Patent Number: 045308144
Section: summary

This invention relates to apparatus for superheating steam. In a conventional nuclear or non-nuclear steam power plant, heat supplied by a reactor core, combustion of fuel, or other means is used to raise the temperature of water until steam at saturation temperature is provided. This saturated steam is then superheated to a desired number of degrees of superheat by means of heat from the same source so that it is at a condition suitable for delivery to a steam turbine. After the steam has performed work as it is expanded through a high pressure turbine, it is generally desirable to deliver it to a low pressure turbine to further expand it as it performs additional work. However, in many such power plants, the steam leaving the high pressure turbine may be at a condition less suitable for delivery to the low pressure turbine without first removing moisture therefrom and reheating it. A moisture separator-reheater is commonly provided between a high pressure and a low pressure turbine to increase the plant efficiency and also protect the turbine blades of the low pressure turbine by reducing moisture in the steam which is to be expanded in the low pressure turbine. The reheater for a fossil power plant may be located within the steam generator and heat supplied by combustion of fuel in the steam generator is thus utilized to reheat the steam to a desired degree of superheat. Throttle steam or steam extracted from the high pressure turbine may also be used to reheat the main steam after it has been exhausted from the high pressure turbine. Likewise in a nuclear power plant, heat supplied by the reactor core may be used to reheat the steam before it is provided to a lower pressure turbine. Current pressurized water reactor steam cycles utilize modest amounts of superheat such as about 30.degree. to 60.degree. F. (17.degree. to 33.degree. C.) for steam delivered to the high pressure turbine and modest amounts of reheat such as about 100.degree. F. (56.degree. C.) of superheat for steam delivered to the low pressure turbine in order to improve heat rate for greater power output and reduced turbine blade maintenance which would otherwise be required of a greater amount of moisture in the steam. It is considered desirable to increase the number of degrees of superheat of steam delivered to the high pressure and low pressure turbines to further reduce moisture therein and to increase plant efficiency. Operators of nuclear power plants may sometimes find that use of electricity has increased over the years such that the power output thereof is less than the power required. Rather than constructing a new power plant, they have increasingly asked whether there is a way to increase the output of the plant in such situations especially when excess turbine generator capacity exists. Moisture separator-reheaters for steam power generation typically employ large cylindrical shells containing moisture separators and heat transfer tubes extending therethrough. The separators are typically of an inertial type and separate water from wet steam exhausted from the high pressure turbine. This steam is then directed to the heat exchange portion of the unit. The heat transfer tubes of the heat exchange portion may employ throttle steam and/or extraction steam to reheat the main steam flow. The water separated from the main steam is then drained at the bottom of the unit while the dried and reheated main steam is directed to the low pressure turbine. A common problem with horizontally disposed moisture separator-reheaters is unequal heat transfer and flow oscillations. The lowermost tubes of a tube bundle are subjected to a high temperature differential while tubes high in a tube bundle receive shell side flow which has already been partially heated by the lower tubes. As a result, the lower tubes may tend to accumulate water until they no longer carry steam along their entire length. Subcooling of the water in the lower tubes may then occur while steam may pass through the entire length of the tubes higher up in the bundle. Such occurrences may create an unstable condition which results in reduced overall heat transfer and potentially damaging cyclical thermal stresses on the tubes and tube sheets. One proposal for solving this problem has involved employing larger diameter tubes at the bottom of a tube bundle and smaller diameter tubes at the top thereof. Design of individual tubes sizes for such an arrangement is difficult and some tubes may be designed too large and other tubes may be designed too small resulting in perhaps even less flow stability. In addition, such an arrangement results in greater expenses for design and construction since varying tubes sizes are required. Another proposal directed to this problem provides in a water separator-superheater structure a plurality of horizontally disposed netting mattresses arranged in superposed stepped relation through which the main steam passes to be dried after which it passes through a single bank of heat exchange tubes which are disposed at an inclined angle in order to promote self-draining. An inlet header is provided at the upper end of the tubes and an outlet header is provided at the lower end of these tubes. Such an arrangement does not utilize the available space to provide heat transfer surface as effectively as may be desired. It is therefore desirable to more effectively utilize the available space within the shell of a moisture separator-reheater to provide a greater heat transfer area therein while also providing adequate means for separating moisture from the steam. Other proposals for solving this problem have included the use of inlet flow control orifices in each tube and the deliberate passage of excess steam through each tube. Although these methods are in some instances reasonably successful in controlling the tendency for tube flow oscillation, the flow orifices reduce steam pressure in the tubes and thus result in less efficient operation. In addition, the flow orifices cannot adjust to the effects of shell side flow changes caused by tube bowing, bundle bowing, or side baffle misalignment. On the other hand, the use of excess steam passage through each tube constitutes a power loss which is also undesirable. It is, therefore, an object of an aspect of the present invention to provide increased output for a nuclear power plant which has excess turbine-generator capacity. It is another object of an aspect of the present invention to provide a peak load or power upgrade device for nuclear power plants. It is a further object of an aspect of the present invention to increase the thermal efficiency of a nuclear power plant. It is still another object of an aspect of the present invention to provide a moisture separator-superheater which more efficiently utilized the heat provided in the reheating steam for more efficient operation thereof. It is yet another object of an aspect of the present invention to reduce the possibility of radiation contamination of such a superheater and of a separately fired vapor generator supplying vapor thereto. It is yet another object of an aspect of the present invention to provide such a superheater which more effectively utilizes the available space within the superheater for providing heat transfer surface. It is still another object of an aspect of the present invention to simplify and reduce the expense of fabrication of such a moisture separator-superheater. It is another object of an aspect of the present invention to reduce the power loss which would otherwise result from a large pressure drop of the main steam as it passes through a moisture separator-superheater. The above and other objects, features, and advantages of this invention will be apparent in the following detailed description of the preferred embodiments thereof which is to be read in connection with the accompanying drawings.