Thermal resistance plays a crucial role in the heat transfer process within a heat exchanger. As a heat exchanger supplier, understanding this relationship is essential for providing high - performance products to our customers. In this blog, we will delve into how thermal resistance affects the heat transfer process in a heat exchanger.
Understanding Heat Transfer in a Heat Exchanger
A heat exchanger is a device that transfers heat between two or more fluids at different temperatures. The basic principle of heat transfer in a heat exchanger is governed by Fourier's law of heat conduction and Newton's law of cooling. Heat can be transferred through three main mechanisms: conduction, convection, and radiation. In most practical heat exchangers, conduction and convection are the dominant modes of heat transfer.
Conduction occurs when heat is transferred through a solid material. For example, in a plate heat exchanger, heat is conducted through the metal plates separating the two fluids. The rate of heat conduction is given by Fourier's law: (Q = - kA\frac{dT}{dx}), where (Q) is the heat transfer rate, (k) is the thermal conductivity of the material, (A) is the cross - sectional area perpendicular to the direction of heat flow, and (\frac{dT}{dx}) is the temperature gradient.
Convection, on the other hand, involves the transfer of heat between a solid surface and a fluid in motion. Newton's law of cooling describes the convective heat transfer rate as (Q = hA\Delta T), where (h) is the convective heat transfer coefficient, (A) is the surface area, and (\Delta T) is the temperature difference between the solid surface and the fluid.
The Concept of Thermal Resistance
Thermal resistance is analogous to electrical resistance in an electrical circuit. Just as electrical resistance opposes the flow of electric current, thermal resistance opposes the flow of heat. The thermal resistance (R) is defined as the ratio of the temperature difference (\Delta T) across a material or a system to the heat transfer rate (Q), i.e., (R=\frac{\Delta T}{Q}).
In a heat exchanger, there are multiple sources of thermal resistance. These include the resistance due to the fluid - side convection on both the hot and cold fluid sides, and the resistance due to conduction through the separating wall.
Fluid - Side Convection Resistance
The convective heat transfer coefficient (h) is a measure of how effectively heat is transferred between the fluid and the solid surface. A low convective heat transfer coefficient implies a high convective thermal resistance. Factors that affect (h) include fluid velocity, fluid properties (such as viscosity, density, and specific heat), and the geometry of the flow passage.
For example, in a Counter Flow Heat Exchanger, the flow pattern of the hot and cold fluids is in opposite directions. This flow arrangement can enhance the convective heat transfer compared to a parallel - flow arrangement, reducing the convective thermal resistance on both the hot and cold fluid sides. Higher fluid velocities generally lead to higher convective heat transfer coefficients and lower convective thermal resistances. However, increasing the fluid velocity also increases the pumping power required to move the fluids through the heat exchanger.
Conduction Resistance through the Wall
The conduction resistance through the separating wall between the two fluids in a heat exchanger is determined by the thermal conductivity (k) of the wall material, its thickness (L), and the heat transfer area (A). The conduction resistance (R_{cond}=\frac{L}{kA}). Materials with high thermal conductivity, such as copper and aluminum, are often used in heat exchangers to minimize the conduction resistance.
For instance, in a Fusion Bonded Plate Heat Exchanger, the plates are made of materials with good thermal conductivity. The thin plates reduce the thickness (L), and the large surface area (A) of the plates helps to lower the conduction resistance, allowing for efficient heat transfer from the hot fluid to the cold fluid through the plate.
Impact of Thermal Resistance on Heat Transfer Rate
The overall heat transfer rate (Q) in a heat exchanger is inversely proportional to the total thermal resistance (R_{total}). The total thermal resistance in a heat exchanger is the sum of the individual thermal resistances on the hot fluid side, the cold fluid side, and the conduction resistance through the wall, i.e., (R_{total}=R_{h}+R_{cond}+R_{c}), where (R_{h}) is the convective thermal resistance on the hot fluid side, (R_{cond}) is the conduction thermal resistance through the wall, and (R_{c}) is the convective thermal resistance on the cold fluid side.
According to the formula (Q=\frac{\Delta T}{R_{total}}), where (\Delta T) is the overall temperature difference between the hot and cold fluids. A high total thermal resistance will result in a low heat transfer rate, meaning that less heat is transferred from the hot fluid to the cold fluid.
If the convective thermal resistance on the hot fluid side is high, for example, due to a low - velocity flow or a viscous fluid, the temperature drop across the hot - fluid boundary layer will be large. This reduces the effective temperature difference available for heat transfer and decreases the overall heat transfer rate. Similarly, a high conduction resistance through the wall or a high convective thermal resistance on the cold fluid side will also have a negative impact on the heat transfer rate.
Strategies to Reduce Thermal Resistance
As a heat exchanger supplier, we focus on developing strategies to reduce thermal resistance and improve the heat transfer performance of our products.
Optimizing Fluid Flow
We can optimize the fluid flow in the heat exchanger to increase the convective heat transfer coefficient and reduce the convective thermal resistance. This can be achieved by using proper flow distributors to ensure uniform flow distribution across the heat exchanger channels. We can also design the flow passages with appropriate geometries, such as corrugated plates in plate heat exchangers, which can enhance turbulence and increase the convective heat transfer coefficient.
Selecting High - Conductivity Materials
Using materials with high thermal conductivity for the separating walls of the heat exchanger is an effective way to reduce the conduction resistance. For example, in addition to copper and aluminum, some advanced alloys are also used in our heat exchangers to balance the requirements of thermal conductivity, mechanical strength, and corrosion resistance.
Cleaning and Maintenance
Over time, fouling can occur on the heat transfer surfaces, which increases the thermal resistance. Fouling is the accumulation of deposits such as scale, dirt, and biological growth on the surfaces. Regular cleaning and maintenance of the heat exchanger can remove these deposits and restore the original heat transfer performance.
Conclusion
Thermal resistance is a critical factor that affects the heat transfer process in a heat exchanger. As a heat exchanger supplier, we are committed to understanding and managing thermal resistance to provide our customers with heat exchangers that offer high - efficiency heat transfer. By optimizing fluid flow, selecting appropriate materials, and ensuring proper maintenance, we can minimize the thermal resistance and maximize the heat transfer rate.
If you are in the market for high - performance heat exchangers, we invite you to contact us for procurement and further discussions. Our team of experts is ready to assist you in selecting the most suitable heat exchanger for your specific application.
References
- Incropera, F. P., & DeWitt, D. P. (2002). Fundamentals of Heat and Mass Transfer. Wiley.
- Shah, R. K., & Sekulic, D. P. (2003). Fundamentals of Heat Exchanger Design. Wiley.