Civil Engineering Reference
In-Depth Information
Fig. 15.6
Air exhaust gas crossflow exchanger. Operating data as for Example 2
Step 3
- The output temperature of fluid 1 is assumed to be
T
1o
¼
40
C; the effectiveness
is calculated as follows:
P
¼
(110
40)/(110
15)
¼
0.736
-
C
min
/
C
max
¼
1; from Fig.
15.5
it follows that NTU
max
¼
2.9
Step 4
Calculate the exchange surface and the outlet temperature of fluid 2:
NTU
¼
A
U
/
C
min
.
¼
2.9
33,488/938
¼
103 m
2
A
¼
NTU
C
min
/
U
P
¼
(
T
2o
15)/(110
15)
¼
(100
40)/(110
15)
¼
0.736
T
2o
¼
85
C
P
(110
15) + 15
¼
0.736
95 + 15
¼
Example 2 has the same energy saving as Example 1.
Example 3
Air-to-exhaust stream exchanger (log-mean method)
A process requires 3,720 Sm
3
/h of air at 145
C, which can be obtained by
heating air from the ambient temperature of 20
C to the desired value by means of
an exhaust gas stream. The exhaust is at 1,000
C and has a flow rate equal to
1,860 Sm
3
/h. The exchanger is a gas-to-gas type with a crossflow arrangement and a
preliminary evaluation of its surface is required (see Sect.
15.4
for the log-mean
temperature method).
Operating data at both sides of the heat exchanger are shown in Fig.
15.6
.
