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T-shaped finned tube|U bend low fin tube

T-shaped Finned Tube is an efficient heat exchange tube formed by the rolling process of a bare tube.

The T-shaped finned tube is an efficient heat exchange tube formed by the rolling process of a light tube. Its structural characteristic is the formation of a series of spiral annular T-shaped tunnels on the outer surface of the tube. When the outer medium is heated, a series of bubble nuclei are formed in the tunnels. Due to the heating state around the tunnel cavity, the bubble nuclei rapidly expand, filling the cavity. Continuous heating increases the pressure inside the bubble, causing it to rapidly eject from the surface of the tube through fine cracks. When the bubble is ejected, it carries a significant flushing force and generates a certain local negative pressure, causing surrounding lower-temperature liquid to flow into the T-shaped tunnel, forming continuous boiling. This boiling method, within a unit of time, takes away much more heat per unit surface area compared to a light tube, making this tube type have high boiling heat transfer capacity. The article provides detailed information on the research progress, working principles, characteristics, heat transfer mechanisms, and applications of T-shaped finned tubes.

T-shaped Finned Tube Table of Contents

1. Introduction

2. Research Progress of T-shaped Finned Tubes

3. Principles of T-shaped Finned Tubes

4. Characteristics of T-shaped Finned Tubes

5. Heat Transfer Mechanisms of T-shaped Finned Tubes

6. Applications of T-shaped Finned Tubes

7. Development and Application of T-shaped Finned Tube Reboilers

Introduction to T-shaped Finned Tubes

Since the invention of the T-shaped finned tube by the German company Wieland-Worke in 1978, scholars both domestically and internationally have started researching the enhanced heat transfer performance and mechanical processing of T-shaped finned tubes. In China, T-shaped finned tubes have been expanded to applications in the refining and petrochemical fields. Industrial application tests of reboilers have been conducted in the Changling Refinery and Chemical General Factorys alkylation unit, as well as in the desulfurization unit of Luoyang Chemical Engineering Company. Production and operation results show that compared to light tube reboilers, T-shaped finned tube reboilers not only save more than 30% of heat exchange area but also exhibit excellent operational flexibility. Under conditions of production overload by 33%, they maintain high heat transfer efficiency and stable operation.

Research Progress of T-shaped Finned Tubes

The T-shaped finned tube, first invented in West Germany in 1978 (known as Gewa-T tube or simply T tube), is one of the four main boiling enhanced surfaces internationally. It significantly improves the boiling heat transfer coefficient and critical heat load compared to light tubes. Its heat transfer performance is close to or surpasses E-tubes, and it has the advantage of easy processing compared to other enhanced surfaces, drawing attention from many researchers.

However, apart from Chongqing Universitys research on the boiling heat transfer of T-shaped flat surfaces processed by wire cutting, there have been no reports from other units on the development of T tubes domestically. Therefore, timely development and research on T tubes are necessary.

Research on T tubes reported so far has mainly focused on comparing the heat transfer performance of several enhanced surfaces. Experimental results show that the boiling heat transfer coefficient of T tubes is 2 to 5 times higher than that of light tubes. Regarding the enhanced heat transfer mechanism of the tube, preliminary discussions have been conducted by Stephan, K., and others, suggesting that the T-shaped structure of the fins restricts the effective escape of bubbles generated in the tunnels between the fins. This causes the bubbles to move upward along the tunnel during which they contact the inner wall of the tunnel more frequently, thus promoting heat transfer, known as the "contact length growth hypothesis." However, this is only a qualitative and intuitive hypothesis without further analysis of the movement of fluids in and out of the tunnels. Marco, P. J., and others experimental studies also indicate that the movement of vapor-liquid in and out of the tunnel greatly affects its heat transfer performance. Their experiments also found that, like other boiling surfaces, T tubes exhibit a significant temperature difference abnormality (boiling delay phenomenon) during the initial boiling. Clearly, the existence of a delay phenomenon will greatly affect the performance of the enhanced surface. A detailed study of the delay phenomenon can provide important reference data for the design and operation of enhanced tube heat exchangers.

