During the lifetime of a heat exchanger its performance will be influenced by what happens on the surface where the heat is exchanged. On the surface deposits of materials can accumulate that reduce the heat transfer and increase the pressure drop. This is referred to as fouling.

A number of questions arise when one designs a heat exchanger:

how much additional surface is needed to cater for fouling?

how much additional pressure drop can be expected due to fouling?

are provisions needed for cleaning (chemical or mechanical)?

is regular cleaning / inspection required

is it possible to reduce the buildup?

which materials of construction are preferred?

The tendency for fouling depends on many variables that influence each other and can be difficult to address with a theoretical model. Allowing for fouling is therefore a matter of experience. A lot of this experience is available in many tabulations of typical fouling factors. The tabulation provides typical fouling factors. Fouling is also frequently specified as a fouling coefficient, which is the reciprocal value ( 1/x) of the factor.
The tabulations give good benchmarks. The designer, however, needs to judge each particular application.

Fouling can be caused by several mechanisms which in fact can happen at the same time (combined). The most important basic mechanisms are:

Crystallization (e.g. Mg- and Ca- bicarbonates)

Decomposition of organic products resulting in tar or cokes

Polymerization and or oxidation

Settlement of sludge, rust or dust particles

Biological deposits

Corrosion

The driving force of crystallization is supersaturation. For most salts the solubility gets higher with increasing temperatures. There are exceptions that have a large bearing on heat exchanger design, such as calcium sulfate. Calcium sulfate scales are hard and adhesive. Calcium and magnesium carbonates derived from the corresponding bicarbonates can also form a scale. Sulfate scales cause more heat transfer loss than carbonates. The scaling that starts in cooling water at 45 ° C has been well documented. As a margin of 5 ° C is considered sufficient cooling water effluent temperature should not be higher than 40 ° C.

Crystallization on a clean surface will start with nucleation. A smooth and scratch free surface will help as a nucleus generally starts at a scratch. A crystal can be flushed from a smooth surface. A high fluid velocity will increase the attrition and can reduce fouling. For cooling water a velocity between 1.8 and 2 m/s is recommended. In turn down situations, try to keep it above 0.8 m/s.

Hot heat exchanger surfaces can be covered with tar or coke deposits that are in fact products of a chemical reaction. This chemical reaction occurs near the hot surface and produces solid particles or very viscous tar that accumulate on the surface.

This is another form of fouling as a result of a chemical reaction. The layer of plastic can be very difficult to remove.

Solid particles can settle on heat exchange surfaces. Many streams contain suspended solids, cooling water is a good example. The settling process is mainly a function of the particle characteristics and the velocity. Increasing velocity will help to flush the particles off the surface.
Some types of particles can bake on the surface and will become more difficult to remove over time.

Algae, fungi and filamentous bacteria can catch suspended nutrients and form slimes that stick on the surface. These slimes may be the starting point for anaerobic bacteria that can cause corrosion, one of these causes pitting underneath the scale by the reduction of sulfates into sulfides (H₂S) in the presence of CO₂2.
Mussels can reduce both flow and heat transfer to an absolute minimum.

The traditional way of dealing with this is by killing the life by chlorination (continuous or a short period with a high concentration). Of course the environmental aspects need to be looked at. Another way is to use copper-nickel materials for the heat exchange surface.

Corrosion is oxidation of metal. Electro chemical corrosion by dissolved oxygen in the fluid is the common form of corrosion. Chemical attack by present acidic solutions is another form of electro chemical corrosion.
This can cause rust scales on the surface, but more importantly can lead to failure of the complete heat exchanger.

To remove fouling three methods are popular:

Mechanical cleaning (brushing, scraping, ...)

Chemical cleaning (solvent or chemical reaction)

High velocity water jets

Already during the design a lot of decisions are taken. If mechanical cleaning is required for one of the fluids it is wise to put that fluid in the tube side. For more considerations about the choice between shell or tube side see fluid allocation. A fixed tube sheet with a front end channel and removable cover (TEMA type A..,) will allow maintenance and cleaning of the tube side with the piping still connected to the nozzles. The shell side, however, can only be chemically cleaned. U tube exchangers will allow pulling of the bundle. The tubes can be mechanically cleaned provided that the tube pattern and pitch provide sufficient space and access to the inside of the bundle. For this a square pitch is required as with this pattern one can 'look between the tubes' through the whole bundle. The distance between the tubes (pitch - diameter) needs to be at least 6½ mm (¼ inch) to allow cleaning access.

Naturally the choice of a material that is resistant to the fluid (and if applied chemical cleaning liquid) will save money and prevent big quantities of rust particles.
To reduce biological fouling, copper nickel alloys will discourage the settlement of biological life. generally 70% to 90% copper and 30% to 10% nickel are used for this purpose (UNS 71500 and UNS 70600).