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How to choose low finned tube?

Low finned tube description

Low finned tubes are generally formed by machining on the outer surface of the light tube with a certain height, a certain distance, and a certain thickness of ribs. Finned tubes are mostly used for heat exchange elements of condensers and evaporators of air conditioners, and low-finned tubes are often used in condensers.

It consists of a light pipe and fins “attached” to it. The structural parameters are mainly the inner diameter and outer diameter of the finned tube, the wall thickness of the finned tube, the fin pitch, the fin thickness and the fin height.

The low finned tube mainly relies on the outer rib of the tube (the ribging coefficient is 2 ~ 3) to expand the heat transfer area. Compared with the smooth tube, it has a larger surface area under the same metal material consumption. It is the first heat transfer enhancement from a straight view, but it is actually an increase in the heat transfer area and an increase in the heat transfer coefficient. The fins can make the heat transfer surface peel off the flow layer, and the heat transfer surface disturbance increases and improves The heat transfer effect strengthens the heat transfer twice.

The main factors affecting the enhanced heat transfer of the ribbed surface are the fin height, fin thickness, fin spacing, and thermal conductivity of the fin material. In addition, since one side of the heat transfer wall surface is expanded into a fin surface, the convective heat transfer on the smooth side and the heat conduction of the base wall all have a certain effect on the total heat transfer. The fin pitch size of the low-fin tube needs to be determined according to the surface tension of the liquid and the shear force on the liquid film generated by the flow.

Low finned tube application

The actual application proves that the low finned tube also has excellent anti-fouling performance, because the dirt is often formed along the edge of the wave crest, and the tube will expand and contract with the temperature change during operation. This “accordion” type The effect of expansion and contraction will prevent the formation of dirt. On the light pipe, the dirt will form a layer of cylinder on the wall of the pipe, without any natural mechanism to prevent the generation of dirt. 

Due to the lower fins, the cleaning method and difficulty of the low-fin tubes are exactly the same as for light tubes. In addition, the low-fin tube is made of the ordinary smooth tube as the blank and is processed by simple rolling. Its mechanical strength and corrosion resistance are no less than the original smooth tube blank, which can fully guarantee the long-term reliable operation of the heat exchanger.

Low finned tube performance parameters

There are two important parameters for low-fin tubes to describe their performance, namely the fining ratio β and the fin efficiency η. The fining ratio is expressed by “β”, and its definition can be derived from the formula: β = total external surface area of the finned tube / external surface area of the original smooth tube; the larger the value of β, the more the heat transfer area of the finned tube expands. 

The thermal performance is also enhanced. In the heat exchange process of the finned tube, assuming that the temperature of the fluid in the tube is higher than the temperature of the fluid outside the tube, the heat is transferred from the root of the fin through the tube wall along with the height of the fin through the heat conduction, and the fin also communicates with the surrounding fluid Convective heat transfer occurs, and eventually, the fin temperature gradually decreases along with the fin height.

 

Murphy low finned tube
Murphy low finned tubes

 

Manufacturing process of low finned tube

The low-fin tube is produced by the rolling method (three-roll oblique rolling). Its working principle is: the smooth tube is lined with a mandrel, the tube material is driven by the roller blade to make a spiral linear movement, and is rolled by the roller The hole pattern composed of the groove and the core rod is gradually processed with fins on its outer surface. In order to facilitate the formation of fins. 

The rolling parts adopt three stages of biting, rolling, and shaping during the deformation process, so that the processed fins are complete, smooth, and regular. The finned tube produced by this method is based on the base tube and The outer fin is an organic whole, so there is no problem of contact thermal resistance loss and electrical corrosion. It has good heat transfer efficiency and strong resistance to deformation.

Influencing factors of low finned tube performance

The structural parameters are mainly the inner diameter and outer diameter of the finned tube, the wall thickness of the finned tube, the fin pitch, the fin thickness, and the fin height, etc.

It is generally used in applications where the heat supply coefficient inside the tube is more than double the heat supply coefficient outside the tube. The most typical application is an oil heat exchanger. For condensation and boiling outside the tube, due to the effect of surface tension, it also has a good effect on strengthening heat transfer. Its processing has been industrialized and has been verified by many refineries.

(1) As far as the heat transfer effect is concerned, the primary and secondary relationship between the structural parameters of the low-fin tube is wing pitch → wing height → wing thickness, and the fin pitch is within 1 ~ 2 mm. The thermal performance increases with the increase of the wing pitch. When the wing pitch exceeds 2mm, the heat transfer performance decreases with the increase; the heat transfer performance decreases with the increase of the wing thickness and increases with the increase of the wing height.

(2) The pressure drop outside the finned tube is significantly affected by the height of the wing. The pressure drop increases geometrically with the increase of the wing height. The influence of the wing distance on the pressure drop is also obvious. The pressure drop increases with the wing distance. As it becomes larger, the pressure drop is hardly affected by the thickness of the wings.

(3) When the fluid flow rate inside and outside the tube increases, the heat transfer volume and pressure drop of the finned tube also increase. When the fluid flow rate outside the tube increases, the increase in pressure drop is significantly greater than the increase in heat transfer volume. When the flow rate increases, the pressure drop outside the tube remains unchanged, and the pressure drop inside the tube increases less

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