Finned tube is a kind of ribbed wall, in power, chemical industry and other industries have a wide range of applications, many spiral heat exchange surface or threaded pipe can also be regarded as finned tube. It has a significant effect on expanding the heat transfer area and promoting turbulence, whether it is single-relative flow heat transfer or phase change convective heat transfer. The structure of finned tube heat exchangers is basically the same as that of general shell and tube heat exchangers. Only the finned tube is used instead of the light tube as the heat transfer surface, because of the heat transfer enhancement and compact structure, it can be made into a compact heat exchanger; Finned tube heat exchangers are also often used to heat or cool gas outside the tube, and steam or water is passed inside the tube, such as air coolers, boiler economizers, radiators, etc.
First, the structure of the fin tube has two types of fins, longitudinal and radial (transverse), and other types are deformations of these two types, such as large spiral angle finned tubes, threaded tubes, etc., the former is close to longitudinal and the latter is close to transverse. The ribs can be in, outside or inside and outside the tube. Rib tubes can be divided into integral fins, welded fins and mechanical connection fins according to different manufacturing methods. Several finned tubes with longitudinal ribs and radial ribs are shown. Transverse vertical integral fin is made of casting, machining or rolling, ribs and tubes, no contact thermal resistance, high strength, thermal shock resistance and mechanical vibration, so heat transfer, mechanical and thermal expansion and other properties are better, but the manufacturing cost is increased, and it is more suitable for low fins; Welded fins are manufactured using processes such as brazing or argon arc welding, and modern welding techniques allow fins of different materials to be joined together with the parent tube and twisted into various shapes. Welded finned tubes have been widely used in industry because of their simple manufacturing, economical and good heat transfer performance and mechanical properties. The main problem of welding ribs is the quality of the welding process, the residue in the weld is not conducive to heat transfer and even causes fracture, and high-frequency welding is often used, and the effect is better; Mechanical connection Finned tubes are usually available in three types: winding, mounted, hot jacketed or expanded. The advantage of mechanical connection fin tube is economical, ribs and tube materials can be arbitrarily combined, the finning ratio can be as large as 30, its disadvantage is that the contact thermal resistance may be increased due to uneven expansion caused by loosening, so the working temperature of the winding type does not exceed 200~250 °C, the embedded heat resistance is better, often used in 250~350 °C occasions, but the manufacturing cost is high, the strength is low. Finned tubes have a wide range of materials, including carbon steel, stainless steel, aluminum and aluminum alloys, tin and copper alloys, titanium, Monel alloys, etc., and sometimes bimetallic fins are used to save precious metals, while adapting to corrosion resistance and other process requirements. The finned tube heat exchanger has no fins at both ends of the tube bundle and the outer diameter is large, so it can be welded or expanded with the tube sheet like the light tube, and a baffle wrench can also be installed if necessary, and a straight section without fins should be made at the baffle. Due to the wide application of finned tubes, diverse materials and manufacturing methods, industrially developed countries have been standardized and serialized, and have special research institutions and manufacturing plants.
Second, the advantages of finned tubes are mainly:
(1) Strong heat transfer capacity Compared with the light tube, the heat transfer area can be increased by 2~30 times, and the heat transfer coefficient can be increased by 1~2 times;
(2) Compact structureDue to the increase of heat transfer surface per unit volume, the heat transfer capacity is enhanced, and compared with the light tube under the same heat load, the finned tube heat exchanger tube has fewer tubes, and the diameter or height of the cylinder can be reduced, so the structure is compact and easy to arrange;
(3) It can make more efficient and reasonable use of materials not only because of the compact structure that reduces the amount of materials, but also makes it possible to flexibly select materials for heat transfer and process requirements, such as inlay or welded finned tubes made of different materials;
(4) When the medium is heated, compared with the light tube, the temperature of the wall of the finned tube under the same heat load is reduced, which is beneficial to reduce the high temperature corrosion and overtemperature damage of the metal surface. Whether the medium is heated or cooled, the heat transfer temperature difference is smaller than when the tube is light, which is beneficial to reduce the fouling on the outer surface of the tube. Another important reason for the reduction of fouling is that the fin tube will not form a uniform overall scale layer along the circumferential or axial direction like the light tube, and the scale formed along the surface of the fin and the tube will break at the root of the fin under the action of expansion and contraction, prompting the hard scale to fall off by itself;
(5) For the phase transformation heat, the heat transfer coefficient or critical heat flux density can be increased. The main disadvantages of finned tubes are high cost and large flow resistance. For example, due to the complex process, the cost of the fin tube of the air cooler reaches 50-60% of the equipment cost; High resistance, resulting in high power consumption. However, if properly shaped, power consumption can be reduced, which is more cost-effective than the benefits of enhanced heat transfer. The application of finned tubes radial finned tube surface area expansion degree is greater than longitudinal finned tube, industrial widely used. It has been clarified that the measures to strengthen convective heat transfer can effectively increase the heat transfer coefficient K when added to the side with weak heat exchange capacity, so the fins of the finned tube should generally be added to the side with a small heat transfer coefficient to be reasonable. The effect is more significant if the α value differs by more than 3 times. For example, air coolers, so the fins are mostly located on the gas side to compensate for the defect of low α value on the gas side, and when the α values on both sides are similar, it is suitable for adding twist iron and spiral spoilers inside and outside the inner and outer wing tubes. Under normal circumstances, the heat transfer coefficient on both sides is very different when the high fin is used, and the low fin internal thread pipe is very effective to prevent the heat transfer crisis in the tube; In view of the excellent anti-fouling ability of finned tubes, it is beneficial to heat exchange equipment such as reboilers with serious fouling conditions.
