QUESTION 6 A small flexible bag contains 0.1 kg ammonia at 10 ? and 300 kPa The
ID: 707573 • Letter: Q
Question
QUESTION 6 A small flexible bag contains 0.1 kg ammonia at 10 ? and 300 kPa The bag material is such that the pressure inside varies linearly with the volume. The bag is left in the sun receiving 75 Watts of radiation, losing energy with an average 25 Watts to the ambient ground and air. After a while the bag is heated to 30C at which point the bag has an inside pressure of 1000 kPa. We would like to know the final volume, the work and heat transfer in the process and the time the whole process takes Vfinal-0.0132m3 WEC-8.484 kJ Q-129.8 kJ t 43.3 minExplanation / Answer
At initial state of - 10°C and 300 kPa
Specific volume
v1 = 0.001534 m3/kg
Internal energy U1 = 133.96 kJ/kg
After heating, state 2 at 30°C
Specific volume
v2 = 0.13206 m3/kg
Internal energy U2 = 1347.1 kJ/kg
Final Volume = mass x specific volume at final state 2
= 0.1 kg x 0.13206 m3/kg
= 0.013206 m3
Work done
W = 0.5 x (P1 + P2) x m x (v2 - v1)
W = 0.5 x (300 + 1000)kPa x 0.1kg x ( 0.13206 - 0.001534) m3/kg
= 8.484 kJ
Heat transfer
Q = m x (U2 - U1) + W
= 0.1 kg x (1347.1 - 133.96) kJ/kg + 8.484 kJ
= 129.8 kJ
= 129800 J
Net energy remains in the bag = energy in - energy out
Q net = 75 - 25 = 50 W
Time t = Q/Q net
= (129800 J x 1min/60s)/(50 J/s)
= 43.3 min
All values are taken from the ammonia tables.
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