Patent Number: 051715180
Section: description

DETAILED DESCRIPTION OF THE INVENTION According to the present invention, a segment of tubing having a first conductivity is removed from a heat exchanger. A thermally conductive material is supported on one of the inner side and the outer side of the tubing segment in thermal contact with the tubing segment wall. A thermocouple is placed in contact with a portion of the thermally conductive material which is not close to the tube wall. Temperature measurement points T.sub.1 and T.sub.2 that are not within the melting temperature range or vaporization temperature range of the thermally conductive material are selected. Preferably, T.sub.1 and T.sub.2 are selected such that the thermally conductive material is a melt at one of T.sub.1 and T.sub.2 and a solid at the other of T.sub.1 and T.sub.2. The tubing segment is brought to an initial temperature T.sub.i that is either higher or lower than both T.sub.1 and T.sub.2, and is kept at the initial temperature T.sub.i for a measured or known period of time. A heating or cooling medium having a temperture appropriate to bring the temperature of the thermally conductive material to T.sub.1, and subsequently to T.sub.2, is placed on the other of the inner and outer sides of the tubing segment in thermal contact with the tubing segment wall. Thermal contact includes direct physical contact, as well as indirect contact that is sufficiently close to provide a measurable amount of heat transfer between the substances. Preferably, the thermally conductive material and the heating or cooling medium are in direct physical contact with the tubing segment wall and are directly opposite each other, separated only by 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 portion of the thermally conductive material in contact with the thermocouple to change from T.sub.1 to T.sub.2 is measured. Optionally, the tubing segment is then removed from contact with the medium after a known amount of time and is placed in contact with another heating or cooling medium having a temperature T.sub.c which will effect a temperature change in the opposite direction, i.e. from T.sub.5 to T.sub.6. The time required for the thermally conductive material to undergo a temperature change in this direction also can be timed. Time measurements of the heating and/or cooling processes preferably are made repeatedly until consistent, reliable data has been obtained. A tubing segment having a wall with different physical and/or heat transfer characteristics than the wall of 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 a different tubing segment. If the original tubing segment is altered, the thermally conductive material and thermocouple preferably, although not necessarily, remain in contact with the tubing segment during the alteration process. If a different tubing segment is used, the thermally conductive material is disposed in thermal contact with the wall of the tubing segment in the same manner as for the original tubing segment, and a thermocouple is placed in contact with the thermally conductive material, away from the tube wall. The time required for the thermally conductive material to undergo a predetermined temperature change from T.sub.3 to T.sub.4 is then measured by contacting the other of the inner and outer wall of the tubing segment, i.e, the wall that is not in contact with the thermally conductive material, with an appropriate heating or cooling medium having a temperature T.sub.b and measuring the time required for the temperature of the thermally conductive material to change from T.sub.3 to T.sub.4 in the same manner as with the original tubing segment. Measurements optionally can be made of temperature changes in an opposite direction, i.e., from T.sub.7 to T.sub.8 after placing a medium having a temperature T.sub.d in thermal contact with the tubing segment or the side of the tubing segment opposite to the thermally conductive material. Time measurements preferably are repeated until consistent and reliable data have been obtained. The same temperature measurement points and heating and/or cooling mediums preferably are used before and after alteration of the tubing segment, i.e. T.sub.a =T.sub.b, T.sub.1 =T.sub.3, and T.sub.2 =T.sub.4, and when applicable, T.sub.c =T.sub.d, T.sub.5 =T.sub.7 and T.sub.6 =T.sub.8 The tubing segment or segments used according to the method of the invention can be formed from any conductive metal or non-metal tubing material. Because the method of the invention does not require calculations based upon tube geometry, the tubing segment or segments can have any shape, thickness and composition. The invention is particularly useful for comparing the heat transfer characteristics of various types of thin-walled metal tubing useful in and/or removed from nuclear reactor steam generators, such as used and unused or cleaned Inconel 600 (International Nickel Company) tubing. The thermally conductive