What is a heat exchange tube

What is a heat exchange tube?

Heat exchange tube is one of the components of heat exchanger. It is a steel tube placed in the cylinder for heat exchange between two media. It has high thermal conductivity and good isothermal properties. It is a device that can quickly transfer heat energy from one point to another, and there is almost no heat loss. Therefore, it is called heat transfer superconductor, and its thermal conductivity is thousands of times that of copper.

Form of heat exchange tube

In addition to the smooth tube, the heat exchanger can also adopt a variety of enhanced heat transfer tubes, such as finned tube, threaded tube, spiral grooved tube, etc. When there is a large difference in heat transfer coefficient between the two sides of the inner diameter of the tube, the fins of the finned tube shall be arranged on the side with low heat transfer coefficient.

Size of heat exchange tube

The commonly used size of heat exchange tube (outer diameter x wall thickness) is mainly Φ 19mmx2mm, Φ 25mmx2. 5mm and Φ 38mmx2. 5mm seamless steel tube and Φ 25mmx2mm and Φ 38mmx2. 5mm stainless steel tube. Standard tube lengths include 1.5, 2.0, 3.0, 4.5, 6.0, 9.0m, etc. The adoption of small tube diameter can increase the heat transfer area per unit volume, compact structure, reduce metal consumption and improve the heat transfer coefficient. It is estimated that the heat exchange tubes of heat exchangers with the same diameter will be Φ Change 25mm to Φ 19mm, the heat transfer area can be increased by about 40% and the metal can be saved by more than 20%. However, the fluid resistance of small diameter is large, which is inconvenient to clean and easy to scale and block. Generally, large diameter tubes are used for viscous or dirty fluids, and small diameter tubes are used for cleaner fluids.

Material of heat exchange tube

Common materials include carbon steel, low alloy steel, stainless steel, copper, copper nickel alloy, aluminum alloy, titanium, etc. In addition, there are some non-metallic materials, such as graphite, ceramics, polytetrafluoroethylene, etc. Appropriate materials shall be selected according to working pressure, temperature and medium corrosivity during design.

Center distance of heat exchange tube

Fig.1 arrangement of heat exchange tubes
As shown in Figure 1, the arrangement forms of heat exchange tubes on tubesheet mainly include equilateral triangle, square and corner equilateral triangle and corner square. Equilateral triangle arrangement can arrange the largest number of tubes on the same tubesheet area, so it is most commonly used, but it is not easy to clean outside the tube. In order to facilitate external cleaning, square or corner square tube bundles can be used.
The center distance of heat exchange tube shall ensure that the tube bridge (clearance distance between two adjacent tubes) has sufficient strength and width when the tube is connected with the tubesheet. When the tube room needs to be cleaned, a channel for cleaning shall be reserved. The center distance of heat exchange tubes should not be less than 1.25 times of the outer diameter of heat exchange tubes. The center distance between the most commonly used heat exchange tubes is shown in the table below.

Center distance of common heat exchange tubes (unit: mm)

Outer diameter of heat exchange tube d0	10	12	14	16	19	20	25	32	35	38	45	50	57
Center distance of heat exchange tube	13-14	16	19	22	25	26	32	40	44	48	57	64	72

How to repair the heat exchange tube in pressure vessel

Leakage position of heat exchange tube

The leakage of heat exchange tube in pressure vessel generally has two positions: one is the connection of tube, and the other is the leakage of tube itself.

Leakage at the connection of heat exchange tube

There are three main connection methods between tube and tubesheet: expansion, welding, welding and expansion.
Due to the fluctuation of temperature and pressure and the uneven temperature difference deformation of the expansion tube, the tube is pulled off and loosened from the tubesheet and leaks. Supplementary expansion can be adopted to eliminate the leakage. If it is invalid, it can be carried out again. The supplementary expansion shall not be more than three times, otherwise the material at the tubesheet hole will be cold hardened and not tight.
If the expansion supplement is invalid, the tube shall be replaced or the welding method shall be adopted.
It should be noted that after welding, the surrounding row of tubes need to be expanded again, because after welding, due to the thermal expansion and cold contraction of tubesheet, the expansion joints of other tubes around will leak. In case of weld leakage between tube and tubesheet, grinding and repair welding can be carried out.

Leakage of heat exchange tube itself

Due to corrosion, abrasion, erosion, deposition corrosion and other reasons, heat exchange tubes often leak due to cracks and perforation.
Generally, the leakage of the tube itself cannot be repaired. If the number of leaked tubes is small, the two ends of the tube orifice can be blocked with conical metal plugs. If the tube side pressure is high, it is more reliable to weld after plugging.
The length of the metal plug is generally twice the inner diameter of the tube, the diameter of the small end shall be equal to 85% of the inner diameter, and the taper shall be 1:10.
The hardness of the plug material shall be lower than or equal to that of the tube. When tube plugging is used to eliminate leakage, the number of tube plugging shall not exceed 10%.
When one end of the stainless steel heat exchange tube is damaged, the tube drawing repair method can also be used, because the elongation of the stainless steel tube is 40% ~ 45%. When the total elongation of the tube drawing does not exceed 12%, the tube can be pulled for many times.

Source: China Heat Exchange Tube Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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