Civil Engineering Reference
In-Depth Information
Step 3
Calculate
Q
/
T
(W/K):
Δ
Q
/
Δ
T
¼
2,344,000/25
¼
93,700 W/K
Step 4
Calculate the exchange surface for different kinds of heat exchangers:
- From Table
15.4
(shell and tube exchanger):
U
¼
938 W/m
2
K (typical value)
T
)
¼
2,344
1,000/(938
25)
¼
100 m
2
- For plate exchanger
U
is assumed to be equal to 4,000 W/m
2
K
Area
¼
Q
/(
U
Δ
23.4 m
2
Area
¼
Q
/(
U
Δ
T
)
¼
2,344
1,000/(4,000
25)
¼
In both cases, if the parameter cost per unit of W/K is known, the cost of the heat
exchangers can be found:
C
(
Q
/
T
)
¼
cost of exchanger
Δ
Although costs are not here included, the plate exchanger, which has the smaller
surface, seems to be the more economical.
The energy saving is calculated with reference to a boiler plant which will heat
the cold water flow from 15 to 85
C as required from end users with an efficiency
equal to 85 % (lower heating value as reference; 41,860 kJ/kg of oil).
In the case of 2,000 h/year recovery, it follows that
ENERGY SAVING
¼
(2,344
3,600)
2,000/(41,860
0.85)
¼
474,320 kg
oil
/
year
¼
474.32 TOE/year.
The economic evaluation is shown in Table
19.4
.
Example 2
Cooling water-to-process water
exchanger
(effectiveness-NTU
method)
The case is similar to that in Example 1; outlet temperatures of both fluids are
supposed unknown (see effectiveness-NTU method in Sect.
15.4
).
Given temperatures:
110
C
T
1i
¼
15
C
T
2i
¼
Step 1
Estimate a value of U coefficient:
938 W/m
2
K (shell and tube)
U
¼
Step 2
Calculate the heat capacity flow rate of the two fluids:
m
1
c
1
¼
8
4,186
¼
33,488 W/K
m
2
c
2
¼
8
4,186
¼
33,488 W/K