material preferably has a higher conductivity than the tubing segment wall within the temperature ranges T.sub.1 -T.sub.2 and T.sub.3 -T.sub.4 in order that the limiting factor in the rate of temperature change will be the heat transfer coefficient of the wall rather than the heat transfer coefficient of the thermally conductive material. The thermally conductive material preferably melts and crystallizes within narrow temperature ranges between T.sub.1 and T.sub.2 and between T.sub.3 and T.sub.4, and has a melting point well below the melting point of the tubing material. For example, low melt metal alloys comprising bismuth and lead and which have melting ranges of less than one degree Fahrenheit are particularly useful. Cerrobend.RTM. (Cerro Metal Products Co.), which contains 50.00% bismuth, 26.70% lead, 13.30% tin and 10.00% cadmium has been used successfully, as it melts in a narrow temperature range well below the melting point of a heat exchanger tube, i.e. at 158.degree. F. The thermally conductive material is conveniently kept at about atmospheric pressure, although the method of the invention can be carried out with the thermally conductive material maintained at another pressure. The thermally conductive material preferably is in the form of a melt when it is placed in contact with the inner or outer wall of the tubing segment. The amount of thermally conductive material that is required will depend in part upon the desired accuracy of the measurements which are made. Preferably, when the thermally conductive material is placed inside the tubing segment, the inner side of the tubing segment is filled almost completely, leaving only a small air space (about 1-5%) to allow for expansion of the thermally conductive material. As a minimum, a sufficient amount of thermally conductive material must be used to ensure that the temperature-sensitive point on the thermocouple is always in contact with only the thermally conductive material and does not contact the tube wall. When the thermally conductive material is placed on the outer side of the tube segment, it is desirable to have a thickness of about 0.25 inch or more along the entire length of the tube. In order to measure the temperature of the thermally conductive material, a thermocouple is positioned in contact with a portion of the thermally conductive material. The thermocouple should not touch the wall of the tubing segment. When the thermally conductive material is placed inside the tubing segment, a particularly useful way for preventing the thermocouple from touching the tube wall is to place the thermocouple at the end of a wire which is bent in an S-shaped curve. In this manner, the thermocouple can be positioned near the center of the tube, with the curves of the "S" abutting opposite side walls of the tube. Preferably, the thermocouple is positioned as far as possible from the ends of the tube, as heat transfer through the seals on the ends of the tube can therefore be considered negligible. When the thermally conductive material is placed inside the tubing segment, the tube preferably is sealed in any manner that will prevent leakage into or out of the interior of the tubing segment. Proper sealing of the tubing segment is particularly important for situations in which the inner walls of the tube may have been in contact with radioactive material, as escape of radioactivity into the heating and cooling mediums is thereby prevented. The tubing segment can be sealed with welded caps, locking fittings, or other airtight and watertight means. When the thermally conductive material is positioned outside the tubing segment, it is preferable to place the thermally conductive material in a sealed enclosure which encompasses the outer wall of the tubing segment, as such an arrangement will allow for the thermally conductive material to be melted. However, it also is possible to use a thermally conductive material on the inner or outer side of the tubing segment which remains in a solid phase throughout the measurement process, and under such circumstances a sealed enclosure is not necessarily required, assuming that the thermocouple is in contact with only the thermally conductive material. In order to bring about temperature changes of the thermally conductive material, heating and/or cooling mediums such as liquid baths are prepared. While a single measurement of the time required for a temperature change between a pair of temperatures T.sub.1 and T.sub.2 can be made using a single medium having a temperature that is higher or lower than both T.sub.1 and T.sub.2, it is preferable to have at least two baths, e.g. a first bath at a temperature T.sub.a that is warmer than both T.sub.1 and T.sub.2 and a second bath at a temperature T.sub.c that is cooler than both T.sub.1 and T.sub.2. By transferring the tubing segment repeatedly between the first and second baths, temperature increases and/or decreases of the thermally conductive material