This invention relates to heat exchangers such as those used in power plants, and, more particularly, to reduction of temperature extremes experienced in rotating-wheel heat exchangers.
In most fossil-fuel fired power plants, the incoming air used in combustion of the fuel is preheated prior to its reaching the burner or combustor. The preheating is accomplished by transferring heat from the hot combustion gas leaving the burner to the cool incoming air entering the burner. The heat transfer has the dual benefits of reducing the fuel consumption of the power plant and also reducing the temperature of the combustion gas so that it may be further treated, as in an electrostatic precipitator, and then exhausted to the atmosphere.
A regenerative rotating-wheel type heat exchanger, also known as a L'jungstrom heat exchanger, is often used to accomplish the heat transfer from a continuous flow of the hot combustion gas to a continuous flow of the cool air. The heat exchanger is formed as a large wheel, often 30 feet or more in diameter, that rotates about its cylindrical axis. Large numbers of heat exchange elements, often provided as thin metal plates to optimize heat transfer, are held in baskets within the heat exchanger wheel. A fixed hot gas plenum passes hot combustion (flue) gas through one portion of the heat exchanger wheel and its heat exchange elements, and simultaneously a fixed cool gas plenum passes cool incoming air through another portion of the heat exchanger wheel and its heat exchange elements. Appropriate controls and seals are provided to regulate the flow of the gases.
The hot gas flow heats the heat exchange elements over which it passes. The rotation of the wheel moves these heated elements to the region through which the cool gas passes, and the cool gas is heated by passing over the heated elements. The rotation of the wheel continues, moving the cooled elements back into the heating zone. The process is continuous, removing heat from the combustion gas and transferring it to the incoming air. The configuration of the wheel and the heat exchange elements, and the rate of rotation of the wheel, are optimized to achieve a maximum heat transfer efficiency.
The wide variations in temperature experienced by the heat exchange elements can create problems in several ways. When the cold end heat exchange elements move back into the hot combustion gas stream, the sulfur-laden combustion gas may condense as sulfuric acid onto the heat exchange elements, causing them to corrode. The elements over which the cool air first passes (the "cold end" elements) are cooled to the lowest temperatures of any of the heat exchange elements, and are therefore the most prone to this problem. The hot end heat exchange elements that always operate at higher temperatures may be coated with catalysts to accelerate specific chemical reactions that reduce the concentration of pollutants in the combustion gas. These catalysts are operable only in certain temperature ranges characteristic of the catalyst materials. Excessive temperature variations during the cycles of rotation may prevent the catalysts from functioning efficiently.
Thus, while rotating wheel and other types of continuous regenerative heat exchangers are widely used, both in power plants and in many other industrial plants and operations, their performance in many cases is limited by the temperature variations of the heat exchange elements. There is a need for an approach to improving the performance of the heat exchangers, such as rotating wheel heat exchangers. The present invention fulfills this need, and further provides related advantages.