 Physics
1 April, 07:08

# Saturated water vapor at 200 kPa is condensed into a saturated liquid via a constant-pressure process inside of a piston-cylinder device. To complete this process, heat is removed from the system and transferred to the environment which is at a constant temperature of 90C. Is this process possible?

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Answers (1)
1. 1 April, 07:40
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The process is not possible

Explanation:

if we want to determine if the process is possible, we can check with the second law of thermodynamics

ΔS≥ ∫dQ/T

for a constant temperature process (condensation)

ΔS≥ 1/T ∫dQ

and from the first law of thermodynamics

ΔH = Q - ∫VdP, but P=constant → dP=0 → ∫VdP=0

Q=ΔH

then

ΔS≥ΔH/T

from steam tables

at P = constant = 200 Kpa → T = 120°C = 393 K

at P = constant → H vapor = 2201.5 kJ/kg, H liquid = 1.5302 kJ/kg

, S vapor = 7.1269 kJ/kg, S liquid 1.7022 kJ/kg

therefore

ΔH = H vapor - H liquid = 2201.5 kJ/kg - 1.5302 kJ/kg = 2199.9698 kJ/kg

ΔS = S vapor - S liquid = 7.1269 kJ/kg - 1.7022 kJ/kg = 5.4247 kJ/kg

therefore since

ΔS required = ΔH/T = 2199.9698 kJ/kg / (393 K) = 5.597 kJ/kg K

and

ΔS = 5.4247 kJ/kg ≤ ΔS required=5.597 kJ/kg K

the process is not possible
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