Challenges
Corrosion, scaling, and biofouling are three important challenges in maintaining the proper functioning of cooling water systems.
In a thermoelectric power plant, electricity is totally (or partially if a gas turbine is employed1) generated through the rotation of a steam turbine driven by high pressure steam, which is obtained from boiler water heated by burning fossil fuels or nuclear by reactions2.  The steam then passes through a heat exchanger (or condenser) and is converted to liquid phase again for further circulation. In a heat exchanger, heat is transferred from steam to cooling water through high thermal conductance alloys such as copper and steel. The warmed cooling water is then pumped to a cooling tower where heat is removed by convective air cooling. The cooled water is then returned to the plant. Figure 1 is a schematic showing the process flow of boiler water and recirculating cooling water.
FIGURE 1. Â The schematic of the process flow of boiler water and recirculating cooling water.
Since the function of cooling water is to condense the steam through the heat exchanger, the heat exchange efficiency in the heat exchanger is very important. Good exchange efficiency relies on the design of the heat exchanger and the selection of high thermal conductance alloys, as well as the smoothness and cleanness of the heat exchanger alloy component surfaces. While the design of the heat exchanger and the selection of high thermal conductance alloys are determined before putting the cooling system into service, the smoothness and cleanness of the alloy surfaces can only be maintained through careful cooling water chemistry management.
Corrosion, scaling, and biofouling are the three main challenges causing the heat exchanger to lose heat exchange efficiency. Severe corrosion, scaling, and biofouling are common problems in cooling systems due to the changing gravity of recirculating cooling water and the difficulty in managing cooling water chemistry.
Corrosion of metals or metal alloys can create corrosion product depositing on the surfaces. In addition to reducing the heat exchange efficiency, penetrating corrosion can result in failure of heat exchanger function by contaminating the high purity boiler water with low quality cooling water.
Scaling is mineral precipitation and deposition from water onto the metal or metal alloy surface. Scale has much lower thermal conductance and reduces the heat exchange efficiency.
Fouling caused by microbiological growth on the surfaces of metal and metal alloys also reduces heat exchange efficiency. Biofilm growth can also cause microbiological influenced corrosion and enhance suspended solid attachment onto the surfaces.
Corrosion, scaling, and biofouling all reduce the inner diameter of heat exchanger tubing and result in more energy for pumping cooling water, which reduces the electricity generation efficiency.
FIGURE 1. Clean heat exchanger copper tubes. (Source: www.process-controls.com)
FIGURE 2. Heat exchanger with fouling problem. The fouling can be caused by corrosion, scaling, and biofilm growth.Â
(Source: (http://upload.wikimedia.org/wikipedia/commons/6/69/Fouling02.jpg)
Footnotes:
1. For a thermoelectric power plant burning natural gas or gasified coal, not only steam turbines but also gas turbines are employed to generate electricity. This type of power plant is called a combined cycle power plant. When natural gas is used, the plant is called natural gas combined cycle (NGCC) power plant. When gasified coal is used, the plant is called integrated gasification combined cycle (IGCC) power plant.
2. The thermoelectric power plant categories include pulverized coal (PC) plant, natural gas combined cycle (NGCC) plant, integrated gasification combined cycle (IGCC) plant, and nuclear plant.
