All Study Guides Heat and Mass Transfer Unit 5
❤️🔥 Heat and Mass Transfer Unit 5 – Heat ExchangersHeat 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 = − k A d T d x q = -kA\frac{dT}{dx} q = − k A d x d T
q q q : heat transfer rate (W)
k k k : thermal conductivity (W/m·K)
A A A : cross-sectional area (m²)
d T d x \frac{dT}{dx} d x d T : 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 = h A ( T s − T ∞ ) q = hA(T_s - T_∞) q = h A ( T s − T ∞ )
h h h : convective heat transfer coefficient (W/m²·K)
A A A : surface area (m²)
T s T_s T s : surface temperature (K)
T ∞ T_∞ T ∞ : 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 = ε σ A ( T 1 4 − T 2 4 ) q = \varepsilon\sigma A(T_1^4 - T_2^4) q = ε σ A ( T 1 4 − T 2 4 )
ε \varepsilon ε : emissivity (0 ≤ ε ≤ 1)
σ \sigma σ : Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴)
A A A : surface area (m²)
T 1 , T 2 T_1, T_2 T 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)
Evaluate heat exchanger performance using key parameters
Heat transfer rate (Q Q Q )
Overall heat transfer coefficient (U U U )
Log mean temperature difference (LMTD)
Effectiveness (ε \varepsilon ε )
Number of transfer units (NTU)
Heat transfer rate: Q = U A Δ T L M T D Q = UA\Delta T_{LMTD} Q = U A Δ T L MT D
U U U : overall heat transfer coefficient (W/m²·K)
A A A : heat transfer surface area (m²)
Δ T L M T D \Delta T_{LMTD} Δ T L MT D : log mean temperature difference (K)
Overall heat transfer coefficient: 1 U = 1 h 1 + t k + 1 h 2 \frac{1}{U} = \frac{1}{h_1} + \frac{t}{k} + \frac{1}{h_2} U 1 = h 1 1 + k t + h 2 1
h 1 , h 2 h_1, h_2 h 1 , h 2 : convective heat transfer coefficients (W/m²·K)
t t t : wall thickness (m)
k k k : thermal conductivity of the wall material (W/m·K)
LMTD: Δ T L M T D = Δ T 1 − Δ T 2 ln ( Δ T 1 / Δ T 2 ) \Delta T_{LMTD} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)} Δ T L MT D = l n ( Δ T 1 /Δ T 2 ) Δ T 1 − Δ T 2
Δ T 1 , Δ T 2 \Delta T_1, \Delta T_2 Δ T 1 , Δ T 2 : temperature differences at the ends of the heat exchanger (K)
Effectiveness: ε = Q Q m a x \varepsilon = \frac{Q}{Q_{max}} ε = Q ma x Q
Q Q Q : actual heat transfer rate (W)
Q m a x Q_{max} Q ma x : maximum possible heat transfer rate (W)
NTU method: N T U = U A C m i n NTU = \frac{UA}{C_{min}} NT U = C min U A
C m i n C_{min} C 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