U-bend heat exchanger tubes are tubes formed into a U shape to provide a return flow path within a heat exchanger. They are used in U-tube shell-and-tube exchangers and in high- and low-pressure feedwater heaters, where the tube material and dimensions are selected for the operating conditions.
In a conventional U-tube bundle, both ends of each tube connect to the same tubesheet. The U-bend accommodates differential thermal expansion between the tubes and shell. Tubes with different bend radii are arranged to suit the bundle layout.
Product Specifications
| Item | Description |
|---|---|
| Product | U-bend heat exchanger tubes |
| Product form | Seamless straight tubes formed into U bends |
| Materials | Carbon steel, alloy steel or austenitic stainless steel, as specified |
| Tube dimensions | Outside diameter and wall thickness |
| Bend dimensions | Centerline bend radius, leg spacing and drawing-defined lengths |
| Delivery condition | Material heat treatment and any required treatment after bending |
| Principal applications | Feedwater heaters and U-tube shell-and-tube heat exchangers |
| Manufacturing requirements | Applicable tube standard, fabrication specification and approved drawing |
The straight-tube product standard defines the starting material requirements. Finished U-bend geometry, tolerances, treatment and inspection should also be specified in the order and fabrication drawing.
Materials and Standards
| Material family | Representative products or grades | Starting-tube standards |
|---|---|---|
| Carbon steel | Seamless cold-drawn low-carbon heat exchanger tubes | ASTM A179/A179M or ASME SA-179/SA-179M |
| Alloy steel | T11, T22 or another grade selected by the design | ASTM A213/A213M or ASME SA-213/SA-213M |
| Austenitic stainless steel | TP304, TP316L, TP321 or another specified grade | ASTM A213/A213M or ASME SA-213/SA-213M |
These are material options for design consideration, rather than interchangeable grades or a universal selection for every feedwater heater. Selection depends on temperature, pressure, fluid chemistry, corrosion conditions and the governing construction code. Other materials may be specified by the project.
Bend Geometry and Dimensions
The drawing should identify the centerline bend radius, distance between the tube legs, straight-leg lengths, end alignment and required tolerances. Length dimensions must clearly distinguish the original straight-tube length from the finished leg length and overall U-tube length.
For a simple semicircular 180-degree bend with parallel legs, the nominal center-to-center leg spacing is twice the centerline bend radius. Other bend profiles should follow the specified drawing.
Bending can cause wall thinning at the outside of the bend and changes in cross-sectional shape. The finished bend must meet the specified minimum wall thickness and limits on ovality, wrinkles and surface defects. The permissible bend radius depends on the material, diameter, wall thickness and forming process.
Heat Treatment and Surface Condition
Treatment after bending depends on the material, forming strain and fabrication requirements. Stress relief or solution heat treatment may be required; the treatment method, temperature range and extent of the treated area should follow the applicable specification.
Tube surfaces should meet the specified cleanliness requirements for assembly and service. For stainless steel, cleaning, pickling or passivation may be specified as appropriate. Tube ends and bends should be protected during storage and transport.
Applications
Feedwater Heaters
U-bend tubes are used in high- and low-pressure feedwater heaters to transfer heat to boiler feedwater. Material selection should account for pressure differences, water chemistry, corrosion and the temperature conditions in each heater section.
Shell-and-Tube Heat Exchangers
U-tube bundles provide two tube passes through a return bend and accommodate differential thermal expansion. They are used where this arrangement suits the thermal duty, pressure requirements and maintenance plan.
Access for mechanical cleaning through the bend is more restricted than in straight tubes. Fouling tendency and the planned cleaning method should therefore be considered when selecting the exchanger arrangement.
Manufacturing and Inspection
Manufacturing starts with qualified straight tubes, followed by bending to the specified radii. Any required treatment, cleaning and final inspection are completed according to the fabrication specification.
Depending on the order, inspection may include:
- Starting-material certificates and heat-number traceability.
- Bend radius, leg spacing, length and end-alignment checks.
- Wall thickness and ovality checks at the bend.
- Surface inspection for cracks, wrinkles and other defects.
- Hydrostatic testing and/or specified nondestructive examination.
- Heat-treatment records and additional tests where required.
The purchase specification should identify which inspections apply before and after bending. Starting-tube test results alone do not document every requirement of the finished U-bend.
Ordering Information
For a quotation, specify:
- Material: required grade and starting-tube standard, including the edition.
- Tube dimensions: outside diameter and wall thickness basis.
- Bend radius: centerline radius for each tube size.
- Lengths: straight-leg lengths and overall dimensions, clearly defined.
- Quantity: number of tubes required for each bend radius.
- Drawing: bend schedule, leg spacing and dimensional tolerances.
- Treatment: requirements for heat treatment after bending.
- Surface condition: cleanliness and any specified surface treatment.
- Inspection: tests, acceptance criteria and certificate requirements.
- Packaging: protection suitable for the finished tube dimensions.