5.1 Types of Heat Exchangers

Heat exchangers are crucial devices in thermal systems, transferring heat between fluids without mixing them. This section dives into the various types, from tubular to plate designs, each with unique features suited for different applications.

Understanding heat exchanger types is key to selecting the right one for a specific job. We'll explore how construction and flow arrangements affect performance, helping you make informed decisions in thermal system design.

Heat Exchanger Types

Construction-based Classification

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• Tubular heat exchangers include double pipe, shell and tube, and spiral tube heat exchangers, each with specific construction features and flow arrangements
  • Double pipe heat exchangers consist of concentric pipes, with one fluid flowing through the inner pipe and the other through the annular space (suitable for small-scale applications)
  • Shell and tube heat exchangers have a bundle of tubes enclosed within a shell, allowing for high pressure and temperature operations (widely used in chemical and process industries)
  • Spiral tube heat exchangers feature a spiral-shaped tube bundle, promoting turbulence and enhancing heat transfer (used in viscous fluid applications)
• Plate heat exchangers consist of a series of plates with flow channels, allowing for counter-current or cross-flow arrangements and easy disassembly for cleaning
  • Gaskets or welding are used to seal the plates and create flow channels
  • Corrugated plate designs promote turbulence and increase heat transfer surface area (commonly used in food and pharmaceutical industries)
• Extended surface heat exchangers, such as plate-fin and tube-fin heat exchangers, employ fins to increase the heat transfer surface area and enhance performance
  • Plate-fin heat exchangers have corrugated fins sandwiched between parallel plates, creating compact and lightweight designs (often used in cryogenic and aerospace applications)
  • Tube-fin heat exchangers have fins attached to the outer surface of tubes, enhancing heat transfer in gas-to-liquid or gas-to-gas applications (air conditioning and refrigeration systems)

Flow Arrangement-based Classification

• Flow arrangements in heat exchangers can be parallel-flow, counter-flow, cross-flow, or a combination of these, each with distinct temperature profiles and effectiveness
  • Parallel-flow arrangement has both fluids entering and exiting the heat exchanger at the same end, resulting in a lower temperature difference and heat transfer effectiveness
  • Counter-flow arrangement has fluids entering and exiting the heat exchanger at opposite ends, achieving the highest temperature difference and heat transfer effectiveness
  • Cross-flow arrangement has fluids flowing perpendicular to each other, resulting in a moderate temperature difference and heat transfer effectiveness (commonly used in automotive radiators and air-cooled condensers)
• Regenerative heat exchangers, such as rotary and fixed matrix types, utilize a heat storage medium to transfer heat between hot and cold fluids alternately
  • Rotary regenerative heat exchangers use a rotating matrix (heat wheel) to store and release heat, efficiently transferring heat between gas streams (power plants and waste heat recovery systems)
  • Fixed matrix regenerative heat exchangers have a stationary matrix that alternately stores and releases heat as hot and cold fluids pass through (used in high-temperature applications)

Heat Exchanger Principles

Working Principles

• Double pipe heat exchangers transfer heat between fluids through the inner pipe wall, with the fluids flowing in either parallel or counter-current directions
  • The overall heat transfer coefficient depends on the fluid properties, flow rates, and pipe materials
  • Suitable for small-scale applications with limited heat transfer surface area requirements
• Shell and tube heat exchangers transfer heat between a fluid flowing through tubes and another fluid flowing through the shell side
  • Baffles are used to direct the shell-side fluid flow and enhance heat transfer by promoting turbulence
  • Various tube configurations (straight, U-tube, or helical) and shell types (E, F, G, H, J, K, or X) are available to suit different applications
• Plate heat exchangers transfer heat between fluids flowing through alternate channels formed by corrugated plates
  • The corrugated plate design promotes turbulence, enhances heat transfer, and reduces fouling
  • Gaskets or welding are used to seal the plates and create flow channels, allowing for easy disassembly and cleaning
• Plate-fin heat exchangers transfer heat between fluids flowing through channels formed by corrugated fins sandwiched between parallel plates
  • The fins increase the heat transfer surface area and promote turbulence, resulting in compact and lightweight designs
  • Various fin types (plain, perforated, or serrated) and flow arrangements (counter-flow, cross-flow, or multi-pass) are used to optimize performance
• Tube-fin heat exchangers transfer heat between a fluid flowing through tubes and another fluid (usually air or gas) flowing across fins attached to the tubes
  • The fins increase the heat transfer surface area on the air/gas side, enhancing overall heat transfer performance
  • Various fin types (plain, wavy, or louvered) and tube arrangements (staggered or in-line) are used to suit different applications

