Energy-Tech Magazine

Additionally, the impact of different materials should be reviewed with respect to expected levels of galvanic corrosion. A retube with different materials and lighter gauge tubes may meet the expected performance criteria, and additional scope may be required in order to mitigate the possible effects of tube vibration. The installation of tubes stakes, and/or additional drilled support plates, may also have to be considered to allow the condenser to operate properly with the different tube material. In addition, changing the tube materials, especially with a lighter (thinner) tube gauge may very well result in a reduction in the overall weight of the condenser, thereby possibly increasing the uplift on the foundations. A foundation analysis to determine the suitability of the existing foundation arrangements should therefore always be performed when the overall weight of a condenser will be altered due to retubing. Re-tubing can utilize the existing tube sheets if it is determined that this feasible. If not, replacement tube sheets (of preferably the same material as the tubes) are

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often supplied and installed. Nevertheless, re-tubing can never increase the available surface area, it can only ever improve the over heat transfer of the bundle, and as such, the scale of any possible improvements in the thermal performance are restricted and still may not meet the needs of a power uprate review.

Modular replacement: In some cases, due to the limitations of re-tubing, the entire removal of the existing tube bundle (tube sheets, tubes and support plates) and replacement with a newly designed and pre-fabricated module, is another option. The module replacement has the highest direct cost, but it typically provides the greatest return on investment due to the potential for significantly increasing the possible output. A modular replacement offers the ability to completely redesign the condenser tube bundle. Modules can be designed to match the existing dimensional parameters of the existing bundles (i.e. same tubesheet dimensions) so that the existing waterboxes can be reused. This limits the scale of the overall improvements in performance, but a review of alternate tube diameters, gauges and materials will often indicate that improvements can be made so that a condenser can meet the needs of a power uprate (which the existing condenser cannot). In some instances, the tube bundle and waterboxes will be replaced. Replacing the waterboxes offers the greatest opportunity for improvements in condenser performance, due to the fact that the modules can be physically larger than the original tubes bundles. Additional surface area can be significantly improved with an increase in bundle size, thereby improving the back-pressure to levels not previously possible. The viability of module replacement typically increases with the size of the unit, but the process is involved and the feasibility should be properly evaluated for power uprate projects. For the modular replacement, it will be necessary to have a proper understanding of the cooling water system and the cooling water circulating pumps curves.

This is required if the tube quantity and/or tube size differ from the original bundle, thereby affecting the bundle pressure drop. It is therefore necessary to properly understand the effects of this revised pressure drop across the module, and based on the existing circulating water pump curves, the subsequent effect on the overall cooling water flowrate based on the overall system pressure drop.