Overall heat transfer coefficient

Definition

The overall heat transfer coefficient is a measure of a heat exchanger's ability to transfer heat between two fluids. It accounts for the thermal resistance offered by the conductive and convective heat transfer processes, as well as any fouling or surface resistance present. This coefficient is crucial for designing and analyzing heat exchangers, as it helps engineers understand how efficiently heat is exchanged between fluids.

5 Must Know Facts For Your Next Test

1. The overall heat transfer coefficient (U) is expressed in watts per square meter-kelvin (W/m²·K) and combines the effects of conduction, convection, and fouling.
2. The formula to calculate U involves adding individual thermal resistances, leading to $$rac{1}{U} = rac{1}{h_1} + R_f + rac{1}{h_2}$$, where $$h_1$$ and $$h_2$$ are convective heat transfer coefficients for each fluid, and $$R_f$$ represents fouling resistance.
3. Higher values of U indicate better heat transfer performance, meaning less thermal resistance and more efficient heat exchange between fluids.
4. The overall heat transfer coefficient can vary with fluid properties such as temperature, flow rates, and physical characteristics like viscosity.
5. Selection of materials and design configurations in a heat exchanger can significantly influence the overall heat transfer coefficient, making material choice critical for efficiency.

Review Questions

• How does the overall heat transfer coefficient influence the design and operation of a heat exchanger?
  • The overall heat transfer coefficient is vital for determining how efficiently a heat exchanger operates. A higher value of U suggests that the heat exchanger can transfer more heat between fluids with less thermal resistance. In designing a heat exchanger, engineers must consider factors affecting U, such as flow rates and material properties, to ensure optimal performance and energy efficiency.
• Discuss how fouling impacts the overall heat transfer coefficient in a heat exchanger system.
  • Fouling introduces additional thermal resistance that decreases the overall heat transfer coefficient in a heat exchanger. As deposits accumulate on the surfaces where heat exchange occurs, they hinder efficient thermal conduction and convection. This reduction in U means that more energy is required to achieve the same level of heating or cooling, ultimately leading to increased operational costs and potential failures if not managed properly.
• Evaluate the relationship between thermal conductivity and the overall heat transfer coefficient in terms of material selection for heat exchangers.
  • When selecting materials for heat exchangers, thermal conductivity plays a critical role in influencing the overall heat transfer coefficient. Materials with high thermal conductivity enhance U by allowing more efficient conduction through their surfaces. Conversely, poor thermal conductors increase thermal resistance, reducing the efficiency of heat exchange. Therefore, an understanding of both thermal conductivity and how it contributes to the overall system performance helps engineers make informed choices that balance cost and efficiency.

Related terms

Thermal conductivity: A property of a material that indicates its ability to conduct heat, typically measured in watts per meter-kelvin (W/m·K).

Heat exchanger effectiveness: A measure of a heat exchanger's performance that indicates the ratio of the actual heat transfer to the maximum possible heat transfer.

Fouling:

The accumulation of unwanted material on the heat transfer surfaces, which can decrease the efficiency of heat exchangers by adding thermal resistance.