Large power plants, by which is meant electrical generating plants, typically include two or more operating units. Each operating unit includes a steam turbine and a source of steam where high pressure steam is admitted into the turbine inlet. The steam turbine assembly normally includes several pressure stages operating at successively lower pressures. The outlet of the lowest pressure stage exhausts into a condenser where the steam is condensed, producing a vacuum which is the discharge pressure of the last turbine stage. The condenser is an indirect heat exchanger where steam passes into one section of the heat exchanger while coolant, usually water, runs on the other side of the metal tubing separating the condensing steam from the coolant. There is accordingly a circulating water circuit where the water is heated in the condenser and cooled in a cooling tower or other heat sink.
The coolant, normally called circulating water, passes between the condenser, where it absorbs heat, and a cooling tower or heat exchanger, where it gives up heat. The condensed steam, normally called condensate, remains separate from the circulating water and is reheated to provide a source of steam to drive the turbine. There is accordingly a steam-condensate circuit where high pressure steam is made in a boiler, the pressure is reduced in the turbine and the steam is condensed in the condenser.
The efficiency of a power plant depends on many variables, one of which is the pressure in the condenser. Those skilled in the art will recognize that the difference between the steam pressure at the turbine inlet and the condensing pressure in the condenser is a measure of the potential work that can be accomplished.
Almost every generating plant ever built has one unit that is more efficient than the other. Almost universally, the more efficient unit is run much more than the less efficient unit. Thus, the more efficient unit will be run continuously, or longer and at higher loads than the less efficient unit, and the less efficient unit will be started, or run at higher loads, only when electrical demand cannot be met by the first unit or when maintenance or construction requires that the more efficient unit be shut down. Thus, the less efficient unit is used only in periods of peak demand or when maintenance or repair work is being done on the more efficient unit.
Disclosures of some interest relative to this invention are found in U.S. Pat. Nos. 3,881,548 and 4,291,537.