emodynamics Exam 1 2/2018 must be shown in order to earn partial credits. team a

Question

emodynamics Exam 1 2/2018 must be shown in order to earn partial credits. team at 125°C and 1 atm (state 1) is placed into a well insulated der-and-piston arrangement and heated to 200°C (state 2), with electricity. How much heat is absorbed per kg of steam, if the pressure outside the piston is that of 1 atm. (30 points) Hint: The absorbed heat increases the internal energy and does work as well against the atmosphere 2. Water at a pressure of 125 kPa and a temperature of 106°C enters a boiler where it is vaporized at a rate of 30 kg/min. )How much heat, in kJ/min, is needed? (25 points) i) If the heating is carried out by condensing saturated steam at 250 kPa, how much steam is needed? (10 points)

Explanation / Answer

1. given Ti = 125 C

Tf = 200 C

adiabatic process at constant pressure

hence

change in internal energy = m*C*dT

C = heat capacity of steam = 1996 J/kg K

m = 1 kg

hence

dT = Tf - Ti

hence

dU = 149700 J

work done by steam = PdV

dV = Vf - Vi

but

PV = nRT

Vf = nRTf/Po ( Po = 1 atm)

Vi = nRTi/Po

now, Vi = nR*(125 + 273.16)/1.01*10^5

for one kg steam, n = 1000/18 = 55.55 mol

hence

Vi = 1.81995 m^3

Vf = 2.162772 m^3

hence

W = 34625.06 J

hence

total jeat input = W + dU = 184325.065578 J

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