1. Synthesis of Heat Exchanger Networks
Part 6
Synthesis of Heat Exchanger Networks

2. 6.2 Simultaneous Synthesis
MINLP Model

3. Superstructure

4. Special Features
Objective function = annualized area cost + annualized fixed unit cost + utility cost.
Objective function is nonlinear, but constraints are linear.
Within each stage of the superstructure, potential exchangers between every pair of hot and cold streams can occur.
It is assumed that the outlets of the exchangers are isothermally mixed, which simplifies the calculation of the stream temperature for the next stage, since no information of flows is needed in the model!!!
The outlet temperatures of each stage are treated as variables.
The isothermal-mixing assumption can be relaxed.

5. Special Features
In most cases, it is OK to select the number of stages as the maximum of hot and cold streams.
The use of by-passes and split streams with 2 or more matches in each branches is not included in superstructure.
A major advantage of this model is its capability of handling constraints for forbidding stream splits.

6. Overall Heat Balance for Each Stream

7. Heat Balance at Each Stage
Note that no variables are required for the branch flows due to the isothermal mixing assumption!

8. Temperature Constraints

9. Hot and Cold Utility Loads

10. Logic Constraints

11. Temperature Approach

12. Objective Function

13. Example 3
Stream
Tin
(K)
Tout
Fcp
(kW/K)
Heat Load
(kW) h
(kW/m^2K)
Cost
($/kW-yr) H1
440
350 22
1980
2.0 - C1
349
430 20
1620 C2
320
368
7.5
360
0.67 S1
500
1.0
120 W1
300

14. Solution Obtained with Sequential Approach

16. Design without stream splits