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DSS, the abbreviation of Duplex stainless steel, is a classification of stainless steels composed of two steel with the center one composed of either austenitize or ferric. These are also known as duplex steels since their chemical structure features two distinct phases, both of which are usually represented by martensite respectively. These steels are very useful in applications requiring extreme toughness since the two phases can be applied together at high temperatures and pressures. The duplex stainless steel is able to obtain sufficient hardness in both its austenitic and martensite phases due to the presence of significant amounts of residual austenite. The commonly used DSS grades are S31803, S32750 and SS32550.

The duplex stainless steel grades

Type	UNS	Sweden	German	France	Japan
Low alloy	UN23（SAF2304）	SS232（SAF2304）	W.Nr.1.4362	UR35N	DP11
Medium alloy	UNS S31500 UNS S31803	SS2376(3RE60) SS2377(SAF2205)	W.Nr.1.4417 W.Nr.1.4462	UR45N	DP1 DP8
High alloy	UNS S32900 UNS S31260	SS2324(10RE51)	W.Nr.1.4460 W.Nr.1.4501		329J1 329J2L
Super duplex	UNS S32750 UNS S32550	SS2328(SAF2507)	W.Nr.1.4410 W.Nr.1.4507	UR47N+ UR52N+

 

Apart from the alloy itself, another important factor that contributes to its corrosion resistance is the nickel content. Nickel is commonly found in higher percentages in most alloys, which makes it an extremely useful component. In comparison to nickel, which is often used in high-performance alloys for its electrical conductivity and ability to form good-quality alloys, nickel is not as frequently used in making high-quality duplex stainless steel. One of the most interesting aspects of nickel alloys is its corrosion resistance ability, which makes it the best alternative for high-performance materials. When mixed with the steel, nickel produces a more stable alloy, which can increase the alloy’s wear-ability and mechanical strength.

Another significant property of this alloy is its high resistance to thermal expansion. It exhibits a high level of thermal expansion resistance despite the expansion resistance ability of austenitic stainless steels, due to its superior mechanical properties. This property gives it an excellent corrosion protection capability, especially during the tempering/stain removal cycle. The excellent corrosion resistance feature of duplex stainless steel enables it to stand up against a wide range of chemicals. It also has high levels of resistance towards oil, grease and other liquids with a high viscosity level.

Apart from the above features, duplex stainless steel is also popular because of its high strength and durability. Its high strength rating of up to 300Kg is made possible through its ability to make use of two-directional mandrel rolls. It is comprised of a hard carbon fiber rolled into strips that are interlaced on both sides and formed into a bar with a mandrel. A further feature that makes it an excellent alloy is that its surface is completely smooth with no ridges.

One of the most important factors that contribute to the durability of duplex stainless steels is their low rate of pitting corrosion resistance. These steels exhibit a low rate of formation of crystalline grains inside the hot alloy. They can be used to build both large and small structures in different industries. Due to their resistance to crystalline grains, they are highly valued by the construction industry.

The mechanical properties of duplex stainless steel offer a number of benefits that make them an excellent choice for a wide range of applications. These properties allow these steels to be used for a variety of applications including precision engineering component building, heat exchangers and sheet metal fabrication. Some other important properties of this type of alloy include high heat tolerance, low density and excellent corrosion resistance. They also offer a number of mechanical properties that contribute to the overall properties of the alloy. These include extreme hardness, toughness, chemical resistance and creep resistance.

The bellows tube heat exchanger is an upgrade based on a straight (bright) tube heat exchanger. The design of the crest and trough of the wave inherits the advantages of the tubular heat exchanger such as durability and safety, and at the same time overcomes the defects such as poor heat transfer capacity and easy scaling. The principle is to improve the total heat transfer coefficient so as to reduce the required heat transfer area, which can save materials and reduce weight under the same heat transfer effect.

Because the bellows body is processed by cold pressing of bright pipe billet, it is generally believed that the bellows body can be strengthened after forming. The external pressure stability experiment shows that the instability of the corrugated heat exchange tube under external pressure first occurs in the straight pipe section, and the corrugated tube will be unstable only if the external pressure continues to rise. This indicates that the stability of the corrugated section is better than that of the straight section and that the critical pressure of the corrugated section is higher than that of the straight section.

Experiments show that the ripple of buckling deformation occurred in the wave trough, especially local single wave trough, generally not more than two troughs instability at the same time, it shows that the stability of the wave crest is better than trough but sometimes also can appear the opposite, in the cold pressing mark process, both trough and the wall thickness of the straight section is constant, cold after the tube is actually shorter.

The existence of wave peaks and troughs in the bellows increases the effect of radial heat exchange convection in the tubes, as shown in Fig below:

Radial convection has a great influence on the total heat transfer coefficient, which is the fundamental reason for the low price and lightweight of the double tube plate bellows heat exchanger. The heat exchange area of the tube body surface of the bellows and the straight tube is large at the same length, but this change is far less than the contribution of changing the coefficient value. It can be clearly seen that the flow velocity of the straight (light) tube is significantly reduced when it is close to the tube wall.

The shell heat exchanger with bellows can make the fluid speed and direction of constant change to form turbulence compared with a straight tube exchanger, making exchange heat with the wall, the boundary effect that affects heat transfer will no longer exist. The total heat transfer coefficient can be increased by 2 ~ 3 times, and the actual operation can even reach 5 times, and the weight is light, which is the reason why the price of bellows heat exchanger is lower than that of the straight tube heat exchanger. According to calculation and practical experience, the total heat transfer coefficient of 1 mm thick bellows is 10% lower than that of 0.5 mm thick bellows. The operation data of hundreds of bellows heat exchangers show that the wall thickness (almost all 0.5 mm) is the main reason for the operation of 10 ~ 14 years without major repair or damage.

In addition, the bellows heat exchanger can effectively resist the impact of a water hammer. The shell of the double tube plate heat exchanger is connected with an expansion joint. If it suffers from the impact of water hammer, the expansion joint will be misplaced. This happens to both bellows and straight tube heat exchangers, and the deformation of the shell may cause the tube to twist. It is because the bellows have more expansion margin, the elastic margin of strain is large when undergoing deformation, that is to say, the ability to resist instability is strong in this case. But in any case, in the process of installation to avoid the occurrence of water hammer, can be taken through the use of Angle sitting valve, delay switch and other measures.

Advantages of stainless steel bellow shell heat exchanger

• High heat transfer efficiency

The special crest and trough design of the bellows makes the fluid flow because of the continuous mutation of the inside and outside section of the tube to form a strong turbulence. Even in the case of a very small flow rate, the fluid can form a strong disturbance inside and outside the tube, which greatly improves the heat transfer coefficient of the heat exchange tube. The heat transfer coefficient is 2~3 times higher than that of the traditional tube heat exchanger.

• No scaling and blocking

The medium inside and outside the bellows is always in a highly turbulent state, which makes the solid particles in the medium-difficult to deposit scale; On the other hand, affected by the temperature difference of the medium will produce a trace of axial expansion deformation, the curvature will change frequently, the dirt and heat exchange tube will produce a large pull force, even if there is scale calm will therefore break off automatically, so that the heat exchanger has always maintained a lasting, better heat transfer performance.

• Automatic compensation

The special structure and shape of bellows can effectively reduce the thermal stress under the condition of being heated without adding expansion joints, thus simplifying the structure of products and improving the reliability of products.

• Long service life

The axial expansion ability is enhanced, which effectively reduces the temperature difference stress and can adapt to the large temperature difference and pressure change, so there will be no leakage caused by pipe mouth rupture. The connection between the baffle plate and the bellows extends the service life of the heat exchanger.