Introduction
The basic function of a heat exchanger is to transfer heat between two fluids while meeting the needs of the industrial process under defined operating conditions. One feature common in many shell-and-tube heat exchangers is the use of U-bend tubing to return fluid flow from one leg of the passage back through the other.
The bend has implications for the arrangement of the tubes within the bundle, as well as for manufacturing and inspection requirements. To specify or review tubes within this configuration, it is necessary to understand not only the required bend shape but also the associated construction requirements, tube dimensions, materials, and applicable inspection criteria.
The Tubular Exchanger Manufacturers Association (TEMA) has issued standards for the design and construction of certain types of shell-and-tube heat exchangers.
What Are the TEMA Standards?
TEMA standards contain engineering details and requirements for shell-and-tube heat exchanger construction within the scope of the publications. Their contents are relevant to design aspects, specific components and dimensions, construction requirements, and other considerations.
It is recommended that the applicable edition of the requirements be confirmed for every project. TEMA is not a substitute for material specifications, pressure-vessel or statutory requirements, or contractual documents issued by the customer. Separate standards may also apply in certain cases.
The requirements for TEMA U bend tubes must be reviewed in relation to the relevant exchanger design, tube material specification, dimensions given in the purchase order, and any other applicable code or standard.
What Should Buyers Know About TEMA Tube Specifications?
TEMA tube specifications should be reviewed within the context of the design of the complete heat exchanger. Tube material, outside diameter (OD), wall thickness, bend details, the length and configuration of the straight legs, and the required dimensional tolerances all influence the suitability of the tube and its inclusion within the bundle.
1. Tube Dimensions and Configuration
The dimensions of the tube must be compatible with the exchanger's design drawings and the proposed arrangement of the tube bundle.
The OD, wall thickness, bend details, and spacing between the legs in the U-bend configuration must correspond to the proposed bundle arrangement to ensure that the tubes can be inserted correctly.
2. Material and Operating Conditions
It is necessary to confirm that the selected material is suitable for the intended operating conditions. The choice of material and its quality will depend on factors including temperature, pressure, and the chemical environment.
The material may be defined by an accepted product standard or by the relevant customer project specification.
The tube specification must include the material grade and its identification documentation. Traceability is particularly important when an order includes multiple material grades or alternative tube sizes.
3. Manufacturing and Acceptance Requirements
Requirements relating to the manufacturing route, heat treatment after bending if specified, dimensional limits, and testing of the finished tube should be clearly defined.
Although TEMA-related construction requirements provide an important reference, it is still necessary to identify the applicable material standard and the inspection plan for the tubes.
Why Do U-Bend Tube Standards Matter?
U bend tube standards provide a basis for defining design, manufacturing, inspection, and acceptance requirements. Without a clearly defined specification, the interpretation of dimensions, tolerances, and testing requirements can vary.
By defining the characteristics for which the manufacturer is responsible and the methods used to judge conformity, standards facilitate the exchange of information between the exchanger designer, tube manufacturer, fabricator, and end user.
However, the term “TEMA compliant” should not be used without a clear basis. When specifying requirements for a project, it is recommended that the applicable standard and edition, material grade, dimensional specifications, and testing and acceptance requirements be identified.
How Do U-Bend Tubes Affect Heat Exchanger Tube Bundles?
Heat exchanger tube bundles consist of groups of tubes arranged within the shell of a heat exchanger to provide the required surface area for heat exchange.
The dimensions of these tubes influence how the arrangement fits within the exchanger, as well as maintenance requirements and the flow characteristics of the fluids.
In this configuration, the tubes include an internal bend to reverse fluid flow rather than requiring an external connection.
In some exchanger designs, U-bend tubes accommodate differences in thermal expansion between the shell-side and tube-side components.
This outcome is achieved by designing the exchanger with consideration for operating conditions, material characteristics, and the forces and limitations imposed during operation.
However, the suitability of a U-bend configuration is not automatic. The designer must consider the intended operating conditions, maintenance requirements, and implications of the change in tube routing.
What Should U-Bend Tube Inspection Cover?
U bend tube inspection is generally defined by the applicable standard and includes checks for features that could pose risks during the intended service, as well as verification of the required dimensions.
The bending process introduces variations that may not be identified or accounted for when inspecting tubing in a straight configuration.
1. Dimensional Inspection
For U-bend tubes, the following dimensions should be checked according to the applicable specification:
- Outside diameter.
- Wall thickness, where specified.
- Bend radius.
- Distance between the two straight tube legs.
- Overall length and width.
- Straight-section length.
- Any other specified dimensions and tolerances.
The surface condition of the bent section must also be evaluated for features relevant to the intended service application.
2. Material and Non-Destructive Testing
Other forms of testing may be performed depending on the requirements.
- Hydrostatic testing: Evaluates pressure retention in accordance with the specified test procedure.
- Eddy current testing: Can help identify certain discontinuities in suitable conductive materials.
- Ultrasonic testing: Can detect certain internal or surface-connected discontinuities, depending on the method and application.
These techniques have different capabilities depending on the material, tube size and shape, and the nature and location of any discontinuities or defects.
Material testing and inspection must be carried out in accordance with the applicable requirements, including the specified acceptance criteria.
Such procedures cannot be assumed to apply merely because TEMA is referenced in the project. They must be defined in accordance with the project documents.
How Should Buyers Specify TEMA U-Bend Tubes?
Before placing an order, buyers should ensure that the applicable edition of the TEMA standard and the exchanger's design requirements are understood and agreed upon.
The purchase specification should clearly address:
- Applicable TEMA standard and edition.
- Tube material and grade.
- Outside diameter and wall thickness.
- Overall tube length and straight-leg dimensions.
- Bend radius and other bend dimensions.
- Dimensional tolerances.
- Heat treatment requirements, where applicable.
- Non-destructive testing and pressure testing requirements.
- Material traceability, inspection records, and other required documentation.
Clear drawings and accompanying documents reduce the possibility of differing interpretations. Where multiple standards apply to a project, the engineering personnel should clarify any conflicts and agree on the appropriate course of action before manufacturing begins.
Sainest Tubes manufactures tubes in U-bend configurations for heat exchanger applications according to specified dimensional and technical requirements. Buyers can review the information in this publication alongside the applicable standards to understand the relevant manufacturing and inspection requirements.
Conclusion
TEMA standards are an important reference for the construction of shell-and-tube heat exchangers. However, attention must also be given to other applicable specifications, material requirements, specific design features, and customer requirements.
Appropriate dimensions, conformity with bend specifications, material traceability, and suitable inspection procedures help ensure that U-bend tubes are manufactured correctly and meet the intended heat exchanger design requirements.