How to increase the use efficiency of finned tube

What is a finned tube

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, to achieve the purpose of improving the heat exchange efficiency. Such a heat exchange tube is called a fin tube.

Fin tube types

The finned tube is divided into square finned tube, spiral finned tube, longitudinal finned tube, spiral serrated finned tube, and inner finned tube according to shape and structure;

The finned tube is divided according to the material: single metal finned tube, bimetallic composite finned tube;

According to different processing techniques, finned tubes are divided into: rolled finned tubes, welded and formed finned tubes, roll-formed finned tubes, and set-shaped finned tubes.

Fin tube advantages

1. Strong heat transfer ability: Compared with the light pipe, 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 structure: due to the increased heat transfer area per unit volume, the heat transfer capacity is enhanced. Compared with the smooth tube under the same thermal load, the finned tube heat exchanger has fewer tubes, and the diameter or height of the barrel can be reduced because of the compact structure and easy layout .

3. The materials can be used more effectively and reasonably: not only because of the compact structure and the reduced amount of materials, but also the possibility to flexibly select materials for heat transfer and process requirements, such as inlays or welded finned tubes made of different materials;

4. When the medium is heated, compared with the light pipe, the temperature of the finned tube wall under the same thermal load has been reduced, which is beneficial to reduce the high-temperature corrosion and overtemperature damage of the metal surface.

Regardless of whether the medium is heated or cooled, the heat transfer temperature difference is smaller than that of the smooth tube, which is beneficial to reduce the scaling of the outer surface of the tube. Another important reason for reducing fouling is that finned tubes do not form along the circumference or the axis like light tubes

The uniform overall scale layer, the scale piece 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, which will cause the scale to fall off by itself.

5. For phase transformation heat, the heat transfer coefficient or critical heat flux density can be used. The main disadvantages of finned tubes are high cost and large flow resistance. Due to the complicated process, the finned tube of the air cooler can cost 50-60% of the equipment cost; the large resistance leads to large power consumption. However, if properly shaped, it can reduce power consumption, which is cost-effective compared to the benefits of enhanced heat transfer.

Finned tube boiler application

As well-known boilers use finned tubes. The finned tubes of the boiler are mainly used for heat transfer. So whether the more finned tubes for the boiler is better, the answer is no.    

Heat exchange area of finned tube

When the heat transfer area of finned tube is doubled, the heat transfer coefficient cannot be doubled, but a discount is happend, generally (0.9—0.7), and the higher the fin, the higher will be the discount value, it even drops below 0.5. We can know that the higher the fin, the lower the efficiency of its use as above mentioned. Then the production cost of the finned tube for each boiler is increased, but its use effect is not increased. Obviously this is not cost-effective.

 Fin ratio of finned tube

 The design of Murphy’s laser welding stainless steel finned tube is mainly to improve the heat exchange efficiency. The finned tube usually adds   fins to the surface of the heat exchange tube to increase the external surface area (or inner Surface area), which is to increase the fin ratio.

 The finned ratio of the finned tube refers to the multiple of the surface area expansion of the surface of the light pipe after fins are installed.  

 That is the total external surface area of the finned tube divided by the external surface area of the original light tube. Generally, after adding   fins, the finization ratio of commonly used finned tubes is more than 10 times. To a large extent, the heat transfer area and heat transfer   efficiency are improved, and the bottleneck problem of the heat exchange effect is solved, that is, the product quality and efficiency are   improved, and the cost is saved.

The calculation method of the fining ratio of the Murphy laser welded stainless steel finned tube:

Calculated with a finned tube of Φ25*2-50/4/1. The size of the base tube is 25*2, the outer diameter of the fin is 50, the distance between the fins is 4mm, and the thickness of the fin is 1mm.

1. First calculate the original light pipe area, S1=1*3.14*0.025=0.0785㎡;

2. Calculate the total external surface area after adding fins. The total external surface area is divided into:

a, the light pipe area between the fins;

b, the top area of the fins;

c, the area of the two sides of the fin;

In the finned tube of this specification, the light tube area seen by a fin = (total length of finned tube-effective length of fin) *π * outer diameter of base tube = 0.0589㎡;

b. Top area of fin = effective length of fin * outer diameter of fin tube * π=0.03927㎡;

c. The area of the front and back sides of the fin = (the outer diameter of the fin?-the base tube of the fin?) *π*number/meter*2=0.7363㎡.

The total external surface area of the finned tube = a + b + c = 0.7363

0.03927+0.0589=0.8345㎡

3. Calculate the fining ratio of the finned tube. The fining ratio is equal to the total external surface area of the finned tube/the area of the original light tube=0.8345/0.0785=10.63.

It can be seen that after the addition of fins, the heat exchange effect of the finned tube is 10.63 times the original, which greatly improves the heat exchange effect of the heat exchanger.

Fin tube processing method

In this way, it can improve the purpose of heat exchange efficiency. The finned tube is a spiral groove with a certain width and depth pre-processed on the steel tube, and then the steel band is embedded on the steel tube on the lathe.

Through the spiral method, the finned tube has a certain pretension, so that the steel belt will be tightly tightened in the spiral groove, thus ensuring a certain contact area between the steel belt and the steel pipe. In order to prevent the springback of the steel belt, both ends of the steel belt are welded to the steel pipe. In order to facilitate inlaying, there should be a certain distance between the steel belt and the spiral groove. The side clearance, this inlay method, can bring better heating efficiency.