can be measured repeatedly. Optionally, the warmer bath has a temperature above the melting point of the thermally conductive material and the cooler bath has a temperature below the melting point of the material. Furthermore, the tubing segment preferably is repeatedly transferred from one bath directly to the other bath at a consistent interval of time, e.g. every 5 minutes, in order to ensure that multiple measurements are made under identical conditions. While a variety of types of mediums can be used, including gases and liquids, water and air are preferred for economic reasons. Preferably, the heating and/or cooling baths are the same substance that the tube is in contact with during its normal use, as the amount of improvement in heat transfer rates resulting from tube alteration may depend upon the type of medium that is used. For example, when the surface of the tubing segment has pores, different mediums may enter the pores at different rates, thereby affecting heat transfer rates through the tube wall. The temperature measurement points T.sub.1 and T.sub.2 are temperatures at which the slope of the applicable temperature versus time heating or cooling curve for the thermally conductive material is relatively steep. For example, T.sub.1 and T.sub.2 preferably are points of maximum slope on a temperature versus time heating curve for the thermally conductive material when T.sub.1 &lt;T.sub.2 T.sub.1 and T.sub.2 preferably are points of maximum negative slope on a temperature versus time cooling curve for the thermally conductive material when T.sub.1 &gt;T.sub.2. When a conventional thermocouple is used, the thermally conductive material preferably will have a rate of temperature change of at least about 2-3 degrees per minute at the temperature measurement points, and more preferably at least 3.5 degrees per minute. The melting and vaporization temperatures of the thermally conductive material are generally not suitable measurement points, as the temperature of the thermally conductive material will change very little, if at all, during the phase change. The same temperature measurement points preferably are used in connection with the original tubing segment and the second tubing segment, as otherwise normalization of one set of data relative to the other may be necessary. Also, it is preferable to use the same heating and/or cooling mediums for measurements made before and after alteration of the tubing segment, in order to avoid having to normalize one set of data relative to the other. Furthermore, use of a generally constant-temperature heating or cooling medium is preferred in order to avoid having to take into account changes in the temperature of the medium, and differences in volume of the medium used for time measurements of tubing segments having different conductivities. It has been found useful in practice to make time measurements between one pair of temperatures, e.g., T.sub.1 and T.sub.2 during heating of the thermally conductive material, and another pair of temperatures, e.g., T.sub.5 and T.sub.6, during cooling of the thermally conductive material, as the temperatures at which the rate of change in temperature of the thermally conductive material is highest can be different during thermally conductive material heating and cooling processes. When Cerrobend.RTM. alloy is used, useful temperature measurement points when the thermally conductive material is heated are about 155.degree. F. and 170.degree. F. During a corresponding cooling process, useful temperature measurement points are 160.degree. F. and 147.degree. F. The method of the present invention can be carried out under boiling conditions, preferably using water as a cooling medium and using a hot oven as a heating medium. When measurement of changes in heat transfer rates at or near the boiling point of water are desired, T.sub.1 preferably is greater than 212.degree. F., T.sub.2 is greater or less than 212.degree. F., and the tubing segment preferably contains a thermally conductive material that melts at a temperature greater than 212.degree. F. The tubing segment can be heated in an oven to a temperature above 212.degree. F. and subsequently cooled in a bath having a temperature of less than 212.degree. F. According to one embodiment of the invention, alteration of the heat transfer characteristics of the tubing segment includes any type of change that will impact the overall heat transfer coefficient of the tubing segment and/or the heat transfer area of the tube wall. Preferably, tubing segment alteration constitutes removal of deposits from the wall of the tubing segment, such as by chemical cleaning techniques. While the tubing segment preferably is cleaned on at least the outer surface, the inner surface of the tubing segment also can be cleaned. It is preferable to avoid temporarily removing the thermally conductive material from the tubing segment for purposes of cleaning. Thus, in situations in which the outer