Heat and Mass Transfer

❤️‍🔥Heat and Mass Transfer Unit 5 – Heat Exchangers

Heat exchangers are crucial devices that transfer thermal energy between fluids at different temperatures. They use conduction, convection, and radiation to move heat without mixing the fluids. Various types exist, including shell and tube, plate, and fin exchangers, each suited for specific applications. Heat exchanger design focuses on maximizing heat transfer while minimizing pressure drop and fouling. Key factors include material selection, flow arrangement, and surface area optimization. Performance analysis involves calculating heat transfer rates, overall coefficients, and effectiveness. Regular maintenance is essential for efficient operation.

Basics of Heat Exchangers

  • Heat exchangers transfer thermal energy between two or more fluids at different temperatures
  • Consist of a surface separating the fluids, allowing heat transfer without direct mixing
  • Classified based on flow arrangement (parallel flow, counter flow, cross flow)
  • Can be further categorized by construction type (shell and tube, plate, fin)
  • Heat transfer occurs through conduction, convection, and radiation
    • Conduction: heat transfer through direct contact of materials
    • Convection: heat transfer through fluid motion
    • Radiation: heat transfer through electromagnetic waves
  • Commonly used in various applications (HVAC systems, power plants, chemical processing)
  • Essential for energy conservation and process efficiency in many industries

Types of Heat Exchangers

  • Shell and tube heat exchangers
    • Consist of a bundle of tubes enclosed within a cylindrical shell
    • One fluid flows through the tubes while the other flows through the shell
    • Widely used in industrial applications due to their robustness and versatility
  • Plate heat exchangers
    • Composed of a series of corrugated plates stacked together
    • Fluids flow through alternate channels formed by the plates
    • Compact design and high heat transfer efficiency
    • Commonly used in food processing and pharmaceutical industries
  • Fin heat exchangers
    • Utilize extended surfaces (fins) to increase the heat transfer area
    • Fins can be attached to tubes or plates
    • Suitable for gas-to-liquid or gas-to-gas heat exchange
    • Often used in air conditioning and refrigeration systems
  • Double pipe heat exchangers
    • Simplest type, consisting of one pipe inside another
    • Fluids flow in a counter-current arrangement
    • Limited heat transfer area, primarily used for small-scale applications
  • Regenerative heat exchangers
    • Store thermal energy in a matrix (usually a solid material)
    • Hot and cold fluids alternately flow through the matrix
    • Examples include rotary regenerators and fixed matrix regenerators

Heat Transfer Mechanisms

  • Conduction
    • Heat transfer through direct contact between materials
    • Occurs due to the kinetic energy transfer between particles
    • Governed by Fourier's law: q=kAdTdxq = -kA\frac{dT}{dx}
      • qq: heat transfer rate (W)
      • kk: thermal conductivity (W/m·K)
      • AA: cross-sectional area (m²)
      • dTdx\frac{dT}{dx}: temperature gradient (K/m)
  • Convection
    • Heat transfer through fluid motion
    • Involves the combined effects of conduction and fluid motion
    • Two types: natural convection (buoyancy-driven) and forced convection (externally-induced flow)
    • Described by Newton's law of cooling: q=hA(TsT)q = hA(T_s - T_∞)
      • hh: convective heat transfer coefficient (W/m²·K)
      • AA: surface area (m²)
      • TsT_s: surface temperature (K)
      • TT_∞: fluid temperature (K)
  • Radiation
    • Heat transfer through electromagnetic waves
    • Occurs between surfaces at different temperatures without a medium
    • Governed by the Stefan-Boltzmann law: q=εσA(T14T24)q = \varepsilon\sigma A(T_1^4 - T_2^4)
      • ε\varepsilon: emissivity (0 ≤ ε ≤ 1)
      • σ\sigma: Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴)
      • AA: surface area (m²)
      • T1,T2T_1, T_2: surface temperatures (K)

Design Principles

  • Maximize heat transfer while minimizing pressure drop and fouling
  • Select appropriate materials based on fluid properties, operating conditions, and cost
    • Materials should have high thermal conductivity, corrosion resistance, and mechanical strength
  • Determine the optimal flow arrangement (parallel, counter, or cross flow) based on the application
  • Consider the heat transfer surface area and geometry
    • Increase surface area using fins, corrugations, or multiple passes
    • Optimize geometry to promote turbulence and enhance heat transfer
  • Ensure proper fluid distribution and minimize dead zones
    • Use baffles, headers, or distributors to achieve uniform flow
  • Account for thermal expansion and mechanical stresses
    • Provide adequate clearances and use expansion joints if necessary
  • Incorporate features for easy maintenance and cleaning
    • Include access ports, removable covers, or cleaning-in-place (CIP) systems
  • Comply with relevant codes, standards, and regulations (ASME, TEMA, API)

