The organic fouling includes microbiological growth or slime on the surfaces of the heat
exchanger plates and pipes. Algae may also formed on these surfaces. This type of fouling is
more significant in open systems whereby oxygen dissolved in the water through the open
storage tank will encourage the microbiological growth.
Scaling is a type of inorganic fouling. It is caused by deposit of inorganic salts (e.g. calcium
sulfate, calcium carbonate) in the water onto the heat exchanger plate surfaces and pipes. Such
phenomenon occurs when the water temperature gets higher, usually > 50°C. This is because
the solubility of the salts becomes lower when the temperature becomes higher. When coupled
with low water velocities (laminar flow) and uneven flow distributions (e.g. at areas of water
stagnation in the heat exchanger), a thin hard layer of scaling will form on the surfaces.
Generally, scaling is not a problem with chilled water systems.
Symptoms:
1. A heat exchanger which has been choked with coarse debris will normally exhibit a
sudden increase in pressure drop. This is due to the sudden block of water flow by the
debris.
2. Both the organic and inorganic surface fouling will show a gradual increase of pressure
drop and a gradual drop in the heat exchanger performance.
Remedy
For both the above symptoms, it is necessary to remove all these fouling material in order to have
an optimum chiller performance. We have mentioned earlier that the coarse debris is relatively
easy to remove by cleaning the filter elements and flushing out with water. Nevertheless,
sometimes some of the larger debris may be stuck in between the plates.
If such blockages are too severe, it may be necessary to change a new BPHE altogether. For
both the surface fouling, a chemical cleaning is needed to remove them.
a. Organic fouling
Use an alkaline cleaning agent like caustic soda or sodium hydroxide (NaOH) at a 5%
concentration. Mix with clean water and then pump in the solution into the heat exchanger and
piping system. An external circulator pump may be used for this purpose.
Refer to the chemical manufacturer recommendations for directions of usage. Additional
surface active substances may need to be added into the mixture for better cleaning
performance.
After cleaning, flush the whole system with clean water to remove
Section 9 Page 109
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