Page 177 - CW E-Magazine (7-11-2023)

Special Report

in the process fluid. Particulate foul- to heat transfer. In other words, it rep- posits on heat transfer surfaces. Infra-
ing results in the formation of a layer resents how much additional thermal red thermography can also be used to
of solid material on the heat exchanger resistance the fouling layer adds to the detect temperature variations on heat
surfaces and can reduce heat transfer heat transfer surfaces. The fouling fac- exchanger surfaces. Bore-scoping and
efficiency and increase pressure drop. tor is expressed in units of m².K/W. endoscopy can be used to inspect the
interior of tubes and other difficult-to-
Scaling fouling When designing heat exchangers, access areas.
Scaling fouling occurs when mine- the fouling factor can be used to calcu-
rals or other inorganic materials in the late the appropriate fouling allowance – Predictive maintenance
process fluid precipitate out and adhere the maximum amount of fouling a heat By analysing data from various sen-
to the heat exchanger surfaces, forming exchanger can shoulder without com- sors and instruments, predictive main-
a hard, crusty layer. Scaling can reduce promising the machine’s efficiency – tenance techniques (built through mode-
heat transfer efficiency and increase and to determine the optimal cleaning lling or digital twins, for example) can
pressure drop. frequency and maintenance schedule. be used to detect and predict the onset
of fouling and schedule maintenance
Corrosion fouling How to detect fouling in heat exchangers? or cleaning before it impacts the heat
Corrosion fouling develops due to Some of the most common methods exchanger’s performance.
the process fluid’s reaction with the to detect fouling in heat exchangers are:
heat exchanger materials. Corrosion How to remove fouling in heat
can result in the formation of local Pressure drop monitoring exchangers?
pits or other irregularities on the heat As fouling builds up on the heat The methods for controlling the
exchanger surfaces, which can trap pro- transfer surfaces, it increases flow problem can be classified as: chemi-
cess fluids and lead to fouling. resistance and causes an increase in cal means (addition of inhibitors),
pressure drop across the heat exchanger. mechanical means, changing the phases
Biological fouling Therefore, the pressure drop is a sound of the solution, electromagnetic fields,
Biological fouling occurs when micro- performance indicator for detecting electrostatic fields, acoustic fields,
organisms, such as bacteria or algae, changes in fouling levels so cleaning or ultraviolet light, radiation or catalytic
grow on the heat exchanger surfaces. maintenance can be scheduled accord- treatment, surface treatment, green
This type of fouling can result in the ingly. additives, fibre as a suspension, etc.
formation of a layer of organic mate-
rial, which eventually reduces overall Heat transfer coefficient measurement In the past, chromate was a success-
heat transfer efficiency and increases The heat transfer coefficient, which ful chemical agent for corrosion protec-
pressure drop. decreases as fouling accumulates, tion and crystal growth control until it
directly indicates the heat transfer effi- was banned. Polyphosphate corrosion
Chemical fouling ciency. Thus, measuring this coefficient inhibitors were introduced for the
Chemical fouling occurs when can be used to identify when corrective replacement of chromate-based additives.
chemical reactions between the chemi- actions are necessary. This inhibitor has a tendency to decom-
cal contaminants in the process fluid pose the foulant in water containing
and the heat exchanger materials result Process fluid analysis high calcium hardness. Various studies
in the formation of unwanted com- Chemical analysis of the process revealed that the deposition of calcium
pounds or deposits. There are several fluid can reveal the presence of fouling phosphate (fouling) could be inhibited
types of chemical fouling, including agents, such as scale or corrosion very effectively by using copolymers
scaling fouling (described above). products, and provide insight into the such as acrylic acid/hydroxypropyl
Chemical fouling can reduce heat trans- fouling’s severity. acrylate or acrylic acid/sulfonic acid in
fer efficiency, increase pressure drop high calcium water containing phos-
and cause equipment failure. Non-destructive inspection and test- phate corrosion inhibitor. The above
ing methods additives were found to be effective in
The Fouling Factor Techniques such as ultrasonic test- varying conditions of pH, surface tem-
The Fouling Factor measures the ing (UT) or radiographic testing have perature, and velocities.
extent to which fouling in a heat exchanger been widely applied to detect and
influences the process fluid’s resistance quantify the thickness of fouling de- In the past, most, chemicals/addi-

Chemical Weekly November 7, 2023 177

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