Applications

• Double pipe heat exchangers are used in small-scale applications, such as oil cooling, steam heating, and process fluid heat transfer
• Shell and tube heat exchangers are widely used in chemical, petrochemical, power generation, and HVAC industries for various applications, including:
  • Oil coolers, condensers, and evaporators in refrigeration systems
  • Feedwater heaters and condensers in power plants
  • Process fluid heating and cooling in chemical plants
• Plate heat exchangers are commonly used in food, dairy, beverage, and pharmaceutical industries for applications such as:
  • Pasteurization and sterilization of milk and other dairy products
  • Heating and cooling of beverages, such as beer and fruit juices
  • Heat recovery in food processing plants
• Plate-fin heat exchangers are often used in cryogenic, aerospace, and automotive applications, including:
  • Liquefaction and vaporization of gases in air separation and LNG plants
  • Fuel cell heat exchangers and cryogenic cooling systems in aerospace applications
  • Charge air coolers and EGR (Exhaust Gas Recirculation) coolers in automotive engines
• Tube-fin heat exchangers are widely used in HVAC, refrigeration, and process industries for applications such as:
  • Air conditioning and refrigeration condensers and evaporators
  • Air-cooled heat exchangers in process plants and power generation
  • Radiators and oil coolers in automotive and industrial engines

Heat Exchanger Advantages vs Limitations

Advantages

• Double pipe heat exchangers are simple, cost-effective, and easy to maintain for small-scale applications
  • Suitable for high-pressure and high-temperature operations due to their robust construction
  • Can handle fluids with large differences in operating pressures or temperatures
• Shell and tube heat exchangers offer high pressure and temperature capabilities, diverse configurations, and easy maintenance
  • Wide range of materials available for construction, suitable for various fluid types and operating conditions
  • Modular design allows for easy capacity expansion or modification
• Plate heat exchangers provide high heat transfer rates, compact designs, and easy cleaning
  • Corrugated plate design promotes turbulence and reduces fouling, enhancing heat transfer performance
  • Easily disassembled for cleaning and maintenance, reducing downtime
• Plate-fin heat exchangers are compact, lightweight, and offer high heat transfer surface area
  • Suitable for applications with space or weight constraints, such as aerospace and cryogenic systems
  • Various fin types and flow arrangements available to optimize performance for specific applications
• Tube-fin heat exchangers enhance heat transfer in gas-to-liquid or gas-to-gas applications
  • Fins increase the heat transfer surface area on the air/gas side, improving overall performance
  • Widely used in HVAC, refrigeration, and process industries due to their versatility and reliability

Limitations

• Double pipe heat exchangers have limited heat transfer surface area and are not suitable for high-volume processes
  • Higher cost per unit heat transfer area compared to other types due to their simple construction
  • Not ideal for applications requiring frequent cleaning or maintenance due to limited accessibility
• Shell and tube heat exchangers can be bulky and expensive for large-scale applications
  • Higher capital cost compared to other types due to their complex construction and material requirements
  • Prone to fouling and requiring regular cleaning, especially when handling dirty or corrosive fluids
• Plate heat exchangers are limited by gasket materials and may not be suitable for high-pressure or high-temperature applications
  • Gasket failures can lead to fluid leakage and cross-contamination, requiring frequent replacement
  • Not suitable for fluids with high particulate content or fibrous materials due to clogging risk
• Plate-fin heat exchangers are more susceptible to fouling and have higher pressure drops compared to other types
  • Narrow flow channels can easily clog with particulates or debris, requiring frequent cleaning
  • Higher pressure drops due to compact design and multiple flow passages, increasing pumping power requirements
• Tube-fin heat exchangers can be prone to fouling and have higher pressure drops on the gas side
  • Fin surfaces can accumulate dirt, dust, or frost, reducing heat transfer performance over time
  • Higher pressure drops on the gas side due to flow resistance through the fin passages, increasing fan power consumption

