Fouling Factors in Heat Exchangers
The fouling factors to be used in the design of heat exchangers are normally specified by the client based on their experience of running the plant or process. If uncontrolled, levels of fouling can negate any benefits produced by careful heat exchanger design. The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger, but they are often overstated by the end user in an attempt to minimise the frequency of cleaning. In reality, if the wrong fouling factor is used, cleaning may actually be required more frequently.
Fouling mechanisms vary with the application but can be broadly classified into four common and readily identifiable types.
Common Types of Fouling
- Chemical fouling: when chemical changes within the fluid cause a fouling layer to be deposited onto the tube surface. A common example of this phenomenon is scaling in a kettle or boiler caused by “hardness” salts depositing onto the heating elements as the solubility of the salts reduce with increasing temperature. This is outside the control of the heat exchanger designer but can be minimised by careful control of the tube wall temperature in contact with the fluid. When this type of fouling occurs it must be removed by either chemical treatment or mechanical descaling processes (wire brushes or even drills to remove the scale or sometimes high-pressure water jets).
- Biological fouling: this is caused by the growth of organisms within the fluid which deposit out onto the surfaces of the heat exchanger. Again this is outside the direct control of the heat exchanger designer, but it can be influenced by the choice of materials as some, notably the non-ferrous brasses, are poisonous to some organisms. When this type of fouling occurs it is normally removed by either chemical treatment or mechanical brushing processes.
- Deposition fouling: this is when particles contained within the fluid settle out onto the surface when the fluid velocity falls below a critical level. To a large extent this is within the control of the heat exchanger designer, as the critical velocity for any fluid/particle combination can be calculated to allow a design to be developed with minimum velocity levels higher than the critical level. Mounting the heat exchanger vertically can also minimise the effect as gravity would tend 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.
- Corrosion fouling: this is when a layer of corrosion products build up on the surfaces of the tube forming an extra layer of, usually, high thermal resistance material. By careful choice of materials of construction the effects can be minimised as a wide range of corrosion resistant materials based on stainless steel and other nickel-based alloys are now available to the heat exchanger manufacturer.
The use of corrugated tubes has been shown in be beneficial in minimising the effects of at least two of these fouling mechanisms: deposition fouling because of an enhanced level of turbulence generated at lower velocities, and chemical fouling. Chemical fouling is reduced because the enhanced heat transfer coefficients produced by the corrugated tube result in tube wall temperatures closer to the bulk fluid temperature of the working fluids.