surface of the tubing segment is to be cleaned, the thermally conductive material preferably is placed inside the tube. Conversely, when the inner surface of the tubing segment is to be cleaned, the thermally conductive material preferably contacts the outer side of the tube. In practicing the method of the invention, time measurements which are accurate to at least the nearest second usually will provide for a useful comparison of heat transfer rates before and after cleaning of the tube walls. The times that are measured are inversely proportional to the overall rate at which heat is conducted from the medium surrounding the tube, through the tube wall and into the thermally conductive material, or in the opposite direction. The time measurements obtained before and after alteration of the segment can be compared qualitatively and/or quantitatively in a variety of ways. In this connection, the percentage change in heat transfer rate due to chemical cleaning can be calculated by taking the difference between the heat transfer times before and after chemical cleaning as measured under identical conditions, dividing this difference by the heat transfer time before alteration of the tubing segment, and multiplying the result by 100. Having generally described the invention, the following examples are included for purposes of illustration so that the invention may be more readily understood and are in no way intended to limit the scope of the invention, unless otherwise specifically indicated. EXAMPLE 1 Filling of Tubing Segment; Selection of Temperature Measurement Points A straight segment of Inconel 600 (International Nickel Company) tubing having an outer diameter of 5/8", and a mass of 450.04 g was obtained. One end of the tube was sealed with a Swagelok.RTM. tube fitting (Crawford Fitting Co.) end cap through which a thermocouple was wired. The tip of the thermocouple was bent to an S-shape with the tip of the thermocouple firmly located in the center of the tube. The sealed end of the tube was placed in a hot water bath having a temperature of 200.+-.5.degree. F. About 40% of the tube was immersed in the hot water bath. Cerrobend.RTM. (Cerro Metal Products Co., Bellefonte, Pa.) alloy was heated to above 210.degree. F. and was poured into the tilted tube. The upper end of the tube was sealed, and the tube was heated in the hot water bath in a generally vertical position until the thermocouple indicated that the thermally conductive material was at a temperature of 180.degree. F. This heating process took about ten minutes. The tube was then lifted vertically from the bath and was air cooled at room temperature, thereby allowing a void to form at the top and allowing air bubbles to be removed from the melt. The tube was opened and a 3/4" void was observed at the upper end of the tube. This void space was desirable in order to allow for expansion or contraction of the Cerrobend.RTM. material. The tubing segment was then transferred repeatedly from a hot bath having a temperature of 190.degree.-200.degree. F. to a cooler bath having a temperature of about 149.degree. F., and measurements of the time required for the temperature of the thermally conductive material to change from 155.degree. F. to 151.degree. F. and from 155.degree. F. to 150.degree. F. were made. The following day, similar measurements were made as the tubing segment was alternately heated from 150.degree. F. to 175.degree. F. in a constant temperature hot bath at 192.2.degree. F. and cooled to room temperature. It was found that repeated measurements of heating time between 150.degree. F. and 175.degree. F. were consistent. Repeated measurements of cooling time lacked consistency, and it was concluded that the inconsistency was a result of subcooling during crystallization of the Cerrobend.RTM. material. It was further concluded that the most accurate results can be obtained when the temperature measurement points are temperatures at which the slope of the temperature-versus-time curve is steepest. The temperature-versus-time curve is relatively flat during a phase change. EXAMPLE 2 Measurement of Time Required for Temperature Change of Thermally Conductive Material Before Cleaning A U-tube 10, shown in FIG. 1, having a 5" radius of curvature was obtained. A wire thermocouple 12 was inserted in the tube, with the temperature-sensing end of the thermocouple extending to the bottom of the curve of the U. The thermocouple 12 was bent into an S-shaped curve in order to keep the temperature-sensing end away from the tube wall. The tube 10 was filled to within 1/2" of each end with melted Cerrobend.RTM. alloy having a temperature of about 210.degree. F. The tube was sealed. Wires 14 were affixed to each end of the tube and were connected to a horizontal immersion control rod 16. The thermocouple 12 was clamped to the rod to prevent the U-tube 10 from swinging from the wires 14. A Primeline Stripchart Recorder #EL195 (Esterline Corp.) (not