Is the overall heat transfer coefficient constant for a certain heat exchanger?

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SUMMARY

The discussion centers on the calculation of outlet temperatures in a counter-flow heat exchanger when the overall heat transfer coefficient (U) and surface area (A) are not explicitly provided. The user inquires whether the previously calculated product of U and A (U*A) can be reused in a second example, given that the only change is the flow rate of one fluid while maintaining the same inlet temperatures and fluid properties. It is established that while the surface area (A) remains constant, the overall heat transfer coefficient (U) may vary with changes in flow conditions, thus necessitating a reevaluation of U for accurate calculations.

PREREQUISITES
  • Understanding of heat exchanger principles, specifically counter-flow configurations.
  • Familiarity with the concept of overall heat transfer coefficient (U) and its significance in thermal calculations.
  • Knowledge of how flow rates affect heat transfer characteristics in fluids.
  • Basic proficiency in thermal analysis and temperature calculations in engineering contexts.
NEXT STEPS
  • Research the impact of flow rate changes on the overall heat transfer coefficient (U) in heat exchangers.
  • Study methods for calculating outlet temperatures in counter-flow heat exchangers using the effectiveness-NTU method.
  • Explore the relationship between heat transfer coefficients and fluid properties in various flow regimes.
  • Learn about the use of computational fluid dynamics (CFD) tools for simulating heat exchanger performance.
USEFUL FOR

Engineers, thermal system designers, and students studying heat transfer who are involved in the design and analysis of heat exchangers, particularly those working with counter-flow configurations.

MarkH748
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Hi guys. Without going into too much detail. Basically I have a counter-flow heat exchanger problem where I need to find one of the outlets temps only given the inlet temps of each fluid. I have no idea how to do it though as I'm not given a value for over heat transfer coefficient (U) or surface area (A). However, in a previous example the same heat exchanger was used and one of the outlet temps was given which allowed me to calculate the value of the product of the overall heat transfer coefficient and the surface area (U*A).

I was wondering if I can use this value again for the 2nd example? I know A is probably constant but is U constant too? All that has been changed for the 2nd example is flow rate of one of the fluids (same inlet temps and fluid properties).

Any help would be greatly appreciated as my understanding of U isn't all that great.

Mark.
 
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