Principle of T-shaped Finned Tubes

The T-shaped finned tube is an efficient heat exchange tube formed by the rolling process of a light tube. Its structural feature is the formation of a series of spiral annular T-shaped tunnels on the outer surface of the tube. When the outer medium is heated, a series of bubble nuclei form in the tunnels. As these nuclei are heated from all sides within the tunnel cavity, they rapidly expand, filling the cavity. Continuous heating causes a rapid increase in pressure inside the bubbles, prompting them to rapidly spray out through fine cracks on the tube surface. When the bubbles are ejected, they carry a significant flushing force and create a certain local negative pressure, causing surrounding lower-temperature liquid to flow into the T-shaped tunnel, forming continuous boiling. This boiling method takes away much more heat per unit surface area within a unit of time compared to a light tube. Therefore, this tube type has higher boiling heat transfer capability.

Characteristics of T-shaped Finned Tubes

1. Excellent heat transfer effectiveness. In R113 refrigerant, the boiling heat transfer coefficient of T tubes is 1.6-3.3 times higher than that of light tubes.

2. Unlike conventional plain tube heat exchangers, where the cold medium only starts boiling when the temperature exceeds the boiling point or bubble point of the hot medium by 12°C-15°C, T-shaped finned tube heat exchangers require only a temperature difference of 2°C-4°C for the cold medium to start boiling. The bubbling is fine, continuous, and rapid, presenting a unique advantage compared to light tubes.

3. Single-tube experiments with fluorine 11 as the medium show that the boiling heat transfer coefficient of T tubes can reach 10 times that of light tubes. Small-bundle experiments with liquid ammonia as the medium result in a total heat transfer coefficient 2.2 times that of light tubes. Industrial calibration of reboilers in C3 and C4 hydrocarbon separation towers shows that at low loads, the total heat transfer coefficient of T tubes is 50% higher than that of smooth tubes, and at high loads, it is 99% higher.

4. The price of T-shaped finned tubes for heat transfer is cheaper compared to aluminum porous surface heat transfer tubes.

5. Due to intense gas-liquid disturbance inside the tunnel and the high-speed ejection of gas along the T-shaped seam, both the inner surface of the T-shaped groove and the outer surface of the tube are not prone to fouling. This ensures long-term equipment use without the heat transfer effect being affected by fouling.

Heat Transfer Mechanism of T-shaped Finned Tubes

To explain the influence of the average opening width on the heat transfer performance of T tubes and the different boiling delay phenomena between T tubes and bare tubes, it is necessary to understand the boiling heat transfer mechanism of T tubes. As mentioned earlier, the key lies in the manner and flow conditions of vapor-liquid inside the tunnels during T tube boiling. Different observations of vapor-liquid movement inside the tunnels at various heat loads reveal the existence of both factors promoting contact length growth and factors challenging it.

At low heat loads, there is a clear presence of vapor-liquid columns inside the tunnels, with periodic upward and downward movements of the vapor-liquid column interface and detachment of vapor bubbles from the tunnels. During one cycle, due to the absorption of heat by the vapor-liquid film between the gas and solid in the tunnel, the liquid in the lower part is squeezed out of the tunnel, and the vapor column volume at the top of the tunnel gradually increases as the vapor-liquid column interface descends. When the vapor column pressure is sufficient to overcome the shape resistance escaping from the slit, bubbles rapidly escape from the slit, and the vapor enters the tunnel quickly, causing the vapor-liquid interface to rise, starting the next cycle. In this stage, heat transfer mainly occurs through the heat conduction of the thin liquid film between the gas and solid in the upper tunnel and the natural convection heat transfer of the liquid circulating in and out of the tunnel in the lower part.

