Dear Dr. Milan Zlatkovikj,

I suggest you to have a look at the following, interesting papers:

-Ash fouling monitoring and key variables analysis for coal fired power plant boiler

Yuanhao Shi,Jingcheng Wang

Thermal Science 19(00):118-118 (2013)

Available on RG at: https://www.researchgate.net/publication/270463798_Ash_fouling_monitoring_and_key_variables_analysis_for_coal_fired_power_plant_boiler

-FOULING IN BIOMASS FIRED BOILERS

Jan Sandberg

Mälardalen University Press DissertationsNo. 116 (2011)

Available at: https://www.diva-portal.org/smash/get/diva2:452326/FULLTEXT02.pdf

-A 540 °C/100 bar biomass boiler concept with improved fouling and corrosion resistance

Tom Blomberg

Eurocorr, Lisbon (2005)

Available on Rg at: https://www.researchgate.net/publication/317168514_A_540_C100_bar_biomass_boiler_concept_with_improved_fouling_and_corrosion_resistance

Best regards, Pierluigi Traverso

As you are no doubt aware there is no clear guidance on fouling factors. Experience is the best and only real guide. However, we base clean surface on a FF = 0.0018m2.K/W and where heavy fouling is expected use 0.007 m2.K/W. If the fouling is very severe, as in incinerators, we use 0.01 m2.K/W. Must emphasise that these are only rules of thumb and amount to enormous increases in surface over a clean unit.

Jack Broughton thanks a lot for those values! indeed, there are numerous construction and operational parameters that influence the magnitude and effect of fouling. since my current work mainly focuses on analysis of available measurements from plant, i was looking for at least size of the values. so i have basic idea, what initial assumptions to make prior to simulations. great is that the values that i was getting up to now are very close to the first value you specified.

Fouling factors are the preferred means of allowing for reduced heat transfer, but some engineers use surface area increases, e.g. 20% over the clean value to allow for depositions. Another approach is to calculate the outlet temperature under clean conditions then add say 25 deg K to this, but i think that that approach is far too arbitrary to be good.

Another issue with surface fouling is that it is often localised in the heat exchanger, e.g. near the entrance or at a specific temperature in the unit. A good book on this subject is T.R. Botts book on Fouling of Heat Exchangers, but it's a bit old now.

Jack Broughton thanks for the book suggestion, will check it.

what i could find from literature, is that due to fears of fouling and its effect on production rate and operation (depending of industry of course), there is serious over-sizing in the projection/design part of the heat exchangers.

i currently analyze operation, where i have fixed surface area for the heat exchangers of interest. of course there is some area increase due to deposits. at this stage i dont analyze in that direction, but we have one proposal to evaluate the condition of the heat exchangers in august, when the plant is making general overhaul.

on the suggestion of 25K adding, i assume you refer to the hot (generally gasses) side. up to now I saw at most temperature difference of 10-15K range, before and after soot-blowing process. this deviation would depend on the nominal values of the process and what is feasible to still have operation.

the point about localization is also valid - still to analyze this, way more measurements are required than what is generally installed in plants. i have more measurements available in the superheaters area compared to the economizer, currently working to see if i can extract something from there.