Performance Analysis

  • Evaluate heat exchanger performance using key parameters
    • Heat transfer rate (QQ)
    • Overall heat transfer coefficient (UU)
    • Log mean temperature difference (LMTD)
    • Effectiveness (ε\varepsilon)
    • Number of transfer units (NTU)
  • Heat transfer rate: Q=UAΔTLMTDQ = UA\Delta T_{LMTD}
    • UU: overall heat transfer coefficient (W/m²·K)
    • AA: heat transfer surface area (m²)
    • ΔTLMTD\Delta T_{LMTD}: log mean temperature difference (K)
  • Overall heat transfer coefficient: 1U=1h1+tk+1h2\frac{1}{U} = \frac{1}{h_1} + \frac{t}{k} + \frac{1}{h_2}
    • h1,h2h_1, h_2: convective heat transfer coefficients (W/m²·K)
    • tt: wall thickness (m)
    • kk: thermal conductivity of the wall material (W/m·K)
  • LMTD: ΔTLMTD=ΔT1ΔT2ln(ΔT1/ΔT2)\Delta T_{LMTD} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)}
    • ΔT1,ΔT2\Delta T_1, \Delta T_2: temperature differences at the ends of the heat exchanger (K)
  • Effectiveness: ε=QQmax\varepsilon = \frac{Q}{Q_{max}}
    • QQ: actual heat transfer rate (W)
    • QmaxQ_{max}: maximum possible heat transfer rate (W)
  • NTU method: NTU=UACminNTU = \frac{UA}{C_{min}}
    • CminC_{min}: minimum heat capacity rate (W/K)

Efficiency and Effectiveness

  • Efficiency: ratio of the actual heat transfer rate to the maximum possible heat transfer rate
    • Affected by factors such as flow arrangement, surface area, and fluid properties
    • Can be improved by optimizing design parameters and minimizing losses
  • Effectiveness: measure of how close the heat exchanger performance is to an ideal counterflow heat exchanger
    • Depends on the flow arrangement, heat capacity rates, and NTU
    • Higher effectiveness indicates better performance
  • Relationship between effectiveness and NTU
    • Effectiveness increases with increasing NTU
    • The relationship is non-linear and depends on the flow arrangement
  • Factors affecting efficiency and effectiveness
    • Fluid properties (viscosity, thermal conductivity, specific heat)
    • Flow rates and heat capacity rates
    • Fouling and scaling on heat transfer surfaces
    • Leakage and bypass flows
  • Strategies to improve efficiency and effectiveness
    • Periodic cleaning and maintenance to minimize fouling
    • Optimize flow distribution and minimize dead zones
    • Use enhanced heat transfer surfaces (fins, turbulators)
    • Implement heat recovery systems to utilize waste heat

Applications in Industry

  • HVAC systems
    • Used for heating, cooling, and dehumidification in buildings
    • Examples include air-to-air heat exchangers, water-to-air heat exchangers, and refrigerant-to-air heat exchangers
  • Power generation
    • Used in steam power plants for condensers, feedwater heaters, and air preheaters
    • Enhance overall plant efficiency by recovering waste heat
  • Chemical processing
    • Used for heating, cooling, and condensing process fluids
    • Examples include shell and tube heat exchangers, plate heat exchangers, and fin heat exchangers
  • Refrigeration and air conditioning
    • Used in condensers, evaporators, and subcoolers
    • Essential for heat rejection and absorption in refrigeration cycles
  • Automotive industry
    • Used in radiators, oil coolers, and air conditioning systems
    • Ensure proper cooling of engine components and passenger comfort
  • Food processing
    • Used for pasteurization, sterilization, and cooling of food products
    • Plate heat exchangers are commonly used due to their ease of cleaning and maintenance
  • Waste heat recovery
    • Utilize waste heat from industrial processes to generate steam, preheat combustion air, or drive absorption chillers
    • Improve overall energy efficiency and reduce environmental impact

Troubleshooting and Maintenance

  • Common issues in heat exchangers
    • Fouling and scaling: accumulation of deposits on heat transfer surfaces
    • Leakage: unintended mixing of fluids due to seal or gasket failure
    • Corrosion: degradation of materials due to chemical reactions with fluids
    • Vibration: caused by improper support, flow-induced vibration, or mechanical issues
  • Troubleshooting techniques
    • Monitor inlet and outlet temperatures and pressures
    • Perform visual inspections for leaks, corrosion, or damage
    • Use non-destructive testing methods (ultrasonic, radiographic) to detect internal flaws
    • Analyze fluid samples for contamination or degradation
  • Maintenance practices
    • Establish a regular cleaning schedule based on the application and fluid properties
    • Use chemical cleaning, mechanical cleaning, or high-pressure water jetting to remove fouling
    • Replace gaskets, seals, and other wear components as needed
    • Perform periodic inspections and tests to identify potential issues early
  • Preventive measures
    • Select materials compatible with the fluids and operating conditions
    • Use filtration systems to reduce particulate fouling
    • Implement water treatment programs to minimize scaling and corrosion
    • Install vibration monitoring and control systems
    • Provide proper support and anchoring to minimize mechanical stress
  • Maintenance planning and documentation
    • Develop a comprehensive maintenance plan with scheduled tasks and inspections
    • Document all maintenance activities, repairs, and replacements
    • Keep records of operating conditions, performance data, and fluid analysis results
    • Regularly review and update the maintenance plan based on the equipment's performance and condition


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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