As the heat load gradually increases, the rate of vapor production accelerates, and the vapor expansion needs to overcome the increased viscosity of the liquid inside the tunnel. The pressure of the vapor phase increases, making it easier for the vapor phase to escape from the tunnel. Therefore, it is also observed in experiments that as the heat load increases, the movement amplitude of the vapor-liquid column interface decreases, and the cycle shortens. When the heat load is relatively high, the periodic growth and detachment process of the vapor phase at the top of the tunnel cannot take away all the heat in time, leading to an increase in the temperature of the tunnel wall and liquid in the tunnel, resulting in the generation and detachment of vapor bubbles on the inner surface of the tunnel. These rising bubbles, as predicted by Stephan, K., and others, move upward along the tunnel due to buoyancy and shape resistance, some rising until they merge with the vapor column at the top, and some detaching from the sides of the tube before merging. Moreover, with higher heat loads, more bubbles quickly detach from the sides of the tunnel, gradually filling the tunnel with bubbles. As shown in Figure 8, the vapor-liquid column interface inside the tunnel gradually becomes difficult to distinguish. At this point, heat transfer mainly involves nucleate boiling inside the tunnel. It can be expected that with further increases in heat load, due to the large production, movement, and detachment of bubbles inside the tunnel, the filled bubbles will eventually merge into a continuous vapor phase, making it difficult for the liquid in the tunnel to maintain bubble boiling. Heat transfer then transforms into the evaporation of the thin liquid film inside the tunnel. When the heat load reaches a certain value, the speed of the liquid entering the tunnel is less than the evaporation speed, causing the inner wall to gradually dry up, leading to a boiling crisis or the transition to a film boiling or burning state on the tunnel surface.

Based on the experimental phenomena and analysis, the heat transfer inside the T tube tunnels can be divided into the following five different stages from low to high heat load:

1. Natural convection heat transfer before the generation of vapor bubbles.

2. Local film evaporation during the periodic growth and detachment of the vapor phase at the top of the tunnel and corresponding convection heat transfer during the liquid circulation in and out of the tunnel.

3. Nucleate boiling heat transfer inside the tunnel.

4. Film evaporation heat transfer on the inner wall of the tunnel.

5. Film boiling or burning heat transfer after the surface of the tunnel dries up.

Applications of T-shaped Finned Tubes

As long as the medium on the shell side is relatively clean, free of solid particles and colloids, T-shaped finned tubes can be used as heat exchange elements to form T-shaped finned tube heat exchangers, improving the boiling heat transfer efficiency on the shell side.

Development and Application of T-shaped Finned Tube Reboilers

The industrial test stand for the "T"-shaped finned tube reboiler is located at the gas fractionation depropanizer tower (Tower 1) bottom reboiler (Exchanger 2) of Sinopec Changle Branchs alkylation unit. The design heat load is 4600 GJ/h. The originally used FLa700-135-40-2 floating-head shell-and-tube reboiler had a heat transfer area of 135 m² with an actual heat transfer coefficient of only 250 W/(m²·K) considering a significant margin for the expected increase in processing capacity in the alkylation unit. When using the "T"-shaped finned tube reboiler, assuming the external boiling heat transfer coefficient of the "T"-shaped finned tube is three times that of a smooth tube, the calculated heat transfer area is 65 m². To provide additional margin, the F LB 700-95-40-2 "T"-shaped finned tube reboiler was selected, with a heat transfer area of 95 m². However, to utilize the existing equipment, only the tube bundle was replaced. The tube plate of F LB700 was modified to F LB700 reboiler tube plate, and the tube bundle was manufactured. The last 1 tube pass had 29 plugged tubes, and the 2nd tube pass had a total of 58 plugged tubes. The actual heat transfer area of the tube bundle is 90.5 m².

After the successful trial production of the T-shaped finned tube reboiler, it was installed in the bottom reboiler (Exchanger 2) of the alkylation units Tower 1 at Sinopec Changle Branch. Since its operation, the performance has been excellent, and its heat load has exceeded the design heat load, meeting production requirements.

T-shaped finned tube|U bend low fin tube

T-shaped finned tube|U bend low fin tube

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