Introduction
U-bend tubes are used in shell-and-tube heat exchangers, condensers, and other industrial equipment which requires a compact fluid circulation and flow reversal arrangement, among other applications. Their technical characteristics are partially determined by the material grade, yet another aspect of performance is defined by the bend geometry and the condition of the tube wall after forming.
The two relevant parameters are the bend radius and the wall thickness. The U bend tube radius determines the severity of the deformation while the wall thickness accounts for pressure containment and mechanical performance. Awareness of the interrelationships between them allows the engineer to specify the correct tubes for their equipment, and adhere to the manufacturing tolerances.
What Is the Bend Radius of a U-Bend Tube?
There is more than one way to characterise the bend radius. The engineering drawing or the purchasing specification should state the radius of the centreline of the tube, although this is not always the case.
The tube bend radius is typically related to the outside diameter and the wall thickness. A larger radius will give a more gradual bend while a smaller radius gives a tighter bend.
How Does Bend Radius Affect Deformation?
During bending, the material on the outside of the bend will be subject to tension while the inner wall will be under compression. The outer wall of the bend may thin out while the inner wall will bulge or wrinkle if the process conditions are inappropriate.
A smaller radius (i.e. a tighter bend) tends to lead to greater deformation during the bending process, although this is governed by the combination of the material properties and process factors.
Why Is Bend Radius Important in Heat Exchangers?
The bend radius of a U-bend tube defines the space occupied by the bent section and contributes to the overall arrangement of the heat exchanger bundle. The radius should be compatible with the design of the equipment. A mismatch will prevent assembly or lead to interference between tubes.
The tube bend radius therefore has to satisfy both manufacturing and engineering constraints. Simply stated, the choice of the smallest possible radius is not always beneficial, as it has to satisfy the combination of material and service requirements.
How Does Wall Thickness Affect U-Bend Tube Performance?
As a result of their relevance to the performance and mechanical integrity of U bend tubes, the wall thickness specifications are a key consideration during the selection process. The specified thickness should be chosen to satisfy the design pressure, temperature, material properties, corrosion allowance if applicable and any other requirements.
The wall thickness of a U bend tube should be selected based on the relevant design code, applicable material specifications, temperature, corrosion considerations and other requirements. The thickness of the material should not be selected purely on the basis of diameter.
Wall Thickness and Pressure Containment
The walls of tubes are typically the weakest link in terms of resisting internal pressure. As a result, the required thickness is governed by the design code, material allowable stresses, temperature, corrosion considerations etc. A tube which is too thin will not support the design requirements while a tube which is too thick will incur additional costs and weight.
Wall Thickness During Bending
The wall thickness of a U bend tube may change during bending. Material stretching on the outside of the bend can lead to local thinning while compression on the inside of the bend can lead to excessive thickening and associated wrinkling. The degree to which this will occur depends on the radius, material, tube dimensions and the process parameters. In critical applications, the specification should require that the minimum wall thickness in the bend region is verified.
Why Uniformity Matters
Wall thickness variation can affect the structural performance and suitability of a tube for a particular application. A tube which meets the design requirements at manufacturing (i.e. nominal wall thickness before bending) may not be suitable after forming if the specification allows for local thinning. As a result, manufacturers and buyers should have an agreement in place which defines the appropriate measurement method, relevant locations and acceptance criteria before manufacturing commences.
What Are the Key U-Bend Tube Dimensions to Verify?
The overall dimensions of a U bend tube are essential in ensuring that it can be installed correctly and serve its purpose in the equipment. The approved engineering drawing should indicate all relevant dimensions, and the applicable tolerances for each. The important tube dimensions include:
- Outside diameter
- Wall thickness
- Bend radius
- Centre-to-centre distance
- Straight-leg length
- Overall geometry
It is important that the drawing indicates the relevant dimension reference. This is particularly relevant in cases where dimensions are specified from the tube centreline, the outside surface or another point.
What Are Tube Bending Tolerances, and Why Do They Matter?
The tube bending tolerances define the permissible deviation between the specified and actual tube geometry. They help to establish whether a given product is suitable for assembly, and therefore for its intended service.
Tolerances will typically apply to the bend radius, the leg spacing, the overall dimensions, straightness, ovality and other characteristics which may be specified. The applicable tolerance is determined by the standard, the drawing, the material, tube size and the customer's requirements.
Ovality is a common characteristic of bent tubes, especially U bend tubes. It is a measure of how much the cross-section of the tube deviates from a perfect circle, and can affect the suitability of the product for assembly and service if it is too great. The extent of ovality is affected by a combination of the bending parameters and tube geometry. Where a limiting ovality is specified in the drawing, the inspection should be carried out with the agreed measurement method and acceptance criteria.
How Should Bend Tolerances Be Established?
The tube bending tolerances should be established at the time of manufacturing. The buyer should not assume general tolerances as different applications have different requirements. The specification should identify the controlling dimensions, the required inspection methods, and whether additional inspection is necessary in the bent region.
How Can Manufacturers Control Bend Quality?
The bend quality can be controlled by ensuring that the combination of the material, tooling, bending parameters and inspection is appropriate for the application.
The manufacturer may inspect the relevant dimensions, inspect the surface of the tube for relevant defects, and carry out wall thickness or other measurements where appropriate. Heat treatment and non-destructive testing may also be specified depending on the material and application requirements.
Sainest Tubes specialises in the manufacture of U-bend tubes for heat exchanger applications where the dimensional requirements and inspection criteria are guided by the relevant specification and customer requirements.
Conclusion
Bend radius and wall thickness are relevant to the manufacturability, geometry and service suitability of U-bend tubes. A tighter bend radius will lead to greater deformation while the wall thickness variation should satisfy the applicable acceptance criteria. By defining the relevant dimensions, tolerances, material requirements and inspection methods, the engineering team can ensure that the finished tubes are suitable for the intended equipment design.