(133e) Enhanced Heat Transfer Technology Application in Crude Units for Saving Energy

A number of applications of enhanced heat transfer (EHT) have been identified that have enabled energy savings with attractive discounted cash flow (DCF) returns. Enhanced heat transfer technology is maturing with a significant increase in applications identified, or such applications are in the process of being installed, or are already installed. This paper discusses a systematic way to identify exchangers in a Crude Unit where application of appropriate enhanced heat transfer technology will result in an increase in the fired heater coil-inlet temperature and thus result in reduced energy consumption.

The heat transfer duty of an existing exchanger can be improved by increasing the UA value of the heat exchanger i.e. increase the overall heat transfer coefficient or increase the area of the heat exchanger or both.

Multiple technologies are available for heat transfer enhancement. These include enhancements to tube side, shell side, or both simultaneously in a shell and tube type heat exchanger. Inside the tube can be bulk mixing devices such as twisted tapes and coiled wires, or near-wall devices such as springs or fins. Outside of the tube can be fins or special baffle designs. Other configurations may twist the tubes or apply enhancement to both sides. Additionally other types of exchangers such as plate or spiral heat exchangers are also available.

For a given network topology, it is possible to generate a relation between the % increase in UA of an exchanger and its impact on coil inlet temperature (CIT) to the furnace. UA sensitivity tables can be generated using the UMIST SPRINT program. Comparing the UA sensitivities for all the exchangers will identify the candidate heat exchangers that can significantly increase the CIT by increasing the UA value of that exchanger.

For these heat exchangers, application of appropriate EHT technology can be screened by incorporating the appropriate heat transfer and pressure drop correlations in rigorous heat exchanger simulators.