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
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
S) in the presence of CO2
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).