6-55
6-78 A hot water stream is mixed with a cold water stream. For a specified mixture temperature, the mass
flow rate of cold water is to be determined.
Assumptions 1 Steady operating conditions exist. 2 The mixing chamber is well-insulated so that heat loss
to the surroundings is negligible. 3 Changes in the kinetic and potential energies of fluid streams are
negligible. 4 Fluid properties are constant. 5 There are no work interactions.
Properties Noting that T < Tsat @ 250 kPa = 127.41°C, the
water in all three streams exists as a compressed liquid,
which can be approximated as a saturated liquid at the
given temperature. Thus,
T1 = 80qC
·
m1 = 0.5 kg/s
h1 # hf @ 80qC = 335.02 kJ/kg
h2 # hf @ 20qC =
H2O
(P = 250 kPa)
T3 = 42qC
83.915 kJ/kg
h3 # hf @ 42qC = 175.90 kJ/kg
T2 = 20qC
·
m2
Analysis We take the mixing chamber as the system,
which is a control volume. The mass and energy
balances for this steady-flow system can be expressed
in the rate form as
Mass balance:
m in m out
'm systemÊ0 (steady)
0 
o m 1 m 2
m 3
Energy balance:
E E out
in
Rate of net energy transfer
by heat, work, and mass
E in
m 1h1 m 2 h2
'E system©0 (steady)
0
Rate of change in internal, kinetic,
potential, etc. energies
E out
m 3h3 (since Q
W
'ke # 'pe # 0)
2 gives
Combining the two relations and solving for m
m 1h1 m 2h2
2
m
m 1 m 2 h3
h1 h3
1
m
h3 h2
Substituting, the mass flow rate of cold water stream is determined to be
m 2
335.02 175.90 kJ/kg 0.5 kg/s 175.90 83.915 kJ/kg
0.865 kg/s
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