Third, the heat transfer calculation of the finned tube or rib wall heat transfer calculation includes the calculation of a single wing (rib) piece and the heat transfer calculation of the entire rib wall. The calculation of a single rib includes the temperature distribution along the rib height, the heat transfer and rib efficiency of the rib, the calculation of the heat transfer area, weight, price after ribification, and the determination of the shape and parameters of the rib, which should be verified according to the thermal stress generated by the rib temperature when determining the rib form and size. The heat transfer calculation of the entire rib wall is to calculate the heat transfer of the entire rib wall under the conditions of ribification and heat transfer coefficient, which does not directly involve the calculation of a single rib but only related to the overall structure of the rib and the tube, and the heat transfer calculation of the light tube wall is basically the same, the difference is only that the heat transfer area and heat transfer coefficient or heat transfer coefficient of the ribbed wall are not the same.
1. Heat transfer calculation of a single wing (rib) fin is characterized by the simultaneous presence of heat conduction of the fin and convective heat transfer with the surrounding medium.
(1) Straight ribs with constant thickness
(2) Straight ribs with variable thickness
(3) Round ribs with constant thickness
2. Efficiency and wing-to-wing ratio of wing (rib) sheets
(1) The efficiency of the fin (rib) piece is the ratio of the actual heat transfer of the fin tube to the heat transfer when the temperature of the rib is assumed to be at the rib temperature.
(2) The finning ratio is the ratio of the heat transfer area of the finned tube to the area of the light tube (without fins).
Fourth, the determination of relevant parameters in fin design
1. Rib height h As mentioned earlier, it is not advantageous to raise the fin under any conditions, and it can be theoretically proved that there is an optimal height for various shapes of fins. Experience shows that when the α values on both sides of the heat transfer wall differ by 2~5 times, the use of low-fin threaded pipe is more appropriate, and the cost is only 25~30% higher than that of the plain tube; When the difference in α values on both sides is more than ten times, high fins can be considered, and the heat transfer area of the fins is large.
2. Wing pitch If not a single fin is considered, but the entire fin tube, the smaller the wing pitch, the larger the heat transfer area on the wing side of the fin tube. However, under different flow rates, the wing distance should be guaranteed a few millimeters to tens of millimeters, so that the s value is greater than the sum of the boundary layers of the adjacent two wing surfaces, because the overlap of the boundary layer will not be conducive to convective heat exchange, so the general natural convection of the wing distance should be greater than the wing distance during forced convection, because the boundary layer of the latter is relatively book, for the longitudinal fin, the longitudinal length should not be too long, so as to avoid the development of the thickness of the laminar bottom layer thickened, so some designs use incoherent intermittent longitudinal wings, which prevents the development of the laminar flow bottom.
3. Wing thickness δ According to research, it is reasonable to maintain the following relationship between wing thickness δ and wing height: δ=2~4mm h=12~16 mm
5. Heat transfer calculation of the entire finned tube (or rib wall).
(1) Heat transfer equation When the heat transfer is stable, the calculation of the heat transfer and heat transfer coefficient of the finned tube can be in the same form as the heat transfer calculation of the light tube Q? KF ? T ? K ? F ?? In the T formula: K′ – represents the total heat transfer coefficient based on the total surface area of the tube, K – is the heat transfer coefficient based on the total surface area of the finned tube, ΔT – the effective average temperature difference between the fluid inside and outside the tube, F′ – the external surface area of the tube, F – the total external surface area of the finned tube.
(2) Calculation of heat transfer coefficient (based on F′) When the wall temperature and heat transfer coefficient are fixed, the heat transfer coefficient of the finned tube is completely consistent except for the thermal resistance of the finned tube.
(3) Empirical value of heat transfer coefficient of finned tube (based on F′)
(4) Pressure drop calculation The heat transfer coefficient of the finned tube air cooler is cooled by the cooling medium, fuel oil, light gasoline, light hydrocarbons, residual oil, tar, air (gas):P=3.43bar Air (gas) :P=6.87bar Hydrocarbon gas: P=1~3.43bar Mechanical jacket water Process water Heat transfer coefficient W/(m2°C) 116~159 337~395 430~535 58~116 29~58 58 116 169~227 67