)
SI (kJ/kg K)
USCS
(Btu/lbm R)
Water
4.22
1.00
Ethanol
2.46
0.588
Aluminum
0.797
.212
Brick
.79
0.189
Specific heat
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Gas has two values
Cv – constant volume process
Cp – constant pressure process
Internal energy as f(T)
Enthalpy as f(T)
k
R
Example
 System contains 2lbm water vapor at
300oF & 20 ft3. The system is
compressed isothermally to 9.05 ft3;
the work of compression is – 90.8
Btu. In the next process, the system
is heated isometrically to 120psi.
Kinetic and potential energy effects
are negligible. Find the heat
transfers in the two processes.
Example
 A gas undergoes three processes in
series returning to its initial state,
called a cycle.
 1  2 compression from 10 psi to 50psi.
Initial volume is 4 ft3. The process is described
by p = C.
 2  3 Isometric process; P1 = P3.
 3  1 Isobaric process.
 Find net work of the cycle.
Example
 Piston/cylinder assembly contains
1.4kg saturated water at 200oC.
Heat is transferred to the system until
only saturated vapor present, and the
volume has quadrupled.
 Find the final volume, temperature,
and pressure; find  U.
First law of thermodynamics
 Q – W = (m/2)(V22 – V12) + mg(z2 – z1 ) + m(u2 - u1)