fin tube enclosures, finned, what is fin

Finned tube classification and specification

In order to improve the heat exchange efficiency, fins are usually added on the surface of the heat exchange tube to increase the outer surface area (or inner surface area) of the heat exchange tube, so as to achieve the purpose of improving the heat exchange efficiency. Such a heat exchange tube is called a finned tube. .

There are many types of finned tubes, and new ones are constantly emerging. Generally, it can be classified according to the following aspects:

1. Classification by processing technology

  1. rolled fin tube (extruded fin tube);
  2. Welding and forming finned tubes (high frequency welded finned tubes, submerged arc welded finned tubes);
  3. roll forming finned tube;
  4. set the finned tube;
  5. casting finned tube;
  6.   tension winding finned tube;
  7. insert tube.

2. Classification by fin shape

  1. Square finned tube;
  2. circular finned tube;
  3. spiral finned tube (spiral finned tube);
  4. Longitudinal Finned Tube
  5. corrugated finned tube;
  6. Helical Serrated Finned Tubes;
  7. needle finned tube;
  8. Tonghao overall plate finned tube (plate fin);
  9. inner finned tube. and many more.

3. According to whether the fin material of the finned tube is the same as that of the base tube, it can be divided into:

  1. single metal finned tube
  2. bimetal composite finned tube

Single metal finned tubes are classified by material

  1. copper finned tube;
  2. aluminum finned tube;
  3. carbon steel finned tube;
  4. stainless steel finned tube;
  5. cast iron (cast steel) finned tubes; etc.

Classification by use

  1. finned tubes for air conditioners;
  2. finned tube for air cooling;
  3. Boiler: finned tubes used for water wall, economizer and air preheater respectively;
  4. finned tubes for industrial waste heat recovery;
  5. other special purpose finned tubes; etc.

As a heat exchange element, finned tubes work in high temperature flue gas conditions for a long time. For example, finned tubes for boiler heat exchangers are used in harsh environments, high temperature and high pressure and in a corrosive atmosphere. This requires finned tubes to have high performance. index.

  1. Anti-corrosion performance (Anti-corrosion)
  2. Wear resistance (Anti-wear)
  3. Low contact resistance (lower contact resistance)
  4. Higher Stability
  5. Anti-fouling ability

High-frequency welded spiral finned tube is one of the most widely used spiral finned tubes. It is now widely used in electric power, metallurgy, cement industry, preheat recovery and petrochemical industries. While wrapping the steel pipe, the skin effect and proximity effect of the high-frequency current are used to heat the steel strip and the outer surface of the steel pipe until the plastic state or melting, and the welding is completed under a certain pressure of the wrapped steel strip. This high-frequency welding is actually a solid-phase welding. Compared with methods such as inlay, brazing (or integral hot-dip galvanizing), it is more advanced in terms of product quality (high welding rate of fins, up to 95%), productivity and automation.

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High frequency welding spiral finned tube specification
Tube diameterD:16~140mm
Tube wall thicknessS: 2~10mm
Tube lengthL:<25 m
Fin thicknessδ:0.5~4.0 mm
Fin heightH:5~30mm
Fin pitchT:3.5~40 mm
Tube materialCarbon steel, alloy steel, stainless steel
Fin materialCarbon steel, alloy steel, stainless steel
Fin typeSerrated fin

