Heat Exchangers

The first heat exchanger, the one that heats the initial stream, uses a floating head vaporizer which manipulates a boiling liquid. The second and third heat exchangers drop a vapor into a partially condensed form. These are both floating head partial condensers, which can be used for heat exchanges featuring phase changes. The final heat exchanger that we deal with is the double pipe heat exchanger that reduces the temperature of the wastewater stream. Heat exchangers come in many forms, including double pipe, multiple pipe, fixed tube sheet, floating head, bayonet, kettle reboiler, scraped wall, and spiral tube exchangers. Each one has different sorts of operating conditions and optimal price/performance comparisons. For example, single pass double pipe heat exchangers can only be so long before the middle of the pipe starts to experience problems—other difficulties may arise in the alternatives.

Considerations for Choosing Exchanger Types

There are several types of exchangers readily available for use in today’s industrial processes, but choosing the correct one for the task is extremely important. The stream characteristics such as phases, corrosion and phase changes all play a part in exchanger design. The simplest of the exchanger types is the double pipe model. Underlying the design of this exchanger is two co-centric pipes with flows running countercurrent to each other. The advantage of this approach is that the exchanger is very inexpensive; however, its limitation to systems without phase changes makes it difficult to use throughout our optimization.

Instead, all but one of the exchangers used in new design is of the floating head shell-and-tube variety. This type of exchanger employs bundles of smaller diameter pipes inside a larger shell to efficiently increase the heat-transfer area. The floating head feature not only allows for easier cleaning, but also in cases of phase change, it eases the stress on the joints due to its flexibility (as compared with the fixed tube sheet design). Therefore, for all but the waste water exchanger, the other exchangers in the system are best handled by floating head shell and tube exchangers.