For each process of a closed system, show the initial and final states on a prop

Question

For each process of a closed system, show the initial and final states on a properly-labeled T-s
diagram, evaluate the change in entropy, and apply the entropy balance and energy balances, as
necessary, to answer the specific questions. In each case, neglect changes in kinetic and potential
energy.

1. R-134a is compressed adiabatically from saturated vapor at 8C to 10 bar, 50C. Is this process
possible? Determine the work, in kJ per kg of R-134a. What is the minimum work input possible for
adiabatic compression from the same initial state to a final pressure of 10 bar?

2. One lb of water is contained in a 40-ft^2 tank, initially at 10 lbf/in^2
An electrical resistance element transfers energy by work of W = - 300 Btu until the final pressure is 20 lbf/in^2
Determine the energy transfer by heat, in Btu. Is the process possible? Reversible?

Explanation / Answer

1

For R-134 a properties, seehttp://www.irc.wisc.edu/properties/ (opens in IE)

At quality = 1 (sat.vapor) and T1 = 8 deg C, we get h1 = 255 kJ/kg and s1 = 927 J/kg-K

At P2 = 10 bar and T2 = 50 C, we get h2 = 283 kJ/kg, s2 = 953 J/kg-K

Since s2 > s1, process is possible.

Work = h2 - h1 = 28 kJ/kg

For min. work, s2 = s1 = 927 J/kg-K

At s2 = 927 J/kg-K and P2 = 10 bar, we get h2 = 275 kJ/kg

Min. work = h2 - h1 = 275-255 = 20 kJ/kg

2.

300 Btu = 300*778.2 ft-lb = 233450.8 ft-lb

Q = W + V*(P2 - P1)

= -233450.8 + 40*(20 - 10)*12^2

= -175850.8 lb-ft

= -226 Btu

Sine heat is lost (negative), the process is reversible.

Related Questions

Navigate

• Browse (All)
• Browse F
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.