Spiral finned tube product specification and parameter table

Tube steel strip Steel pipe wrapping per meter finned tube/m
Size Weight 螺距 Size Weight(kg/m) Length Weight heat exchange area(m2)
(kg/m) (mm) (mm) (m) (kg)
25×2.5 1.39 5 1.2×12.5 0.118 23.56 2.77 0.686
25×2.5 1.39 8 1.2×12.5 0.118 14.73 1.73 0.458
32×3 2.15 8 1.2×12.5 0.118 17.48 2.06 0.549
32×3 2.15 8 1.2×15 0.141 18.46 2.61 0.668
32×3 2.15 10 1.2×12.5 0.118 13.98 1.65 0.459
32×3 2.15 10 1.2×15 0.141 14.77 2.09 0.555
38×3.5 2.98 8 1.2×12.5 0.118 19.83 2.34 0.627
38×3.5 2.98 8 1.2×15 0.141 20.81 2.94 0.758
38×3.5 2.98 10 1.2×12.5 0.118 15.87 1.87 0.525
38×3.5 2.98 10 1.2×15 0.141 16.65 2.35 0.630
42×3.5 3.32 8 1.2×12.5 0.118 21.40 2.52 0.679
42×3.5 3.32 8 1.2×15 0.141 22.38 3.16 0.818
42×3.5 3.32 8 1.2×20 0.188 24.35 4.59 1.125
42×3.5 3.32 10 1.2×12.5 0.118 17.12 2.02 0.569
42×3.5 3.32 10 1.2×15 0.141 17.91 2.53 0.680
42×3.5 3.32 10 1.2×20 0.188 19.48 3.67 0.926
51×3.5 4.10 8 1.2×12.5 0.118 24.94 2.94 0.795
51×3.5 4.10 8 1.2×15 0.141 25.92 3.66 0.952
51×3.5 4.10 8 1.2×20 0.188 27.88 5.25 1.294
51×3.5 4.10 10 1.2×12.5 0.118 19.95 2.35 0.668
51×3.5 4.10 10 1.2×15 0.141 20.73 2.93 0.794
51×3.5 4.10 10 1.2×20 0.188 22.31 4.20 1.068
51×3.5 4.10 12 1.2×12.5 0.118 16.62 1.96 0.584
51×3.5 4.10 12 1.2×15 0.141 17.28 2.44 0.688
51×3.5 4.10 12 1.2×20 0.188 18.59 3.50 0.916
Material: ordinary carbon steel, stainless steel, corrosion-resistant steel Base pipe: seamless steel pipe, high frequency electric welded steel pipe

Product display:

U-finned tube
High frequency welded finned tube

finned tube
seamless tube finned tube

Industrial Finned Tubes High Frequency
Welded Finned Tubes

Industrial Finned Tube Radiator
Composite Finned Tube

Introduction to the classification of fins and characteristics

Fins are the basic heat transfer elements, and their function is to expand the heat exchange area and improve the efficiency of heat transfer. The fin can be regarded as the extension and expansion of the partition; secondly, the different forms of the fin make the air form a strong turbulence in the flow channel, and make the flow boundary layer and the thermal boundary layer break and reorganize, thereby strengthening the heat exchange; Finally, the fins can also improve the overall strength of the radiator, effectively expanding its application range. Commonly used fin structures include straight fins, shutter fins, sawtooth fins, porous fins and corrugated fins [1].

fin tube enclosures, finned, what is fin

Typical fin structure

Many scholars have made extensive and in-depth research on fins. In this paper, the application environment of fins is used, according to the aspects of inside and outside the tube; heat exchange between liquids, heat exchange between liquid and gas, heat exchange between gas and gas, etc. The fins are classified and the characteristics of various fins are elaborated.

1. Structural form and characteristics of fins inside and outside the tube

In heat exchangers and many heat exchange equipment, the convective heat transfer coefficients of the fluids on both sides of the heat transfer wall are often very unbalanced, so it is necessary to install fins on the side of the heat transfer wall where the convective heat transfer coefficient is small.

There are two types of finned tubes used in finned tube heat exchangers: inner finned tubes and outer finned tubes, among which the external finned tubes are more commonly used. The outer finned tube is generally machined to form fins with a certain height, a certain distance and a certain thickness on the outer surface of the light tube. The types of finned tubes include spiral finned tubes, set finned tubes, rolled finned tubes, and plate-finned finned tubes .

Among them, the spiral finned tube is widely used in the occasions where liquid or gas-liquid two-phase working medium is inside the tube and gas is outside the tube. At the same time, due to its compact structure, the metal consumption is reduced, so the use of spiral tube bundle finned tube economizers in electric field boilers can greatly save operating costs, and has been rapidly popularized and applied at home and abroad .

In order to improve the shortcomings of the spiral-shaped finned tube, which is easy to accumulate dust and difficult to clean, the H-shaped finned tube has been proposed in recent years. H-type fin tube, also known as H-type fin tube, is to weld two steel sheets with arcs in the middle symmetrically with the light pipe to form fins (fins or butterflies), and the front shape is quite similar to the letter “” H”. Due to the special groove structure on the fin surface, part of the heat exchange area in the inlet and tail separation areas of the fin surface is removed, reducing the influence of the heat transfer deterioration of the inlet and tail separation areas on the heat transfer of the entire fin, thereby improving the The average convective heat transfer coefficient and fin efficiency of the fins are improved to achieve the purpose of enhancing heat transfer, and avoid the common problem of fin burning caused by unreasonable structural design of spiral fin tube bundles .

