The operating temperature of a heat pipe refers to the temperature of the working fluid inside the pipe or the temperature of the heat transfer medium outside the pipe?
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Newcomer’s question: Many publications specify the operating temperature ranges for heat pipes using different working fluids. For example, the operating temperature range for molten steel heat pipes is 30–250 °C. Does this temperature range refer to the reasonable temperature that the working fluid inside the pipe can withstand, or does it refer to the temperature of the heat medium outside the pipe? If it is the former case, is it possible to change the range of temperatures experienced outside the heating section of the heat pipe by adjusting the length ratio between the heating section and the heat dissipation section, as well as the heat transfer coefficients? For example, the operating temperature range for a molten steel heat pipe is 30–250 °C; I want to use it in an environment where the temperature of the heat source is much higher than 250 °C. Could the heating section of the heat pipe be made very short while the heat dissipation section is made very long, so that the heat pipe can function at higher heat source temperatures?
Three questions. First: The operating temperature range of 30–250 degrees for steel melt heat pipes refers to the temperature of the steam inside the pipe, that is, the temperature of the working fluid. The lowest value is determined by the startup performance of the heat pipe, while the highest value can be determined by considering the pressure of saturated steam at the highest temperature (when the superheated steam has reached its carrying limit), as well as using GB150 standards to calculate the pressure resistance of the pipe. Factors such as weld coefficients, manufacturing processes, thermal expansion, and linear stability must also be taken into account. Secondly, it is not a viable approach to use tubes on the cold side that are significantly longer than those on the hot side; the steam on the hot side may condense completely before reaching deep into the cold side, causing the extra length of tubes on the cold side to perform unnecessary work and increasing costs. During design, strive to make the heat flux on the hot side greater than that on the cold side. Third: The heat transfer coefficient is a value obtained through experimental testing, as well as an empirical value; each company has its own set of formulas for calculating it, and these formulas are not altered manually. Moreover, the heat transfer coefficient has a high degree of complexity; factors such as thermal resistance, mass flow rate, thermal conductivity, and the medium on the hot side all have a significant impact on it, making it difficult to control its value. If the temperature on the hot side is too high, causing the steam temperature inside the tube to exceed the allowable value, the heat flow regulation capability of the heat pipe can be considered. 1. Reduce the number of fins per unit length in high-temperature areas to decrease the heat exchange area, thereby reducing the heat flow rate, or use plain tubes instead. 2. Reducing the length of the hot-side tube is feasible, and the length ratio between the hot and cold sides needs to be considered comprehensively based on experience. 3. Increase the length-diameter ratio of the tube to reduce heat flow. 4. Appropriately reduce the external flow velocity of the hot-side tube without causing dust accumulation. 5. Use other heat exchange equipment in areas with higher temperatures. Heat pipes have their limitations, but they are highly practical; they can be used in a flexible, safe, and efficient manner across various industries. Their potential is enormous, and it awaits our efforts to explore and develop them.
There is another issue related to the molten steel heat pipe: if it is to be used in applications with higher temperatures on the hot side, can this be achieved by increasing the heat absorption capacity of the condensation section? For example, by lowering the temperature of the medium outside the heat pipes in the condensation section, or by appropriately increasing the length ratio between the hot and cold sides, as long as it is feasible for the heat pipes
It is theoretically feasible as long as the various limits of the heat pipe are not reached; it’s best to calculate these limits oneself