How to select clad tubesheet for heat exchanger?

Why Choose a Bimetallic Composite Tubesheet for Shell and Tube Heat Exchangers?

The primary reason for choosing a bimetallic composite structure over a single-metal material for the tubesheet of a shell and tube heat exchanger is to achieve a balance among economy, reliability, and optimal performance while meeting demanding operating conditions.

The media on the tube side and shell side of a heat exchanger are usually different, with significantly varying corrosiveness. For example, the tube side may carry highly corrosive seawater or chemicals, while the shell side may carry relatively mild steam or oil.

Drawbacks of Single-Metal Tubesheets:

If the entire tubesheet is made of a high-grade corrosion-resistant material (such as Hastelloy, titanium, or duplex stainless steel), the cost would be prohibitively high because the tubesheet is a very thick and large forging. If only inexpensive carbon steel is used, the side exposed to the corrosive medium would quickly corrode and perforate, leading to equipment failure, extremely high repair costs, and production disruptions.

Advantages of Bimetallic Composite Tubesheets:

✂ Base Layer: uses carbon steel or low-alloy steel with sufficient strength and rigidity. This layer bears the main structural pressure and weight at a low cost.

✂ Cladding Layer: uses a material with excellent corrosion resistance, typically on the side exposed to the corrosive medium. This ensures corrosion resistance while significantly reducing material costs.

For heat exchangers with large diameters and high pressures, composite sheets with high-strength and inexpensive low-alloy steel as the base layer are commonly used. The larger the tubesheet size, the more economical it becomes.

Under What Circumstances Is a Composite Tubesheet Generally Chosen Over a Single-Metal Tubesheet?

① Single stainless steel tubesheets with a diameter greater than 700 mm and a thickness greater than 55 mm.

② Single titanium or nickel-alloy tubesheets with a diameter greater than 450 mm and a thickness greater than 35 mm.

Classification of Composite Tubesheets:

①By Cladding Layer Location

✂ Single-Side Cladding: The most common form, where the cladding is only on the side exposed to the corrosive medium (usually the tube side).

✂ Double-Side Cladding: Less common, where both sides of the tubesheet are clad. Used in extremely harsh conditions where both the tube side and shell side media are highly corrosive and require different materials for resistance.

②By Material

③By Composite Method

✂ Welded Composite Tubesheet

Uses overlay welding technology for cladding. A corrosion-resistant alloy weld material is deposited in one or more layers on the base metal surface to form the cladding layer.

Advantages:

• Strong bonding
• Can produce very thick cladding layers

✂ Explosively Bonded Composite Tubesheet

Uses the ultra-high-pressure shockwave generated by the instant explosion of high-energy explosives to propel the cladding metal at extremely high speed against the base metal. The impact generates immense pressure and heat, causing plastic deformation, melting, and flow of the metals at the interface, achieving metallurgical bonding.

This method is suitable for metals with significant differences in melting points and thermal expansion coefficients that are difficult to bond by hot rolling.

Advantages:

• Extremely high bond strength and excellent interfacial shear resistance
• Fewer size restrictions on sheets, allowing the production of large and heavy composite sheets

✂ Roll-Bonded Composite Tubesheet

It can be divided into cold roll bonding and hot roll bonding:

Hot Roll Bonding: The base and cladding metals are cleaned and stacked together, heated to a certain temperature, and subjected to enormous pressure through a large rolling mill. This causes significant plastic deformation in the same direction, and under high temperature, high pressure, and deformation, interfacial atoms diffuse to form metallurgical bonding.

• Advantages: High bond strength, high production efficiency

Cold Roll Bonding: Uses cold rolling to create atomic bonding or mechanical interlocking between metal layers.

• Advantages: Good surface quality, high bond strength

✂ Explosive-Roll Bonded Composite Tubesheet

This method combines explosive bonding and hot rolling processes, cleverly avoiding the drawbacks of both while leveraging their advantages. Explosive bonding is performed first, followed by hot rolling.

Advantages: Inherits the high bond strength of explosive bonding while eliminating interface inhomogeneity through rolling, resulting in excellent consistency in plate performance.

The rolling process improves the microstructure of the metal, enhancing toughness, fatigue strength, and pressure-bearing capacity.

Heat Exchanger Clad Tubesheet