Modern Engineering Thermodynamics

9.6 Heat Exchangers 291
(T H )
(T H ) in
in
(T
(T H ) C ) out out
(TH ) out
(T C ) out
(T C )
(T C ) in
in
0 x
L
0 x
L
(a)
(b)
FIGURE 9.8
Temperature profiles inside single-tube, single-pass heat exchangers: (a) parallel flow; (b) counterflow.
TABLE 9.1 Typical Ranges for the Overall Heat Transfer Coefficient (U)
Fluids Used Btu/(h · ft 2 · R) W/(m 2 · K)
Water and
Water 200–250 1140−1420
Gasoline 60–100 340−570
Fuel oil 15–25 85−140
Compressed air 10–30 57−170
Steam and
Water (liquid) 250–400 1420−2270
Fuel oil (light) 60–90 340−510
Fuel oil (heavy) 15–25 85−140
Compressed air 5–50 28−280
Kerosene and
Water 25–50 140−280
Oil 20–35 110−200
and
Heat exchanger entropy production rate when both fluids are incompressible liquids
_S P = _m H c H ln T
out
+ _m C c C ln T
out
T in H
T in C
(9.25)
And, if both fluids are ideal gases with constant specific heats, then Eqs. (6.22) and (7.37) give
and
_m H c p ð
H T in − T out Þ H
= _m C c p ð
C T out − T in
Heat exchanger entropy production rate when both fluids are ideal gases
_S P = _m H c p
H ln T
out
− R H ln p
out
T in H
p in H
+ _m C c p C ln T
out
− R C ln p (9.26)
out
T in C
p in C
Þ C
EXAMPLE 9.5
A single-tube, single-pass heat exchanger is used to cool a compressed air flow of 0.200 kg/s from 90.0 to 75.0°C. The
cooling fluid is liquid water that enters the heat exchanger at 20.0°C and leaves at 40.0°C. If the overall heat transfer coefficient
is 140. W/(m 2 · K) and all flow streams have negligible pressure drop, determine the required heat exchanger area and
the entropy production rate for (a) parallel flow and (b) counterflow. Assume the compressed air behaves as an ideal gas
with constant specific heats.
(Continued )