shown) was calibrated and loaded with chart paper. A first water bath 18 was prepared and kept at a constant temperature of 144.2.degree. F., and an agitator 20 was arranged to heavily agitate the bath. A thermocouple 22 was placed in the bath to continually monitor its temperature. A second water bath was prepared and kept at a constant temperature of about 186.5.degree. F. with an agitator set for minimum agitation. The tube was placed in the first constant-temperature bath overnight in order to ensure that it reached equilibrium with the bath. The next day, the tube was placed in the second bath for about 15 minutes in order to melt the Cerrobend.RTM., and was subsequently cooled in the first bath for 5.0 minutes. Temperature measurement points were selected at which the rate of change in temperature of the thermally conductive material was found to be high when it was subjected experimentally to heating processes and cooling processes, i.e. 155.degree. F. and 170.degree. F. were selected for the heating process and 160.degree. F. and 147.degree. F. were selected for the cooling process. Temperature measurements were then taken as the tubing segment was repeatedly heated in the second bath from 155.degree. F. to 170.degree. F. and subsequently cooled in the first bath from 160.degree. F. to 147.degree. F. The tubing segment remained in each bath for 5 minutes before being transferred to the other bath. The heating and cooling time in seconds for each measured temperature range are provided on Tables 1 and 2 below, along with average times for the heating and cooling processes. Differences in the times required for heating the thermally conductive material and the times required for cooling the material were attributed to both the difference in .DELTA.T for the heating and cooling processes, and to the differences in the conductive properties of liquid metal and solid metal in the tube. EXAMPLE 3 Measurement of Time Required for Temperature Change of Thermally Conductive Material After Cleaning The U-shaped tube used in Example 2 was cleaned on a polishing wheel to remove the magnetite coating from its outer surface, and was buffed with a moist cloth. The tests conducted in Example 2 were then repeated, using the same constant-temperature water baths, temperature measurement points and Stripchart Recorder. The temperature of the first bath was maintained at 144.2.degree. F. The temperature of the second bath was maintained at 186.6.degree. F. Before time measurements were taken, the tube was warmed in the first bath for about 90 minutes and was then placed in the second bath for about 15 minutes in order to melt the Cerrobend.RTM.. The tubing segment was subsequently cooled in the first bath for five minutes. The measurements of temperature and time collected are shown on Tables 1 and 2 below. Calculations of the percent decrease in heat transfer time based both on (1) total average times, and (2) average times with the highest and lowest data points excluded, are also provided. As indicated in Tables 1 and 2, the percent decrease in heat-transfer time due to the chemical cleaning of the tube used in Examples 2-3 was between 5 and 6%, indicating that it would be economically beneficial to clean the steam generator tubes. Furthermore, in a power-limiting situation, cleaning the tubes could result in a 5-6% increase in power generation. Similar results were obtained for measurement taken of heating and cooling processes. As will be apparent to persons skilled in the art, various modifications and adaptations of the structure above described will become readily apparent without departure from the spirit and scope of the invention, the scope of which is defined in the appended claims. TABLE 1 ______________________________________ HEAT TRANSFER TIME FOR TEMPERATURE INCREASE OF U-SHAPED HEAT EXCHANGER TUBE Heating Time Test Number 155-170.degree. F. ______________________________________ Heat Transfer Time Before Cleaning 1 114 2 116 3 117 4 122 5 117.5 6 117 7 115.5 8 116 9 119 10 118 11 118 12 116 13 118.5 AVG 117.3 AVG w/o HI/LO 117.1 Heat Transfer Time After Cleaning 14 111.5 15 109 16 110 17 111 18 107 19 113 20 111 21 114 22 108 23 109.5 24 106 25 112 26 113 27 111 28 117 AVG 110.9 AVG w/o HI/LO 110.8 Decrease in Heat Transfer Time AVG 5.5 AVG w/o HI/LO 5.4 ______________________________________ TABLE 2 ______________________________________ HEAT TRANSFER TIME FOR TEMPERATURE DECREASE OF U-SHAPED HEAT EXCHANGER TUBE Cooling Time Test Number 160-147.degree. F. ______________________________________ Heat Transfer Time Before Cleaning 1 187 2 181 3 193 4 184 5 181 6 192 7 183 8 184 9 183.5 10 181 11 186 12 184 13 179.5 AVG 184.5 AVG w/o HI/LO 184.2 Heat Transfer Time After Cleaning 14 175 15 175 16 176 17 175.5 18 178 19 171 20 173.5 21 172 22 176 23 181 24 167 25 178 26 174 27 172 28 177 AVG 174.7 AVG w/o HI/LO 174.8 Decrease in Heat Transfer Time AVG 5.3 AVG w/o HI/LO 5.1 ______________________________________