Emerging Issues and Needs in Power Plant Cooling Systems

The majority of the electricity generated in the United States is produced by steamdriven turbine-generators. A very important step in this power generation process is the condensation of exhaust steam from the final, low-pressure turbine. When the steam condenses, the rapid decrease in vapor-to-liquid specific volumes creates a vacuum at the turbine outlet (monitored as turbine backpressure) that increases power generation efficiency. The conventional low-pressure steam turbine-generator can operate over a modest backpressure range (typically 1.0 to 5.0 or 5.5 in. Hga), but the design point for optimum efficiency is usually at the lower end of this range (2.0 to 3.5 in. Hga). Operating at backpressures greater than the design point reduces generation efficiency, and operating beyond a maximum backpressure limit is prohibited by warranty terms specified by the turbine manufacturer. Because lower turbine backpressures are achieved when the steam condensate temperatures are lower, designing and operating a cooling system that can consistently and continually remove the heat of condensation at those low temperatures is essential. Therefore, the cooling system should be considered an integral part of the power generation process that can have a major influence on the overall power plant performance and availability.