The Externally Fired Combined Cycle (EFCC) is a clean coal technology power generation cycle that has the potential to compete with the currently cheaper and cleaner gas fired cycles. The critical component in the EFCC is an Ultra High Temperature Heat Exchanger (UHTHE) at maximum temperatures in the region of 1500C^ . A conceptual design of the UHTHE utilises bayonet element ceramic tubes in order to eliminate thermal stresses in the tubes and to facilitate sealing between the tubes and the tubesheets. A computer code known as COHEX has been developed to model the heat transfer and fluid flow processes in the heat exchanger by calculating the shell-side and tube-side outlet conditions for given hardware geometry and inlet conditions. A 250 kW capacity laboratory scale heat exchanger was designed and built to carry out an array of tests for validation of !t.e COHEX code. Comparison of the results showed that COHEX performed well, to within – I 0%, at predicting trends as heat exchanger inputs were varied, which validates the fundamental theory used in the code. However, the inaccuracy was apparent in the prediction of outlet values. This has been attributed to assumptions and equations used in COHEX that were made on the basis of modelling larger scale heat exchangers; precipitating errors when modelling the laboratory scale heat exchanger used in this study. A study was undertaken to assess the feasible process intensification devices (PIDs) for use in the heat exchanger based upon current ceramic fabrication technology. The dimensions of annular ring type PID were optimised for heat transfer and pressure drop through a computational fluid dynamics (CFD) study. Results of this study were used to perform experimental tests using PIDs on the heat exchanger which showed an average (over the smooth tube case) of a 23% increase in heat transfer enhancement with a 132% increase in pressure drop across the whole bayonet tube. The 180 ^ annular bend within the bayonet tube end-cap was identified as generating complex fluid flows. Further particle image velocimetry (PIV) experimental and theoretical (CFD) studies have identified the existence of recirculation zones and vortex shedding at the inlet to the annulus. The fluid dynamics theory utilised in COHEX has been shown to be oversimplistic at modelling the pressure drop incurred in the end-cap. The study concludes that COHEX, on condition of further validatory tests, is a viable tool to be used in the design of large scale heat exchangers such as the UHTHE. Also, that simple annular ring PIDs significantly enhance the heat transfer between the shell-side and tube-side flows.