ENGR 207 ELEMENTARY THERMODYNAMICS FALL 2005 QUIZ # 8A PANKAJ R. CHANDRA, Ph.D., P.E. A stream of refrigerant-134a at 1 MPa and 12 o C is mixed with another stream at 1 MPa and 60 o C. If the mass flow rate of the cold stream is twice that of the hot one, determine the temperature and the quality of the exit stream. Solution: m 1 +m 2 =m 3 =3m 2 & m 1 =2m 2 Q + W = m 3 h 3 - m 1 h 1 - m 2 h 2 (0) (0) m 1 h 1 + m 2 h 2 = m 3 h 3 2m 2 h 1 + m 2 h 2 = 3m 2 h 3 h 3 = ( 2h 1 + h 2 ) / 3 h 1 = h f @12 o C = 66.18 kJ/kg h 2 = h @ 1 MPa, 60 o C = 291.36 kJ/kg h 3 = (2 x 66.18 + 291.36) / 3 = 141.24 kJ/kg At 1 MPa , h f = 105.29 kJ/kg and h g = 267.97 kJ/kg. Thus the exit stream is a saturated mixture. T 3 = T sat @ 1 MPa = 39.39 o C x 3 = ( h 3 - h f ) / h fg = (141.24 – 105.29) / (267.97 – 105.29) = 0.221

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ENGR 207 ELEMENTARY THERMODYNAMICSFALL 2005QUIZ # 8APANKAJ R. CHANDRA, Ph.D., P.E.

A stream of refrigerant-134a at 1 MPa and 12oC is mixed with another stream at 1 MPa and 60oC. If the mass flow rate of the cold stream is twice that of the hot one, determine the temperature and the quality of the exit stream.

Solution:

m1+m2=m3=3m2 & m1=2m2

Q + W = m3h3 - m1h1 - m2h2

(0) (0)

m1h1 + m2h2 = m3h3

2m2h1 + m2h2 = 3m2h3

h3 = ( 2h1 + h2 ) / 3

h1 = hf @12oC = 66.18 kJ/kg

h2 = h @ 1 MPa, 60oC = 291.36 kJ/kg

h3 = (2 x 66.18 + 291.36) / 3 = 141.24 kJ/kg

At 1 MPa , hf = 105.29 kJ/kg and hg = 267.97 kJ/kg. Thus the exit stream is a saturated mixture.

T3 = Tsat @ 1 MPa = 39.39 o C