Plant operators are acutely aware of the damages caused by scale and corrosion in recirculating cooling systems and usually focus their attention on controlling these serious problems. However, there are other water related factors which adversely affect cooling system efficiency, such as the growth of microorganisms and the accumulation of silt and other foulants. With a few relatively simple procedures, plant operators can often uncover these problem conditions at an early stage and can take prompt corrective action.
Where might these occur and what kind of problems can be caused by these microorganisms? Slime-forming bacteria, algae, fungi and other microorganisms often grow profusely in cooling water systems. Bacteria may thrive anywhere in the system, but algae require sunlight and, therefore, are usually found growing in the exposed areas of the cooling tower, top deck, peripheral fill and sump. The presence of algae does not necessarily indicate a problem but, if growth is heavy, distribution holes can become plugged resulting in poor water distribution through the fill. Further, when a mass sloughs off, there is a possibility it will get into the piping and restrict system water flow. The presence of one type of organism is often an indication that another may be present. Bacteria can also thrive in heat exchangers where they can’t be seen and they, too, pose the threat of restricted flow. They also form an insulating layer on heat transfer surfaces and cause an increase in the corrosion rate of metal equipment. Fungus is a lower form of plant life which will often attack cooling tower wood and cause deterioration.
System fouling by algae and bacteria may be indicated by these symptoms:
- Rising temperatures of process water in the heat exchanger
- Decrease in Delta T across a heat exchanger or cooling tower
- Higher pumping pressures
- Lower vacuum pressures
It is important to keep in mind that factors other than the presence of microorganisms can cause these same symptoms. Proper interpretation by someone knowledgeable in cooling system operation is necessary.