585
Ethylene glycol is cooled by water in a heat exchanger. The rate of heat transfer in
the heat exchanger and the mass flow rate of water are to be determined.
Assumptions
1
Steady operating conditions exist.
2
The heat exchanger is wellinsulated
so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid
is equal to the heat transfer to the cold fluid.
3
Changes in the kinetic and potential
energies of fluid streams are negligible.
4
Fluid properties are constant.
Properties
The specific heats of
water and ethylene glycol are
given to be 4.18 and 2.56 kJ/kg.
°
C,
respectively.
Analysis
(
a
)
We take the ethylene
glycol tubes as the system, which is a
control volume. The energy balance
for this steadyflow system can be
expressed in the rate form as
Then the rate of heat transfer becomes
(
b
) The rate of heat transfer from glycol must be equal to the rate of heat transfer to the
water. Then,
596
Two streams of water are mixed in an insulated chamber. The temperature of the
exit stream is to be determined.
Assumptions
1
This is a steadyflow process since there is no change with time.
2
Kinetic
and potential energy changes are negligible.
3
There are no work interactions.
4
The
device is adiabatic and thus heat transfer is negligible.
Properties
From the steam tables (Table A4),
h
1
2245
h
f
@ 90
°
C
= 377.04 kJ/kg
h
2
2245
h
f
@ 50
°
C
= 209.34 kJ/kg
Analysis
We take the mixing chamber as the system, which is a control volume since
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 Spring '11
 kkk
 Thermodynamics, Energy, Heat, Heat Transfer

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