8 kg of refrigerant 22, initially at 30 degrees Celsius is contained inside a cl

Question

8 kg of refrigerant 22, initially at 30 degrees Celsius is contained inside a closed rigid tank that has a pressure gage installed through the tank wall which reads 800 kPa. The Tank is fitted with a paddle wheel that transfers energy to the refrigerant at a constant rate of 0.3 kW. Heat transfer from the refrigerant to its surroundings occurs at a rate Kt, in kW, where the constant K is in kW per minute and t is time in minutes. After 30 minutes of stirring, the pressure gage reads 1100 kPa. No overall changes in kinetic or potential energy occur. For refrigerant 22, determine:

B) The heat transfer, in kJ

Explanation / Answer

At initial state : P1=800kPa , T1=30 C find u1and v1

At final state : P2=1100 kPa v2=v1 --> find T2 first and then u2

Apply first law at the tank

Q = m(u1-u2)   where Q = 0.3 - Kt      (m=8kg, t =30 min)

Solve for K

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