By Eugene le Roux, FSAIRAC, and Eamonn Ryan
Would you say that heat exchangers are widely used?
Eugene le Roux. © RACA Journal
Predicting the unknown: designing a new heat exchanger
But what if the heat exchanger hasn’t been built yet? How can engineers predict the value of the total heat transfer coefficient, ℎtot, which is key to determining performance?
This is where things become challenging — and even described as a ‘black art’ in thermal engineering. The total heat transfer coefficient is influenced by several factors:
- Fluid flow rates and flow regimes (laminar, turbulent)
- Thermophysical properties (viscosity, thermal conductivity, specific heat)
- Surface condition and geometry (pipe diameter, roughness, fins)
In early design stages, engineers use empirical correlations, experimental data, and computational simulations (CFD) to estimate performance. But these predictions carry uncertainties that often require real-world validation.
Phase change and constant temperature processes
The situation becomes even more complex when phase changes are involved. For example, during condensation or evaporation, the temperature of the fluid can remain nearly constant, even as massive amounts of heat are absorbed or released. These processes are typical in:
- Air conditioning condensers (gas to liquid)
- Refrigeration evaporators (liquid to gas)
- Steam generators and condensers in power plants
During condensation, heat transfer coefficients can be significantly higher than in single-phase heat transfer. For instance:
- Ammonia condensing on water: ℎ ≈ 850 – 1400 W/m2 ⋅ K
- Other refrigerants: often half that range
This variability must be taken into account in the design and selection of materials and geometries.
Conclusion: a balance of science and art
Heat exchangers are foundational to modern engineering systems — from cars to chemical plants. While many aspects of their operation can be measured and calculated with confidence, designing new systems or working with phase changes introduces complexities that require experience, intuition, and often trial-and-error.
Ultimately, mastering heat exchangers demands both an understanding of the underlying physics and the ability to navigate the real-world uncertainties of materials, fluids, and operation.
