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Fin tube in heat exchanger

Under normal circumstances, the spacing and height of the fin tube heat exchanger mainly affect the fining ratio, which has a great relationship with the film heat transfer coefficient of the medium inside and outside the tube. If the heat transfer coefficients of the inner and outer films of the tube differ greatly, a fin tube with a larger fin ratio should be selected, such as steam heating air. When there is a phase change in one side of the medium, the difference in heat transfer coefficient will be greater, such as the exchange of hot and cold air. 

When the hot air drops below the dew point, a fin tube heat exchanger can be used. In the case of heat exchange between air and air without phase change, or heat exchange between water and water, a bare tube is usually more suitable. Of course, low-finned tubes can also be used, because at this time, the heat supply coefficient is weak, and strengthening either side of them will have a certain effect. However, the effect of too large fin ratio is not obvious. The perfect situation is that the contact area inside and outside the tube is strengthened at the same time, and threaded or fluted tubes can be used.

The fin pitch mainly considers factors such as dust accumulation, dust formation, and easy cleaning, and at the same time, it must strictly comply with the equipment’s pressure drop requirements. When arranging, the distance between the tubes is not easy to be too large, generally more than 1mm is suitable for tube layout. In the process of heat exchange, when air flows through the finned tube heat exchanger, the front and back sides of the fin are mainly involved in heat exchange. 

There is only a small amount of radiant heat transfer in the middle of the two finned tubes, and the heat transfer effect is not obvious. Since this part has no fins and resistance, the air is easy to penetrate. In the process of air heating, the unheated cold air will neutralize with the heated hot air passing through the middle of the fins, which reduces the heat exchange effect. Compared with foreign finned tube heat exchangers, tubes The spacing is only 0.5mm larger than the outer diameter of the fins, which shows the importance of the tube spacing when the fin tubes are arranged.

The tube arrangement of the fin tube heat exchanger should be arranged as an equilateral triangle as far as possible. The hot air passing through the first row encounters resistance in the second row and there is a certain rebound wind, so that the 360° heat exchange of the entire fin tube does not have a dead angle. Therefore, the isosceles triangle arrangement should be avoided, and the square arrangement should not be used as much as possible, unless there are special requirements.

The pressure drop on the air side is a very important parameter in the design, which has a great relationship with the arrangement of the fin tube. When designing the fin tube arrangement, the ratio of the narrow gap flow surface and the windward surface should be calculated, so as to calculate according to the front wind speed The air mass flow rate is calculated, and the friction coefficient is calculated for the dynamic viscosity of air at different temperatures

Murphy fin tube for fin tube heat exchanger

What is the effect of fin thickness on the performance of fin tube heat exchanger

Fin tube heat exchanger is a commonly used heat exchanger structure in air conditioners. In the air conditioning industry, the fins of fin tube heat exchangers have different thicknesses, usually 0.095mm and 0.105mm. What is the impact on the performance of the fin tube heat exchanger? Let’s take a look at the factors affecting the performance of finned tube heat exchangers.

1. The larger the heat exchanger tube diameter, fin width and fin pitch, the greater the influence of fin thickness on heat transfer performance;

2. Compared with flat fins, the thickness of the fins has a greater impact on the heat transfer performance; the thickness of the fins has no effect on the heat transfer performance corresponding to the number of rows;

3. The fin thickness of the heat exchanger with small pipe diameter and small pipe spacing has little effect on the heat exchange performance.

Several Methods to Improve the Total Heat Transfer Coefficient of Finned Tube Heat Exchanger

Fin tube heat exchangers are not suitable for all heat exchange occasions. When the heat transfer coefficients on both sides of the tube are equal, finned tube heat exchangers can be omitted, such as oil-oil, water-water, gas-gas The heat exchange.

When the heat transfer film coefficients on the two sides of the heat transfer surface are very different, it is suitable to use a finned tube heat exchanger, because at this time, the smaller side becomes the main aspect of heat transfer control. When designing finned tube heat exchangers, the heat transfer coefficient on the smaller side should be maximized. It is mainly advantageous to make the film coefficients on both sides roughly equal.

The methods to increase the film coefficient mainly include the following aspects:

1. Reduce the cross-sectional area of the passage to increase the flow rate, add baffles in the passage or promote the degree of turbulence, which is generally used in the tube side structure of the finned tube heat exchanger.

2; Adding fins on the tube wall can not only increase the degree of turbulence, but also increase the heat transfer area. The main heat transfer function of the finned tube heat exchanger is based on this.

3; Use enhanced heat transfer surfaces, such as groove surfaces of various shapes, or porous surfaces, which can obtain a considerable heat transfer film coefficient for heat transfer processes with phase changes such as condensation and boiling. . Since such enhanced heat transfer reduces metal consumption, but at the same time increases the processing difficulty and production cost, it is less used in finned tube heat exchangers, unless there is a specific installation space restriction.

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