Dry-out counterintuitive phenomenon

A second post because it is my grand opening :)
The other day I had a long discussion with a customer. They had an existing model working and wanted to test some different scenario. This is something that I see more and more happening in the industry but this will be another post. One test was to run the same design with high fouling. Fouling can cause a severe reduction of performance both by reducing the duty and increasing the pressure drop. You can model both effect, but most of the time it is very difficult to know the conductivity and the thickness of the fouling so only the reduction of duty is calculated.
The fouling was added and the heat exchanger was performing better. This could only mean one thing the modelling was wrong... or there was a different phenomenon occurring. I suspect that I need to say that one side of the exchanger was boiling. Boiling heat transfer coefficients are usually very high compare to single phase. We could spend a lot of posts about boiling, but in 1 sentence: the hot side is providing a heat flux through the material which is used to boil the cold side.

The type of boiling depends of a lot of parameters including the heat flux mentioned earlier. If this heat flux is too big (the limit is very creatively called “critical heat flux” – we are engineers...) a phenomenon called dry-out occurs. During dry-out, the liquid is heated so quickly that a film of vapour is created next to the material. This vapour creates a layer with a very low heat transfer coefficient. Modern tools will calculate the critical heat flux and modify the local heat transfer coefficient to take account of the high resistance layer.

Now what is happening if you add fouling to the previous case? The heat reaching the fluid is reduced because of the extra resistance of the fouling and you could fall under the critical heat flux, and get back to a region with a high boiling heat transfer coefficient.

So in this particular case, adding fouling was improving the performance of the heat exchanger because the performance was restricted by a dry out zone.