Article By Giuseppe Tommasone
Common types of fouling
Chemical fouling takes place when chemical changes within the fluid cause a layer of fouling to be deposited on to the tube surface. A common example of this phenomenon is the sealing caused in a kettle or boiler as a result of ‘hardness’ salts depositing on to the heating elements as the solubility of the salts reduce with increasing temperature. This sort of phenomenon is outside the control of the heat exchanger designer but can be minimized by careful control of the tube wall temperature, which is in contact with the fluid. When this type of fouling occurs it must be removed by either chemical treatment or mechanical de-sealing processes (wire brushes or even drills to remove the scale, or sometimes even through high-pressure water jets).
Biological fouling is caused by the growth of organisms within the fluid, which deposit on to the surfaces of the heat exchanger. This type of fouling is once again outside the direct control of the heat exchanger designer but can be influenced by the choice of materials as some, notably non-ferrous brasses, are poisonous to certain organisms. When this type of fouling occurs it can normally be removed by either chemical treatment or mechanical brushing processes.
Deposition fouling occurs when particles contained within the fluid settle out on to the surface when the fluid velocity falls below a critical level. This phenomenon can be largely controlled by the heat exchanger designer as the critical velocity for any fluid particle combination can be calculated. This allows a design to be developed with minimum velocity levels that are higher than the critical level. Mounting the heat exchanger vertically can also minimize the effect as gravity tends to pull the particles out of the heat exchanger, away from the heat transfer surface even at low velocity levels. When this type of fouling occurs it is normally removed by mechanical brushing processes.
- larger heat transfer coefficients;
- smaller heat transfer surfaces required;
- lower fouling due to high fluid turbulences (self-cleaning effect);
- significantly less installation and maintenance space required;
- a lighter weight
- a simplified cleanability especially for plate heat exchangers;
- lower investment costs;
- a closer temperature approach;
- pure counter-flow operation for plate heat exchangers.
Fouling inside a heat exchanger can be reduced by:
- using an appropriate heat-exchanger design;
- making the correct selection of heat-exchanger type;
- choosing the correct mitigation methods (mechanical and chemical);
- choosing the correct heat exchanger surface modification/coating
I have my own personal experience with regards to this fouling factor discussion. Working for a large oil & gas EPC in Italy, I was told to design a heat exchanger with standard fouling factors for a monoethanolamine gas sweetening job despite my argumentation to the contrary. After installation, I received a phone call to inform me that the unit was not performing according to specifications. I suggested opening up the plate heat exchanger and taking away 50% of the plates and then restarting. After this the performance was perfect.