Heat exchangers, crucial in transferring thermal energy between fluids at different temperatures. Shell and tube heat exchangers are particularly dominant in industrial applications due to their robustness and versatility in handling various operating conditions.

1. Introduction to Heat Exchangers

A heat exchanger is a device designed for the transfer of internal thermal energy between two or more fluids with different temperatures. Typically, these fluids are separated by a heat transfer surface to prevent mixing. Heat exchangers find applications across various industries, including process, power, petroleum, transportation, air-conditioning, refrigeration, cryogenic, heat recovery, alternate fuels, and more. Examples of heat exchangers encountered in daily life include automobile radiators, condensers, evaporators, air preheaters, and oil coolers. Heat exchangers can be classified in numerous ways based on their design and applications.

Photo 1: A heat exchanger

 

A heat exchanger typically comprises heat-exchanging elements, such as a core or matrix containing the heat transfer surface, along with fluid distribution elements like headers, tanks, and inlet/outlet nozzles or pipes. Generally, heat exchangers have no moving parts, though exceptions exist, such as rotary regenerators with a rotating matrix or scraped surface heat exchangers featuring a rotary element with scraper blades inside the heat transfer tube. The heat transfer surface comes into direct contact with fluids for heat transfer via conduction. The section of the surface that separates the fluids is termed the primary or direct contact surface. Additional surfaces like fins may be added to increase heat transfer area. 

2. Shell & Tube (Tubular) Heat Exchangers

In process industries, shell and tube heat exchangers hold a predominant position, surpassing all other types in terms of usage. Over 90% of heat exchangers employed in industrial settings adopt the shell and tube configuration. Recognized as the “workhorses” of industrial process heat transfer, they are the preferred choice due to established design and manufacturing procedures applicable to a diverse range of materials. With many years of successful service, coupled with the availability of codes and standards for design and fabrication, they are produced in an extensive array of sizes and styles. Shell and tube heat exchangers exhibit virtually no limitations concerning operating temperature and pressure.

3. Advantages of Tubular Heat Exchangers over Plate Heat Exchangers:

1.Pressure and Temperature Handling:

    • Cylindrical tubes are thicker and can withstand higher pressures and temperatures.

2.Pass Arrangement:

    • Shell-and-tube exchangers often have longer flow lengths, arranged in 2- or 4-pass units.

3.Overall Heat Transfer Rates:

    • Small shell-and-tube exchangers in light industrial service may have higher overall heat transfer rates due to a higher pressure drop available per foot of travel.

4.Variable Design Factors:

    • Tube-side pressure drop can be varied by changing tube diameter, length, or number of passes.
    • In plate-fin units, heat transfer rates and pressure drops can be varied through changes in plate dimensions, number of passes, and herringbone stampings.

In summary, plate exchangers offer advantages in terms of compactness, heat transfer efficiency, and design flexibility, while tube exchangers, particularly shell-and-tube, provide strength and stability, making them suitable for high-pressure and high-temperature applications. The selection between the two depends on specific requirements and operating conditions.

4. Fabrication of Shell & Tube Heat Exchangers

The production of tube heat exchangers involves several key steps. The specific process may vary depending on the design, materials, and intended application, but here is a general overview of the production steps for tube heat exchangers:

