5.2 Overall heat transfer coefficient and fouling factors
3 min read•july 31, 2024
Heat exchangers are crucial in many industries, transferring heat between fluids. The (U) is key, measuring how well heat moves through the system. It considers all resistances, from fluids to walls to fouling layers.
Fouling, the buildup of unwanted deposits, is a major issue in heat exchangers. It adds resistance, reducing efficiency over time. Engineers use fouling factors to account for this in design, ensuring exchangers perform well even as deposits accumulate.
Overall Heat Transfer Coefficient
Definition and Significance
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- The overall heat transfer coefficient (U) measures the total to heat transfer in a heat exchanger
- Accounts for individual resistances of fluids, wall, and fouling layers
- Expressed in units of W/(m^2•K) or Btu/(hr•ft^2•°F)
- Represents the rate of heat transfer per unit area per unit between hot and cold fluids
- Quantifies the overall heat transfer performance of a heat exchanger
- Considers combined effects of , , and fouling
- Higher U indicates better heat transfer performance and more efficient heat exchange between fluids
- Critical parameter in the design and analysis of heat exchangers
- Directly influences required heat transfer area and overall effectiveness
Calculating Heat Transfer Coefficient
Resistance Network Approach
- Overall heat transfer coefficient calculated using resistance network approach
- Total thermal resistance is the sum of individual resistances in series
- For a simple double-pipe heat exchanger:
- 1/U = 1/h_i + R_w + 1/h_o
- h_i and h_o are convective heat transfer coefficients of inner and outer fluids
- R_w is the thermal resistance of the wall
- 1/U = 1/h_i + R_w + 1/h_o
- For a with multiple tubes:
- U_o = 1 / [A_o / (h_i•A_i) + R_w + 1/h_o]
- U_o is overall heat transfer coefficient based on outer (shell-side) surface area
- A_o and A_i are outer and inner surface areas
- U_i = 1 / [1/h_i + R_w•(A_i/A_o) + A_i / (h_o•A_o)]
- U_i is overall heat transfer coefficient based on inner (tube-side) surface area
- U_o = 1 / [A_o / (h_i•A_i) + R_w + 1/h_o]
Calculation Requirements
- Calculation of overall heat transfer coefficient requires:
- Knowledge of fluid properties (viscosity, thermal conductivity)
- Flow rates of fluids
- Geometrical characteristics of the heat exchanger (dimensions, surface areas)
- For a :
- Overall heat transfer coefficient calculated considering thermal resistances of plates and convective heat transfer coefficients of fluids on both sides of plates
- Accurate calculation essential for proper heat exchanger design and performance evaluation
Fouling and Heat Exchanger Performance
Concept of Fouling
- Fouling refers to accumulation of unwanted deposits on heat transfer surfaces over time
- Examples: scale, corrosion products, biological growth
- Creates an additional thermal resistance layer on the heat transfer surface
- Reduces effective heat transfer area
- Increases overall thermal resistance
- Leads to a decrease in overall heat transfer coefficient and reduction in heat exchanger performance
Impact on Heat Exchanger Operation
- Fouling causes several adverse effects on heat exchanger operation:
- Reduced heat transfer efficiency
- Increased pressure drop
- Reduced flow rates
- Potential blockages in flow channels
- Impact depends on type and thickness of fouling layer, operating conditions (temperature, flow velocity, fluid composition)
- Regular cleaning and maintenance necessary to mitigate effects of fouling
- Restores heat transfer performance to optimal levels
Fouling Factors for Fluids
Definition and Units
- Fouling factors (fouling resistances) quantify thermal resistance added by fouling layer on heat transfer surface
- Expressed in units of (m^2•K)/W or (hr•ft^2•°F)/Btu
- Specific to type of fluid, heat exchanger material, and operating conditions
Determination Methods
- Fouling factors can be determined experimentally
- Measure change in overall heat transfer coefficient over time as fouling accumulates
- Empirical correlations and tables available in literature
- Provide typical fouling factors for various fluids and heat exchanger materials under different operating conditions
- Choice of depends on:
- Fluid velocity
- Temperature
- Fluid composition
- Expected level of fouling based on experience or industry standards
Design Considerations
- Conservative fouling factors often used in heat exchanger design
- Account for worst-case scenario
- Ensure adequate heat transfer performance even with fouling present
- Regular monitoring of overall heat transfer coefficient and comparison with design value
- Helps assess actual fouling condition
- Determines when cleaning or maintenance is required
Key Terms to Review (20)
ASME Standards: ASME Standards are guidelines developed by the American Society of Mechanical Engineers that establish norms and best practices for mechanical engineering, particularly in design, construction, and maintenance. These standards ensure safety, reliability, and efficiency in the engineering processes, and they are widely recognized and adopted across various industries, including heat exchange systems.
Biofouling: Biofouling refers to the accumulation of microorganisms, plants, algae, and animals on wet surfaces, particularly in marine environments. This process can significantly impact the performance and efficiency of heat exchangers and other heat transfer equipment by creating thermal resistance, which reduces the overall heat transfer coefficient. Understanding biofouling is crucial for maintaining system efficiency and preventing operational issues.
Conduction: Conduction is the process by which heat energy is transferred through a material without any movement of the material itself, primarily occurring via collisions between neighboring atoms and molecules. This thermal energy transfer plays a crucial role in various physical processes, influencing heat distribution, energy efficiency, and the design of thermal systems.
Convection: Convection is the transfer of heat through a fluid (liquid or gas) caused by the motion of the fluid itself. This process occurs when warmer, less dense regions of the fluid rise while cooler, denser areas sink, creating a continuous circulation that facilitates heat transfer. Understanding convection is essential for analyzing energy and mass transport phenomena, as it connects various aspects of thermal dynamics, fluid behavior, and transport properties.
