Chapter 4-examples

# Chapter 4-examples - Example 1 Two rigid tanks with H2O are...

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Example 1: Two rigid tanks with H 2 O are connected by a valve. The valve is now opened until and the two tanks come to the same state at the temperature of surroundings at 25 o C. The the final pressure and the amount of heat transfer are to be determined. Tank A 0.2 m 3 400 kPa X=0.8 Tank B 200kPa 250 ° C 0.5 m 3 Step 1: determine the system: the content of both tanks consists the system; this choice allows for a) control mass analysis b) zero boundary work . Step 3: perform analysis using thermodynamics principles: 1 st law, ) ( 1 , 1 , 2 1 2 B B A A total net system out in u m u m u m U U U Q E E E + = = Δ = Δ = Step 4: identify all the unknowns, 1 , 1 , 2 , , , , , B B A A total u m u m u m Step 5: use specified conditions to determine all necessary states State 1: Tank A: P A =400 kPa Look up saturated table Æ v f =0.001084, v g =0.4625 m 3 /kg x=0.8 v A = v f + x v fg = 0.3702 m 3 /kg u A,1 =u f + x u fg = 2163.75 kJ/kg Tank B: P B =200 kPa, T B B B =250 o C Æ compare to saturated state T sat =250 o C , P sat =3.973 Mpa P B < P B sat Æ Tank B contains superheated vapor Æ look up table of superheated vapor v B = 1.1988 m3/kg, u B B,1 = 2713.2 kJ/kg Filling up details: m A = V A /v A,1 =0.2m 3 / 0.3702 m 3 /kg = 0.540 kg m B = V B B B /v B,1 =0.5m 3 / 1.1988 m 3 /kg = 0.417 kg m total = m A +m B = 0.957 kg B Next, we need to determine the final state T 2 =25 o C.We need 2 independent intensive properties.

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