Heat Exchanger Selection

Selection Factors

• Consider factors such as fluid properties, operating conditions (temperature, pressure, and flow rates), available space, maintenance requirements, and cost when selecting a heat exchanger
  • Fluid properties, such as viscosity, density, thermal conductivity, and fouling tendency, influence the heat transfer performance and material compatibility
  • Operating conditions determine the required heat transfer rate, pressure drop limitations, and material selection based on temperature and pressure ratings
  • Available space and weight constraints may dictate the use of compact or lightweight heat exchanger designs, such as plate-fin or printed circuit heat exchangers
  • Maintenance requirements, including cleaning, repair, and replacement, should be considered to ensure long-term performance and minimize downtime
  • Capital and operating costs, including initial investment, energy consumption, and maintenance expenses, should be evaluated to optimize the total cost of ownership
• Evaluate the heat transfer performance, pressure drop, and fouling resistance of different heat exchanger types for the specific application
  • Calculate the required heat transfer rate and compare it with the performance of various heat exchanger types using appropriate correlations and design methods
  • Assess the pressure drop limitations and pumping power requirements for each heat exchanger type, considering the allowable pressure drop and energy consumption targets
  • Analyze the fouling resistance and cleaning requirements of different heat exchanger types based on the fluid properties and operating conditions to ensure long-term performance
• Assess the compatibility of heat exchanger materials with the working fluids to ensure long-term reliability and prevent corrosion or degradation
  • Select materials that are resistant to corrosion, erosion, and chemical attack by the working fluids, considering factors such as pH, chloride content, and dissolved gases
  • Evaluate the thermal and mechanical properties of materials, such as thermal conductivity, strength, and expansion coefficient, to ensure reliable operation under the specified conditions
  • Consider the use of protective coatings, cathodic protection, or sacrificial anodes to enhance the corrosion resistance of heat exchanger materials in aggressive environments

Design Considerations

• Analyze the scalability and modularity of the heat exchanger design to accommodate future capacity expansions or modifications
  • Consider the use of modular heat exchanger designs, such as shell and tube or plate heat exchangers, that allow for easy capacity expansion by adding or removing units
  • Assess the feasibility of using multiple heat exchanger units in parallel or series arrangements to meet the required heat transfer capacity and provide operational flexibility
  • Evaluate the impact of future process changes, such as increased production rates or new product introductions, on the heat exchanger design and sizing
• Consider the ease of installation, operation, and maintenance of the heat exchanger in the given industrial setting
  • Assess the required installation space, piping connections, and support structures for the heat exchanger, considering the layout and accessibility of the installation site
  • Evaluate the operating procedures, control strategies, and monitoring requirements for the heat exchanger to ensure safe and efficient operation
  • Consider the maintenance activities, such as cleaning, inspection, and repair, and their frequency to minimize downtime and ensure long-term performance
• Consult industry standards, guidelines, and best practices when selecting and designing heat exchangers for specific applications to ensure compliance and optimal performance
  • Follow relevant industry standards, such as ASME (American Society of Mechanical Engineers), TEMA (Tubular Exchanger Manufacturers Association), or API (American Petroleum Institute), for heat exchanger design, fabrication, and testing
  • Refer to application-specific guidelines and best practices, such as those provided by trade associations or equipment manufacturers, to ensure optimal performance and reliability
  • Engage with experienced heat exchanger designers, manufacturers, and service providers to leverage their expertise and support in selecting and designing the most appropriate heat exchanger for the given application