Patent Number: 051715180
Section: summary

BACKGROUND OF THE INVENTION The invention generally relates to a method for determining rates of heat transfer, and more particularly relates to a method for determining differences or changes in the rate of heat transfer through tube walls. In nuclear reactor steam generators, a gradual decrease in outlet pressure has been noted. This pressure loss has been attributed in part to fouling on the secondary side of the steam generator heat-transfer tubes. In order to determine whether it would be economically advantageous to remove these deposits from the tube walls, particularly from the outer walls of the tubes, an accurate method is needed to determine the change in the rate of heat transfer through a tube wall before and after it has been cleaned. It is well known that the rate of heat transfer through the wall of a tube can be determined by pumping hot water through the tube and measuring the quantity of heat that is transferred to a fluid outside the tube. However, this technique requires an expensive calorimeter, and can be both difficult and costly when the tube is radioactively contaminated, as the water that is pumped through the tube is likely to become radioactively contaminated. Other direct techniques that are known for measuring changes in heat transfer rates do not take into account the specific geometry of a steam generator tube. Removal of deposits from the surface of a tube will reduce the heat transfer area of the tube. For example, the removal of a 10 mil thick deposit from the outer surface of a 0.5 inch OD tube will reduce the outer surface area of the tube by 4%. Therefore, in order to determine the difference in the rate of heat transfer through a tube wall before and after cleaning, it is necessary to consider both the change in the heat transfer coefficient of the tube wall due to the removal of deposits, and any changes in the total heat transfer area of the surface of the tube that would result from the cleaning process. SUMMARY OF THE INVENTION An object of the invention is to provide an accurate method for determining changes in the rate of heat transfer through a tube wall that occur when deposits are formed on, or removed from, the inner and/or outer surfaces of the tube wall. Another object of the invention is to provide an inexpensive method for measuring changes in heat transfer rates through the wall of a heat exchanger tube. Another object of the invention is to provide a method for determining the differences in heat transfer rates through the walls of two or more different heat exchanger tubes. Yet another object of the invention is to provide a method for determining differences or changes in the rate of heat transfer through a nuclear reactor steam generator tube wall which does not involve significant problems of radioactive waste disposal. Other objects will be in part obvious and in part pointed out more in detail hereinafter. The present invention is a method for determining differences or changes in the heat transfer characteristics of tube walls, such as changes in heat transfer rates due to the removal of deposits which form on inner and/or outer surfaces of the tube wall. According to the method of the invention, a segment of tubing material is selected. An appropriate quantity of a thermally conductive material is supported on one of the inner side and outer side of the tubing segment in thermal contact with the wall. Two temperature measurement points T.sub.1 and T.sub.2 are chosen, each of which is outside the melting temperature range and vaporization temperature range of the thermally conductive material under the applicable pressure conditions, i.e. the pressure at which measurements are made according to the invention. A heating or cooling medium having an initial temperature T.sub.a is placed on the other of the inner side and outer side of the tubing segment in thermal contact with the wall. The temperature T.sub.a is appropriate to cause the temperature of the thermally conductive material to change from T.sub.1 to T.sub.2 as a result of heat transfer to or from the medium through the tubing segment wall. While the tubing segment is in thermal contact with both the thermally conductive material and the heating or cooling medium, the time required for the temperature of the thermally conductive material to change from T.sub.1 to T.sub.2 is determined. The tubing segment is subsequently removed from thermal contact with the heating or cooling medium. A tubing segment having different heat transfer characteristics than the original tubing segment is then obtained, either by altering the physical and/or heat transfer characteristics of the original tubing segment or by selecting an entirely different tubing segment. If not already present, an appropriate quantity of a thermally conductive material is supported on one of the inner and outer sides of the tubing segment wall in the manner described above. Two temperature measurement points T.sub.3 and T.sub.4 are chosen, each of which is outside the melting temperature range and vaporization temperature range of the thermally conductive material under the pressure conditions at which measurements are made according to the invention. The temperature measurement points are selected such that T.sub.3 &lt;T.sub.4 when T.sub.1 &lt;T.sub.2, and T.sub.3 &gt;T.sub.4 when T.sub.1 &gt;T.sub.2. A heating or cooling medium having a temperature T.sub.b appropriate to cause the temperature of the thermally conductive material to change from T.sub.3 to T.sub.4 is placed on the other of the inner and outer sides of the tubing segment in thermal contact with the wall. While the tubing segment is in thermal contact with both the thermally conductive material and the heating or cooling medium, the time required for the temperature of the thermally conductive material to change from T.sub.3 to T.sub.4 is determined. The times required for the temperature of the thermally conductive material to change from T.sub.1 to T.sub.2 and from T.sub.3 to T.sub.4 are then compared in order to determine the change in the rate of heat transfer through the wall of the tube as a result of the alteration process, taking into account any differences in T.sub.1 and T.sub.3, T.sub.2 and T.sub.4, T.sub.a and T.sub.b, and the nature of the mediums used to bring about temperature changes. Furthermore, when two different tubing segments are involved, the sizes and weights of the tubing segments, the weight of thermally conductive material in contact with each tubing segment, the positions of the thermally conductive material and heating or cooling medium relative to the tubing segment wall, and any other experimental variables also are considered. The method of the invention is particularly useful to determine the beneficial effect of chemically cleaning the outer surface of thin-walled tubing, such as steam generator tubing used in nuclear reactors. In order to obtain particularly accurate results, and to avoid having to normalize one set of data relative to the other, T.sub.1 =T.sub.3, T.sub.2 =T.sub.4, T.sub.a =T.sub.b, the same type and volume of heating or cooling medium is used for both measurements, and the medium is kept at a generally constant temperature while time measurements are made. The thermally conductive material preferably is on the same side of the tubing segment wall for each time measurement. In an advantageous embodiment, the thermally conductive material has a higher conductivity than the tubing segment. Multiple time measurements preferably are made between T.sub.1 and T.sub.2 and between T.sub.3 and T.sub.4 to confirm the accuracy of the results which are obtained. While the method of the invention is particularly useful for determining differences in heat transfer characteristics of tubing material, it also can be applied to walls of other types of structures. The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others as described in the following detailed disclosure.