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The pipe that connects the expansion tank to the oil pipe, known as the expansion tube, and the point where it connects to the oil pipe is called the pressure constant-point. The pressure H at the constant-pressure point is the height difference (oil column) between the liquid level in the expansion tank and the constant-pressure point. Theoretically, regardless of the head of the circulating oil pump, the pressure at this point remains constant. If the pressure at this point increases, the oil in the system will flow into the expansion tank; as a result, less oil reaches the pump, and the pump’s pressure will drop in order to maintain the original pressure. So theoretically, the oil circulating in the system does not enter the expansion tank. It should be noted here that the heat transfer oil will not enter or exit the expansion tank through this pipe unless the entire heating system is functioning properly. In physics, heat is transferred through three mechanisms: conduction, convection, and thermal radiation. In this case, thermal radiation is not present in this issue. Regarding heat conduction, since the expansion tube has a certain length, the amount of heat conducted is quite small. Therefore, from a physical perspective, only the convection of the heat transfer oil can raise the temperature of the oil in the expansion tank. In a heat transfer oil heating system, convective heat transfer of the heat transfer oil is possible, as the expansion tube is arranged at an angle. The heat transfer oil in the lower part has a higher temperature and lower density, while the oil in the upper expansion tank has a lower temperature and higher density; this results in convection of the oil, causing the temperature of the oil in the expansion tank to rise. The larger the pipe diameter, the shorter its length, and the more vertical it is, the more heat is transferred by convection. Apart from the situations described in physics, what can raise the temperature of the oil in the expansion tank is the entry of heat-conducting oil into it. So there are three possibilities: issues with the quality of the heat transfer oil, problems during operation and use, or defects in the design and installation could cause the heat transfer oil to enter the expansion tank. Let’s start with the operational aspects. Under normal heating conditions, there is only one operation that can be performed on the expansion tank, and that is adding oil. No other operation involves releasing the hot oil from the system and then pumping it into the expansion tank. Therefore, normal heating operations should not cause the temperature of the oil in the expansion tank to rise. Based on the above analysis, there are three reasons for the increase in the temperature of the oil in the expansion tank: issues with the quality of the heat transfer oil; the poor quality of this oil, with a high content of low-volatility substances, which results in more volatile gases being carried along with the heat transfer oil as it enters the expansion tank. Convective heat transfer. Problems with unreasonable technical design.