7.4.3 ENERGY CONSERVATIONTHROUGH OPTIMIZATION OF HEAT EXCHANGER DESIGN
Traditional design of heat exchangers involves many trials by changing one variable at a time and using a trial–error or a graphical method to meet design specifi cations.
The design variables may include Inside heat transfer coeffi cient
•
Outside heat transfer coeffi cient
•
Temperature difference
•
Tube surface area
•
With the combination of exergy analysis and life cycle analysis, an exergy optimiza-tion of a heat exchanger can be obtained. There is a trade-off between exergy saving during operation and exergy consumption during construction of a heat exchanger (Cornelissen and Hirs, 1999; Unuvar and Kargici, 2004).
7.4.4 ENERGY CONSERVATIONTHROUGH HEAT
EXCHANGER NETWORK RETROFIT
Process integration technology for improving energy effi ciency has been widely used around the world. The fi nancial benefi t comes from both reduced energy costs and debottlenecking for increase in throughput. It can also reduce fl ue gas emission. Sys-tematic methods for the design of heat exchanger networks have been developed (Wang et al., 1990; Silva and Zemp, 2000; Smith, 2000). The network retrofi t usually starts by identifying the bottlenecking exchangers within an existing heat exchanger network structure using thermodynamic methods. To overcome the network pinch, a modifi cation in the network structure is required by relocation of an existing heat exchanger to a new duty or addition of a new exchanger or change of stream splitting arrangement.
However, it is not straightforward to identify the most appropriate structural modifi cation. The relationship among heat transfer coeffi cient, pressure drop, and exchanger area is complex. The retrofi t area target should be implemented as a nonlinear optimization problem to minimize the requirement for additional area.
Mathematical models are usually needed to identify the most benefi cial structural changes (Smith, 2000). The minimum temperature difference in heat exchangers can be optimized, and trade-off between the capital costs and the energy saving revenue can be determined by mathematical models (Wang et al., 1990). In addition, besides the increased heat transfer coeffi cients, pressure drop constraints due to the addi-tional heat exchanger area during retrofi t of heat exchanger networks should also be considered (Silva and Zemp, 2000).
On one hand, heat transfer enhancement technology can increase the overall heat transfer coeffi cient of a heat exchanger, thus reducing the temperature difference required for a given heat exchange load and increasing the exergy effi ciency. On the other hand, fouling increases heat transfer resistance and decreases the overall heat transfer coeffi cient, thus increasing the temperature difference required for a given heat exchange load and decreasing the exergy effi ciency. Therefore, it is necessary to frequently remove the fouling layers on the metal surface of a heat exchanger. Energy can also be saved by retrofi tting the heat exchanger network.
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