Spiral Finned Tube as Heat Exchanger used in Boiler Economizer, Air Preheater, Waste Heat Boiler

Spiral Finned Tube as Heat Exchanger used in Boiler Economizer, Air Preheater, Waste Heat Boiler

Spiral finned tube

Spiral finned tube is an high efficient heat transfer element with spiral fins. Its heat transfer area is several to several dozen times as large as that of a bare tube. It can enhance heat transfer, reduce flow resistance and metal consumption, thus improving the economy and operation reliability of the heat exchange equipment. At present, spiral finned tubes are widely used in various boilers. Spiral finned tubes can be manufactured by various ways. There are several manufacturing methods for finned tubes used in heat exchange equipment such as boilers and pressure vessels:high frequency resistance welding spiral finned tubes, brazing and integral spiral finned tubes. The manufacturing methods, performance evaluation index, application and advantages of integral spiral finned tubes are to be introduced in detail.

General introduction

Spiral finned tubes are widely used in heat recovery of boiler economizer, air preheater, and waste heat boiler, as well as heat exchange equipment in chemical industry, pressure vessel and other fields. The capacity of units used in boilers has also increased from small to large, and their applications are increasing day by day. At present, many professional manufacturers in China can produce high frequency resistance welding, brazing spiral finned tubes, and the production equipment and technology of finned tubes have been developed, designed and manufactured from importation, digestion and absorption to self-development. The quality has reached the requirements of advanced foreign standards.

Advantages of using finned tubes:

1. the heat transfer area is increased in the effective space to improve the heat transfer effect.

2. Reduce the occupancy space of heat transfer surface and reduce its volume, especially for fast-loading boilers.

3. Reduce the equipment cost and improve the safety of equipment.

4. Due to the decrease of water side pressure drop, the operation cost is reduced.

5. The stiffness of finned tubes are increased, and the seismic resistance of the tube is improved.

Manufacturing method

Spiral finned tubes can be manufactured by various ways. At present, there are several main manufacturing methods for finned tubes used in heat exchangers such as boilers and pressure vessels.

High frequency resistance welding spiral finned tube

High frequency resistance welding spiral finned tube is kind of high frequency current imported into the welded parts. By using the resistance heat generated by the current through the contact surface of the welded joints and adjacent areas, the welded contact surface can reach the plastic state of melting or semi-melting, then appropriate pressure is applied to the welded contact surface, to complete welding work of finned tubes.

Brazed spiral finned tube

Brazing spiral finned tube is a kind of metal filler with a melting point lower than that of the welded part, and then heating the brazing temperature. When the welded part is not melting, the solder melting well wets and brazes, and forms the brazing joint depending on expansion and dispersion of the contact point of the welded part, thus completing the welding of the spiral finned tube.

Integral spiral finned tube

Integral spiral finned tube is formed by extrusion and tie of thick-walled tube(billet tube) under the condition of medium frequency continuous heating.It is known as the latest method of manufacturing finned tube in domestic market. It is developed by a domestic company and the researched and developed integral finned tube as well as the self-manufactured integral finned tube manufacturine equipment have been applied for patents.

Performance evaluation index

There are three main manufacturing methods of spiral finned tube mentioned above. For the convenience of analysis and comparison, spiral finned tube is divided into welded finned tube(high frequency welding, brazing) and integral spirl finned tube to compare.

Welding rate

Also known as fusion rate

Weling rate is an assessment index of fin weld width and total length. JB/T 6512-92 “Technical conditions for manufacturing high frequency resistance welding spiral finned tube of boilers stipulates that the weld fusion rate in the direction of steel band width is not less than 80%. Although the length direction is not mentioned, it can be understood that no less than 80% of the total length of the fin is qualified.

The local unfused length of the weld shall not be greater than the diameter of the tube and not more than 50mm, and the number of unfused places shall not exceed 2 per meter, otherwise the repair welding shall be carried out.

HG/T 3181-1989 “High frequency resistance welding spiral finned tube” stipulates that the total length of the actual weld is not less than 90% of the total fin length, and the average width of the weld is not less than 80% of the fin width.

The actual welding rate of high frequency resistance welding spiral finned tube can reach 90-95%. The welding rate of brazed spiral finned tube is slightly higher than that of high frequency resistance welding, but the welding rate is not easy to check.

Intergral spiral finned tube, as its name implies, is generated from the tube. There is no welding problem of the fin, and no inspection of the welding rate is needed.

Tensile strength of weld

JB/T 6512-92 stipulates that the tensile strength of welded speciments shall not be lower than 196MPa.HG/T 3181-92 does not regulate this.

High frequency resistance welding spiral finned tube, the weld tensile strength of the manufacturers has reached 200MPa, or even 300MPa. The weld tensile strength of brazed spiral finned tubes can basically meet this requirement.

The integral spiral finned tubes are extruded and rolled from billet tubes, so there is n problem of the weld tensile strength. The strength of the joints between fins and tubes is equal to the tensile strength of the corresponding tubes, even slightly higher.

Post weld heat treatment

The weld heat affected zone of high frequency resistance welding spiral finned tube is very low. The foreign standard stipulates that the heat affected zone is less than 0.8mm. The actual measured heat affected zone of some domestic manufacturers is less than 0.5mm. Therefore, some standards do not specify whether spiral finned tubes are heat treated after welding for high frequency resistance welding. JB/T 6512-92 standard stipulates that stress relief heat treatment should be carried out after welding of finned tubes made of alloy steel.

The production process of integral spiral finned tube is actually the re-extrusion and rolling process of the original thick-walled billet tube at high temperature. After extrusion and rolling at high temperature, there is no need for stress relief heat treatment at all.

Heat Transfer Effect

The heat transfer efficiency of high frequency resistance welding and brazing spiral finned tube is at least 4 times higher than bare tube. The heat transfer effect of open-tooth finned tube is better than that of the intact fin tube. However, it should be noted that the weld rate between fin and tube is not 100%, and the standard stipulation is only more than 80%. Thermal resistance exists in the unfused part, which will affect the heat transfer.

100% contact between the fins of integral spiral finned tube and the main tube, together with the small R transition between the root of the fin and the main tube, this not only increases the stiffness and pressure capacity of the fins, but also facilitates the heat transfer. The heat transfer effect is at last 5-6 times higher than that of bare tube, which is obviously better than that of high frequency resistance welding and brazing spiral finned tubes.