  • Design and Engineering:
    • Define the heat exchanger requirements based on application, fluid properties, temperature, and pressure conditions.
    • Develop detailed engineering drawings and specifications.
  • Material Selection:
    • Choose materials for the tubes, shells, headers, and other components based on the operating conditions and compatibility with the fluids involved.
  • Tube Fabrication:
    • Cut tubes to the required lengths.
    • Expand or roll the tubes to ensure a tight fit within the tube sheets.
  • Tube Sheet Fabrication:
    • Fabricate tube sheets, which are usually made of a material compatible with the tubes.
    • Drill holes in the tube sheet to accommodate the tubes.
  • Assembly:
    • Insert the tubes into the tube sheet, ensuring a secure and leak-tight fit.
    • Weld or mechanically bond the tubes to the tube sheet.
  • Shell Fabrication:
    • Fabricate the shell or casing that will house the tubes.
    • Weld or connect the shell components.
  • Baffle Installation:
    • Install baffles inside the shell to direct the flow of fluid and enhance heat transfer.
    • Baffle design depends on the specific requirements of the heat exchanger.
  • Tube Bundle Installation:
    • Insert the assembled tube sheet and tube bundle into the shell.
    • Secure the tube bundle in place, ensuring proper alignment.
  • Header Installation:
    • Install headers or manifolds at the ends of the tubes to facilitate fluid inlet and outlet.
    • Weld or connect the headers securely.
  • Welding and Finishing:
    • Perform additional welding to ensure the integrity of the heat exchanger.
    • Inspect welds for quality and perform any necessary finishing processes.
  • Testing:
    • Conduct pressure tests to ensure the heat exchanger can withstand the specified operating pressures.
    • Check for leaks and structural integrity.
  • Surface Treatment:
    • Apply any required surface treatments, such as painting or coating, for corrosion resistance.
  • Quality Control:
    • Perform quality control inspections at various stages of production to ensure compliance with design and safety standards.

Photo 2: Heat Exchanger Quality Inspection

 

  • Documentation:
    • Generate documentation, including “as-built” drawings, material certifications, and test reports.
  • Shipping and Installation:
    • Package the heat exchanger for transportation.
    • Install the heat exchanger at the designated location.

Throughout the production process, it’s essential to adhere to industry standards and regulations to ensure the safety, reliability, and efficiency of the tube heat exchanger. The specific steps and details may vary based on the type and complexity of the heat exchanger being produced.

5. Materials used in Tubular (Shell & Tube) Heat Exchangers

Tube heat exchangers are designed to handle a variety of fluids and operating conditions, and the materials used in their construction depend on factors such as the type of fluids, temperature, pressure, and corrosion resistance requirements. Here are some common materials used in tube heat exchangers:

  • Tubes:
    • Copper: Excellent thermal conductivity and corrosion resistance. Commonly used in HVAC and small-scale applications.
    • Aluminum: Lightweight, good thermal conductivity, and corrosion resistance. Used in automotive and air-cooled applications.
    • Stainless Steel: Offers corrosion resistance and is suitable for a wide range of fluids. Various grades are available, such as 304, 316, and duplex stainless steel.
  • Tube Sheets:
    • Carbon Steel: Often used for tube sheets in less corrosive environments.
    • Stainless Steel: Used when corrosion resistance is a critical factor.
  • Shell:
    • Carbon Steel: Commonly used for the shell in less corrosive environments.
    • Stainless Steel: Used when corrosion resistance is required.
  • Baffles:
    • Carbon Steel: Provides structural support.
    • Stainless Steel: Used for corrosion resistance.
  • Gaskets:
    • Rubber: Commonly used for sealing purposes in less demanding applications.
    • Nitrile, EPDM, or Viton: Used for higher temperature and chemical resistance.
  • Expansion Joints:
    • Stainless Steel Bellows: Provides flexibility to accommodate thermal expansion and contraction.
  • Headers and Nozzles:
    • Carbon Steel or Stainless Steel: Depending on the application requirements.
  • Insulation:
    • Mineral Wool or Fiberglass: Provides thermal insulation to minimize heat loss.

It’s important to note that material selection depends on the specific conditions of the application. For example, corrosive environments may require the use of corrosion-resistant alloys, and high-temperature applications may necessitate materials with good thermal stability. Additionally, considerations for factors like cost, manufacturability, and ease of maintenance play a role in material choices for tube heat exchangers.

6. What is Novelty Steel Offering?

Novelty Structures offers contract heat exchanger fabrication for various industries.

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