Pipe Size Analysis

An analysis is done to determine the effects that changing the internal pipe diameter has on the cold filling characteristics. The internal diameter of the pipe is decreased by 10 % and 20 % and also increased by 10 % and 20 % relative to the test case scenario. The five internal diameters tested are then 12.64 mm, 14.22 mm, 15.80 mm, 17.38 mm and 18.96 mm. The wall thickness is maintained for these simulations by adjusting the outer pipe diameters accordingly. A thinker tube wall may be required, but would not affect the heat transfer analysis due to the relatively high thermal conductivity of the steel. The critical inlet molten salt temperature for which the receiver tube just does not fully freeze is determined for each case. The molten salt outlet temperature at the maximum receiver length of 3.5 m at a time of 5 s is also determined. This combination will provide an indication of the effectiveness of the filling procedure for each pipe diameter. The internal pipe areas are also plotted for comparison with these two temperatures since the pipe area is a more useful comparison than the pipe diameter. Second order trend lines are included for each of the result sets to provide a better visual comparison of how a change in internal area affects the two temperatures. The resulting graph can be seen in Figure 6.8.

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Figure 6.8: Critical inlet molten salt temperatures, outlet molten salt temperatures and internal pipe areas with trend lines for five different

internal pipe diameters

From Figure 6.8 it can be seen that as the internal pipe area increases, the critical inlet temperature and the outlet temperature at 5 s both decrease. The outlet temperature is expected to decrease as the inlet temperature is decreased, which means that the internal pipe area does not necessarily affect the outlet temperature directly. By considering the trends, however, it can be seen that the outlet temperature trendline seems to start levelling out earlier than the inlet temperature trendline. It can, thus, be said that the outlet molten salt temperature will become independent of the internal pipe area if the internal pipe area is large enough. This is an indication that the internal pipe area has a direct effect on the outlet pipe temperature. The effect is that a large internal pipe diameter will result in a higher outlet molten salt temperature for a specific inlet temperature. From these observations, it can be concluded that a large internal area will improve the cold filling characteristics by increasing the outlet molten salt temperature. This is because the volume of salt in the pipe increases by a third order, while the pipe circumferential area is only increased by a second order. As a result, increasing the internal pipe area will result in more energy being carried through the pipe relative to the heat lost to the environment, resulting in a higher outlet temperature.

From Figure 6.8, it can also be seen that the critical inlet molten salt temperature decreases at a similar rate to the rate at which the internal pipe area increases. This can be said by noting that the gradients of these two properties’ trend lines are similar, but negatives of one another. The minimum required inlet molten salt temperature can, thus, be lowered by increasing the internal pipe diameter. The effect of increasing the internal pipe diameter on the critical inlet molten salt temperature does, however, decrease due to the second order nature of the trends. Increasing the area of a small pipe will result in a large decrease in critical inlet molten salt temperature, but increasing the area of a large pipe will result in only a small decrease in critical inlet molten salt temperature. From these observations, it can be concluded that a large internal pipe diameter will improve

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the cold filling characteristics by decreasing the critical inlet molten salt temperature.

Even though a large internal pipe diameter will result in favourable cold filling characteristics, the system as a whole should still be taken into consideration. If the internal pipe diameter is increased, the molten salt in the pipe will not be heated by the solar field to a temperature as high as it would have for a smaller diameter pipe. The advantages of increasing the internal pipe diameter for the cold filling characteristics also diminish as the internal area is increased. The relationship between the internal pipe diameter and cold filling characteristics should, therefore, not be treated in isolation. The relationship between the internal diameter and the outlet molten salt temperature while the solar field is active should also be considered, while also taking into account how these changes will affect the whole plant’s efficiency. In the past, only the amount of salt passed to the outlet of the receiver panel and the temperature at the outlet were considered as thermal optimization parameters to determine the receiver pipe sizes. If cold filling is considered as an operating method for a new plant, the effects of pipe size on the cold filling characteristics should also be considered as an optimization parameter during the receiver design.