Question

Under what conditions is the thermal resistance of the tube in a heat exchanger negligible?

Short answer

Answer: The thermal resistance of the tube in a heat exchanger can be considered negligible when the tube material has high thermal conductivity, the tube walls are thin, the surface area for heat transfer is maximized, and there is a large temperature difference between the fluids.

Step-by-step solution

Understanding Thermal Resistance

Thermal resistance is a measure of a material's ability to oppose or resist the flow of heat. In a heat exchanger, tubes conduct heat between two fluids, typically exchanging heat between a hotter fluid and a colder fluid. The tube's thermal resistance will determine how effectively heat is transferred between the fluids.

Factors Affecting Thermal Resistance

Several factors can affect the thermal resistance of a tube in a heat exchanger. These include: 1. Material: Different materials have different thermal conductivities. Higher thermal conductivity materials offer lower thermal resistance and are thus more effective in transferring heat. 2. Thickness of the tube walls: Thicker tube walls provide greater thermal resistance because the heat has to travel a longer distance to transfer between the fluids. Thin walls have lower thermal resistance, allowing for more effective heat transfer. 3. Surface area: A larger surface area for heat transfer will generally lead to lower thermal resistance, as heat transfer will take place across more points. This can be achieved by increasing the number of tubes, increasing tube diameter, or using a corrugated or finned tube design. 4. Temperature difference: The greater the temperature difference between the two fluids, the lower the thermal resistance. This is because the driving force for heat transfer is proportional to the temperature difference.

Conditions for Negligible Thermal Resistance

The thermal resistance of the tube in a heat exchanger can be considered negligible under certain conditions: 1. The tube material has excellent thermal conductivity: A material with high thermal conductivity, such as copper or aluminum, will provide minimal resistance to heat flow. 2. Thin tube walls: As tube wall thickness decreases, the thermal resistance decreases. It is essential, however, to choose a tube wall thickness that provides sufficient mechanical strength and durability for the intended application. 3. Maximum surface area: By maximizing the surface area for heat transfer, the tube's thermal resistance is minimized. This can be achieved through various heat exchanger designs, such as increasing the number of tubes, optimizing tube diameter, or using advanced techniques like corrugated or finned tubes. 4. Large temperature difference between fluids: When there is a significant difference in temperature between the fluids, it creates a strong driving force for heat transfer. In such cases, the thermal resistance of the tube becomes less significant in determining heat transfer rates. In conclusion, the thermal resistance of the tube in a heat exchanger can be considered negligible when the material has high thermal conductivity, tube walls are thin, the surface area for heat transfer is maximized, and a significant temperature difference exists between the fluids.