The experimental study on the convective heat transfer and resistance characteristics of corrugated inner finned tubes shows that the comprehensive performance of finned tubes is generally stronger than that of plain tubes. The finned tube structure is reasonably selected according to the diameter of the tube .

2. Structural form and characteristics of heat exchange fins between liquids

When there is forced convection of liquid inside and outside the tube, if the heat transfer coefficients on both sides of the heat exchange wall are large, it is not necessary to use finned tubes. For example, in a water/water heat exchanger, when using hot water to heat cold water, the heat transfer coefficient on both sides is high enough, and it is not necessary to use finned tubes, but in order to further enhance heat transfer, threaded tubes or corrugated tubes can be used instead of bare tubes; power plants Condenser, the water vapor is condensed outside the tube, the working medium is water, and the heat transfer coefficient on both sides is very high. Under normal circumstances, finned tubes are not required.

3. Structural form and characteristics of heat exchange fins between liquid and gas

When there is forced convection of gas outside the tube and forced convection of liquid working medium inside the tube, the heat transfer coefficient outside the tube is much smaller than that in the tube. Therefore, the heat transfer thermal resistance outside the tube becomes the main thermal resistance (also known as the control thermal resistance) that affects its total heat transfer. In this case, the enhancement of heat transfer in the space outside the tube is usually achieved by means of expanded surfaces, ie ribbed surfaces. Studies have shown that when 2<1, it is ineffective to design the heat transfer surface as a ribbed surface; when 2>5, the heat transfer surface can be designed as a ribbed surface to enhance heat transfer.

When there is forced convection of gas inside the tube and forced convection of liquid working medium outside the tube, the heat transfer coefficient inside the tube is much smaller than that outside the tube. Therefore, the heat transfer resistance in the tube becomes the main thermal resistance that affects its total heat transfer. In this case, it is necessary to increase the expansion surface in the tube, that is, rib or add a spoiler to increase the turbulent intensity and thus enhance the heat transfer performance on the gas side.

The ribbed surface can not only increase the total effective area participating in convective heat transfer and reduce the heat transfer resistance on this side, but also make the wall temperature on the rib side closer to the temperature of the fluid on the same side. Expanding the heating surface is an effective way to strengthen heat transfer. The application of expanding the surface is an important measure to reduce the volume of the heat exchanger, reduce the weight of the heat exchanger and improve the efficiency of the heat exchanger. Because of this, the research and design of extended heat transfer surface are increasingly widely used in industry.

The use of convective heating surfaces with expanded surfaces inside and outside the tube can increase heat transfer, save metal consumption, and reduce ventilation resistance and working fluid flow resistance. It has become the development direction of boiler and heat exchanger convective heating surfaces. more and more widely used. There are various types of extended surfaces, and circular, square and spiral finned tubes are often used in convection heating surfaces, finned tubes with longitudinal fins, and membrane convection heating surfaces composed of longitudinal finned tubes. Wait. Due to the different heating areas increased by various extended surfaces, the degree of disturbance to the fluid is also different, so their effects on heat transfer enhancement are also different.

In the study of strengthening the heat transfer on the surface of the fins, people have proposed various methods to strengthen the heat exchange, mainly including the following: First, to enhance the turbulence intensity on the air side, which can be achieved by continuously changing the direction of the airflow. For the purpose of heat exchange, the fins are mainly punched into a corrugated shape, resulting in a corrugated fin type. The second is to use the discontinuous fin surface to gradually disconnect the fin surface along the airflow direction to prevent the development of the air laminar boundary layer on the fin surface, so that the boundary layer is continuously destroyed on each surface, and a new one is formed in the next punch. The boundary layer is continuously used, and the leading edge effect of the punching strip is continuously used to achieve the purpose of strengthening heat exchange. There are slot-shaped fins and shutter-shaped fins that belong to this kind of fins .

Due to the simplicity and convenience of the structure and manufacture of the flat-fin heat exchanger, the durability of the use and its good applicability, so far, the heat exchangers (such as the ammonia-cooled fan evaporator, Surface air coolers, etc.) still widely use rectangular flat fins as extended surfaces. Rectangular flat fins have the advantages of simple and compact structure, good for defrosting, and easy manufacture. At the same time, because they only rely on increasing the heat transfer area to strengthen heat transfer, the heat transfer effect is poor, especially in the fluid phase change heat in the tube. In the heat exchanger where the air outside the tube is forced to flow heat, although fins are added on the air side, the thermal resistance is still the main thermal resistance in the entire heat transfer process .

