Encrustation, crudding, deposition, scaling, scale formation, slagging, sludge formation…number of labels you can give to this term . In my engineering career i also had to face this problem in the industry. In engineering language basically it is deposition of unwanted compound on the surface of the solid. this so called solid compound is nothing else but heat exchanger tube surface, simple pipe surface or may be boiler tube surface. Why it is unwanted because it will decrease the process efficiency in many ways described below. Number of equipments and utilities have to deal with this problems like:
piping and flow channels:
Fouling greatly reduces the flow of fluid causing large pressure drop. It will cause great power as well as money loss.
(This figure is taken from Portuguese mine showing build up of scaling in pipe).
Fouling will drop the heat transfer between the walls resulting in loss of heat flux.
Greatly reduces the efficiency of turbines a results in mechanical failure.
Incorrect amount injected, component inefficiency,may cause failure.
Venturi tubes, orifice plates:
incorrect measurement of flow rate
Pitot tubes in airplanes:
Incorrect indication of airplane speed
Lot of other subjects regarding fouling but i mentioned few.
very common type of fouling in chemical industry:
Scaling or precipitated fouling is one of the biggest challenge in chemical industry. It not only disturbs the operation but can create life threatening conditions for the labor. For example most of the accidents regarding boilers are occurred due to scaling. Fouling is mainly attached with temperature and pressure variation. suppose if the temperature of salt solution increases it will cause saturation resulting in separation of salt from water.this salt will deposit on the surface in the form of limescale. let us consider a graph showing variation of solubility of salt with temperature.
From the graph we can easily understand that the solubility of calcium sulfate is decreasing with increasing temperature which means at higher temperature the probability of scale deposition will be high if water is not properly treated.
Cost analysis associated with fouling:
Despite the very large costs associated with fouling problem, very less research has been done to sort it out.Detailed study is required to evaluate the cost efficiency.costs related with fouling can be broken down into following categories
To nullify the fouling effect we usually keep excess surface area than design basis. It is usually around 10-15%, with average around 35%. It will increase the initial capital cost, require larger area for extra space, increase transport and installation costs.
Definitely there will be extra fuel costs in the presence of fouling. what fouling does is that it reduces the heat flux. so in order to meet our temperature requirements we have to burn more energy which will increase our cost.
Time is money in industry but also in our real world as well.To remove the scaling we have to shut down the plant at definite intervals which will reduce production and indirectly will reduce revenue.
Last one but important one!!!
According to Pritchard and Thackery about 15% of maintenance costs are related to boilers and heat exchangers and out of which half of the cost is related to fouling.
Fouling can be very expensive in refineries and petrochemical plants since it increases fuel consumption, results in discontinue operation and production losses, and this will increases maintenance costs. Panchal (Argonne National Laboratory), based on the study of Van Nostrand, re-calculated the energy and economic losses associated with heat exchanger fouling for the US refineries, as more than $2 billion per annum. The maintenance costs in the USA were revised because they have increased abruptly due to recent environmental regulations.