Several issues in the process calculation of shell-and-tube finned tube heat exchangers

The heat exchange area generally refers to the medium that is to be processed; for example, in the case of a gas cooler, it is the heat exchange area on the gas side that matters. If the flow rate is given in standard cubic meters, no conversion is necessary – simply use the physical properties under standard conditions. The density is 1.293. In my opinion, the gas should flow outside the tubes, so the heat exchange area refers to the total area of the finned tubes. In the case of composite tubes, the heat exchange area is calculated per meter. For finned tubes, heat transfer calculations need to take into account the type of fins, and the formula for the heat transfer coefficient is derived from experimental data; experiments are required to determine this coefficient. Apart from the formula, most of the values can be calculated. Refer to principles of thermal engineering and heat transfer theory for more details

I think Hualongxu is an expert in this area; please ask Hualongxu to come and explain it to everyone.

The first issue is that when the gas flows along the outer fins, different formulas are used due to the variations in the parameters of these outer fins (such as wall thickness, spacing, and fin height). The design of outer fins is relatively well-developed; it is recommended to refer to Ha’s book on air cooler design, which is available on the forum. The second issue is that the heat transfer coefficient of the gas inside the tube can be determined using classical heat transfer formulas; however, due to the existence of different types of internal finned tubes, parameters such as the equivalent heat transfer diameter vary slightly, so there is no single exact formula for calculation. It can also be seen from this that for small fin channels, the equivalent diameter is, from a heat transfer perspective alone, the smaller the better ； However, it is necessary to take the equipment pressure drop into comprehensive consideration; therefore, the internal fin design is not always the best option – for example, the internal spiral fin tubes used in previous years. . . . In conventional gas-liquid heat exchange, the gas flow velocity is high; therefore, the design of the flow channels means that it is not possible to focus solely on achieving high heat transfer rates. Evaluations should be made based on criteria such as pump power and pressure drop energy consumption. The third question: essentially, this question is determined by what is meant by the equivalent pipe diameter when introducing a formula; generally, it is the diameter of the base pipe that is used. Fourth question: I don’t understand your intention. If you are calculating the initial thermal property parameters of the substance, what is the rationale behind using the operating pressure under different working conditions? One more thing: in your operating conditions, with such high gas pressure, internal fins are generally a viable option, after all, the issue of pressure resistance is relevant here. As for the advantage of improved heat transfer, it isn’t always evident in conditions with lower pressure.

Come and learn about it*. May I ask Hualongxu: Under what circumstances is it appropriate to use internal fins? (For gas-liquid heat transfer, gas-gas heat transfer, as well as flow rate and pressure need to be taken into consideration.)

It should be clear first that the mechanism by which internal fins and external fins enhance heat transfer is the same: the medium with a relatively low heat transfer coefficient (such as ordinary liquids, as conventional gases have a low heat transfer coefficient) is placed on the fin side. Therefore, most scenarios involving heat exchange between gases and liquids are suitable for the use of internal fins. However, there are exceptions for certain media; for instance, when saturated water vapor is used to heat water, or when the heat generated by the condensation of gaseous materials is used to heat heat transfer oil. Although these are both gas-liquid heat exchange scenarios, the heat transfer coefficient associated with condensation on the gas side is higher than that on the liquid side. In such cases, it is not very effective to use the fins on the gas side to enhance heat transfer. As for the flow rate and pressure mentioned, generally, a high gas flow rate requires a sufficiently large cross-sectional area as well as control of the pressure drop along the flow path within a reasonable range; in this regard, it is consistent with the design concept of conventional shell-and-tube heat exchangers. Therefore, flow velocity poses no limitation for use as internal fins. As for pressure, just like the external fins, it is also limited by its pressure resistance capacity, though the range is much wider; the pressure resistance range for the heat exchange tubes with internal fins is from vacuum to 40 BAR. For gas-to-gas heat exchange, when both sides have low heat transfer coefficients, it is common to enhance only one side by using fins on the inside or outside of the tube. In such cases, it is better to enhance both sides by using double fins on both the inside and outside of the tube ； I have worked on practical project implementations in this area, and the performance was not ideal; however, the thermal energy utilization efficiency of the gas materials on both sides was quite good. As an additional point, there is a very practical issue, namely the issue of equipment investment and returns. When comparing the investment costs of shell-and-tube heat exchangers using conventional tubes, the price of heat exchangers with internal finning is lower to achieve the same process parameters.

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