Because the corrugated fins can increase the length of the air flow channel and can sufficiently mix the air flow, they are also widely used in air conditioning and refrigeration. The corrugated fins can change the direction of the airflow, greatly increase the air heat exchange area, and enhance the fluid disturbance. Due to the formation and separation of the vortex, the continuous development of the thermal boundary layer is thinned or destroyed, and its heat exchange characteristics are effectively improved. Strengthening also brings a larger resistance loss, but the increase in heat transfer is greater than the increase in resistance. Under wet conditions, the resistance of the slotted fins increases more, and the air volume of the system will decrease. At this time, a corrugated fin heat exchanger can be considered, and the fin spacing should not be too small .

Slotted fins have efficient heat transfer performance. When the fluid passes through the slotted fins, vortices first appear downstream, and as the Reynolds number increases, the vortex appearance point moves upstream. When the fin spacing is reduced, vortices begin to appear at smaller Reynolds numbers. This phenomenon shows that the heat transfer coefficient is enhanced due to the generation of vortices at a small fin spacing. For continuous corrugated fins, the slits help to eliminate lateral vortices and mix the fluid more fully, thereby improving the flow and heat transfer performance of the fins; at the corners and gaps of the slit fins, the local heat transfer coefficient changes The maximum value of the heat transfer coefficient occurs at the upstream corner or the leading edge of the fin, and the minimum value occurs at the downstream corner or the trailing edge of the fin [9]. Under dry conditions, try to use fins with a large heat transfer coefficient, such as slotted fins, but due to the large resistance of slotted fins, when the same heat exchange is required, try to use a larger windward area. instead of a larger number of rows to take full advantage of the enhanced fin without increasing its fan power.

The louver-shaped fin is a form of fin with a large heat transfer coefficient. When the fin heat exchanger needs to operate alternately under dry and wet conditions, a hydrophilic coating can be added to the surface of the fin, which is effective for heat transfer. The performance impact is minimal, but it can greatly reduce the air flow resistance in wet conditions, which is better for louver fins. In this case, louver-shaped fins can be used as much as possible.

4. Structural form and characteristics of heat exchange fins between gases

When there is forced convection of gas inside and outside the tube, if the heat transfer coefficient on both sides of the tube is very small, in order to enhance heat transfer, fins should be installed on both sides at the same time. If the structure is difficult, fins may not be added on both sides. In this case, if fins are only added on one side, there is no obvious effect on the increase of heat transfer.

For example, in a traditional tubular air preheater, the inside of the tube is forced convection of air, and the outside of the tube is forced convection of flue gas. It belongs to the convection heat transfer between gases, the heat transfer coefficient on both sides is very low, and it is difficult to add fins in the tube, so a bare tube can be used; in the heat pipe air preheater, although the flue gas is still used to heat the air, due to the Both flue gas and air flow outside the tube, so finned tubes can be conveniently used on both the flue gas side and the air side, which greatly increases the heat transfer.

5. Influence parameters of fins

Fin Height: Increasing the fin height will increase the external surface area, but this parameter is limited by the following factors. Fin height affects the fundamental parameter of mass flow, which affects heat transfer and pressure drop; manufacturing constraints for monolithic fins are much more limited than for sawtooth finned tubes; fin efficiency decreases, and again, monolithic fins The fins drop more severely than serrated fins; for fins with a height of less than 12 mm, integral helical fins are recommended. If other parameters are kept unchanged, and only the height of the fin is increased, the cost of the heat exchanger will first decrease, then remain unchanged, and finally increase again. offset by gas flow rate, lower fin efficiency, and lower fluid permeability.

Fin thickness: Smaller fin thickness can lead to higher fin density, but it also reduces fin efficiency and reduces structural stiffness. The minimum fin thickness is usually 0.9 mm, for example, when dealing with corrosive/lubricating fluids or high temperature fluids, thicker fins are required, up to 4.2 mm thick, and for small diameters, there will be some restrictions. The most commonly used fin thickness is 1.2 mm.

Fin density (spacing): In order to obtain the maximum external surface area per unit tube length, the highest allowable fin density needs to be used, but too high fin density brings problems such as excessive pressure drop, incomplete gas penetration, and increased dirt.

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