Heat Exchanger UA value - Query

[zamakaze]

For heat exchangers in simulations, I have often seen that sometimes the UA value is often held constant. Like its taken from a max/design case and kept constant for other cases like turndown. However, is this truly the correct approach? Given that the overall heat transfer coefficient (U) is influenced by film coefficients (h), which themselves depend on Reynolds number and flow velocity, wouldn't operating the exchanger in turndown mode inherently alter the U value? Shouldn't we account for variations in U rather than assuming a fixed UA, especially at lower flow rates where changes in flow regime might impact heat transfer performance?

 

[pierreick]

Hi,
From where this statement? " UA value is often held constant"
Consider this link to support your work.

https://pdhonline.com/courses/m371/m371content.pdf

Pierre

 

[georgeverghese]

Yes, this is a simplification that is often used when rating HX for some operating cases other than design flow. Qualitatively, the result with this simplification is directionally correct. See if it necessary to find a new UA value for the operating case you need to study. When in rating mode on a process simulator, if you have punched in the mechanical details of the HX, the program would calculate the new UA value.

 

[TiCl4]

To be honest, the only time I've seen UA calculated as a whole unit is on existing heat exchangers where you need to track fouling over time. Since the A is fixed, you don't really need to look up the area on each individual heat exchanger. Instead, you divide the heat load by the LMTD to get UA, the trend that value over time to determine when cleaning is needed.

I don't see the value in using UA to evaluate turndown. Any case that transfers less heat due to turndown should inherently be within the heat exchanger's capability, except extreme cases, perhaps, of very shear-thinning liquids, I guess.

For new heat exchangers, UA have to be evaluated separately, because A is the ultimate goal of the analysis (how much area does this thing need?).

 

[georgeverghese]

@TiCl4,
Modelling of heat exchangers in turndown with UA held constant as an approximation, enables the derivation of new terminal temperatures leaving the HX, albeit at a lower heat transfer duty, which then has some effect on downstream units.