Flow Rate: Flow rate is the volume of fluid that passes through a given surface per unit of time, typically expressed in units like liters per second or cubic meters per hour. In heat transfer applications, understanding flow rate is crucial as it directly influences the heat transfer performance and efficiency of systems, impacting calculations involving the overall heat transfer coefficient and fouling factors.
Fouling Factor: The fouling factor is a correction factor used in heat exchanger design and analysis that accounts for the decrease in heat transfer efficiency due to the accumulation of unwanted deposits on heat transfer surfaces. This build-up can significantly reduce the overall heat transfer coefficient, impacting system performance and efficiency. Understanding the fouling factor is crucial for selecting appropriate heat exchanger types and ensuring effective operation over time.
Fouling Resistance: Fouling resistance refers to the ability of a surface to resist the accumulation of unwanted materials, such as deposits or biological growth, which can hinder heat transfer processes. This resistance is critical in maintaining efficiency in heat exchangers and is directly tied to the overall heat transfer coefficient, which quantifies how effectively heat is transferred across different media. A high fouling resistance minimizes the impact of fouling factors, ensuring optimal performance and longevity of thermal equipment.
Fourier's Law: Fourier's Law states that the heat transfer rate through a material is proportional to the negative gradient of temperature and the area through which heat flows. This principle is fundamental in understanding how thermal energy is conducted in materials, linking thermal conductivity to temperature differences, and laying the groundwork for analyzing heat transfer processes across various mediums.
Heat Exchanger Effectiveness: Heat exchanger effectiveness is a measure of a heat exchanger's ability to transfer heat relative to its maximum possible heat transfer capability. It is defined as the ratio of the actual heat transfer to the maximum heat transfer that could occur under ideal conditions, providing insights into the efficiency of the heat exchanger's performance. Understanding this concept is essential, as it relates directly to how well heat exchangers operate in practice, factoring in parameters like overall heat transfer coefficients and how fouling can affect performance.
Newton's Law of Cooling: Newton's Law of Cooling states that the rate of heat loss of an object is directly proportional to the temperature difference between the object and its surroundings, provided that this temperature difference is small. This principle helps in understanding how heat transfer occurs between an object and its environment, linking to momentum, energy, and mass transport concepts through a common framework that describes how substances exchange thermal energy.
Overall Heat Transfer Coefficient: The overall heat transfer coefficient is a measure that quantifies the total heat transfer through a composite system, taking into account conduction, convection, and sometimes radiation. It is crucial in analyzing how effectively heat moves through various materials and interfaces in processes like heat exchangers, evaporators, and condensers.
Plate Heat Exchanger: A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two or more fluids. The design allows for a high surface area to volume ratio, promoting efficient heat transfer while minimizing the space required for operation. This efficiency makes plate heat exchangers popular in various applications, including food processing, HVAC systems, and chemical processing.
Radiation: Radiation is the process by which energy is emitted as particles or waves, often in the form of electromagnetic waves such as infrared radiation, visible light, and ultraviolet light. This energy transfer mechanism is vital in understanding various physical processes, as it influences temperature distribution, heat transfer efficiency, and the interaction between matter and energy.
Scaling: Scaling refers to the accumulation of deposits, often mineral-based, on heat transfer surfaces, which can significantly reduce the efficiency of heat exchangers and other thermal systems. This buildup obstructs the flow of heat and fluids, leading to decreased performance and increased energy consumption, which connects directly to the overall heat transfer coefficient and fouling factors in thermal systems.
Shell-and-tube heat exchanger: A shell-and-tube heat exchanger is a type of heat exchanger that consists of a series of tubes, one set carrying the hot fluid and the other carrying the cold fluid, with the two fluids separated by a shell. This design allows for efficient heat transfer between the fluids, making it ideal for various industrial applications. The effectiveness of this system depends on factors like the overall heat transfer coefficient and potential fouling, which can affect its performance over time.
Specific Heat Capacity: Specific heat capacity is the amount of heat energy required to raise the temperature of one unit mass of a substance by one degree Celsius. This property is crucial for understanding how materials respond to heat transfer and can influence the efficiency of thermal systems, as well as the rate at which materials conduct or store heat.
Tema: Tema is a term used to describe the overall heat transfer coefficient, which measures the efficiency of heat exchange in a system. It plays a crucial role in understanding how heat moves through different materials and interfaces, factoring in the influence of fouling, which can reduce the effectiveness of heat exchangers. Recognizing the importance of tema helps engineers design more efficient thermal systems by accounting for both heat transfer and potential obstructions caused by fouling.
Temperature Difference: Temperature difference is the measure of the variation in temperature between two points or systems, typically expressed in degrees Celsius or Fahrenheit. It is a crucial factor that drives heat transfer, as heat naturally flows from regions of higher temperature to regions of lower temperature. Understanding temperature difference is essential for optimizing energy efficiency and performance in various thermal systems, influencing calculations for heat exchangers and overall heat transfer rates.
Thermal resistance: Thermal resistance is a measure of a material's ability to resist the flow of heat through it. This concept is crucial for understanding how heat transfers through different materials and geometries, influencing overall heat transfer efficiency in various applications. It plays a significant role in designing systems where heat management is essential, like insulation in buildings or heat exchangers in industrial processes.
U-value: The U-value, or thermal transmittance, measures the rate at which heat is transferred through a building element, such as walls, roofs, and floors. It is a crucial indicator of a material's insulating properties and is expressed in watts per square meter Kelvin (W/m²K). Understanding U-values helps in evaluating energy efficiency, designing effective thermal insulation, and improving overall building performance.