Industrial cooling systems, such as cooling towers and heat exchangers, are designed to keep processes in acceptable temperature ranges in an efficient manner. Often, water is used as a cooling medium, due to its availability, relative chemical safety and low cost. However, the cost of water, the system maintenance for both fresh and recycled water, and the environmental requirements of returning cooling water back into the rivers and seas, mean that the water’s physical and chemical properties must be understood and controlled.

The consequences of using low quality cooling water manifest themselves in reduced flow, damaged process equipment and inefficient cooling. Ultimately, this results in increased downtime for unscheduled maintenance, products out of specification, and potential delays due to supply chain issues for hard-to-find parts.

Heat exchanger cleaning is expensive and time consuming. Source: Hammelmann Oelde/CC BY-SA 3.0

Cleaning up cooling water occurs in several stages. Physical contamination must be removed, the chemistry must be adjusted and biological growth must be limited. In addition to these steps, cooling water should be protected from unplanned phase changes and system preventative maintenance should include inspecting and repairing or replacing cooling loop components as needed.

Filtration

The first step in treating water is physical filtration. Filtration often occurs in several stages, ranging from large grates at water intakes in rivers and lakes scaling down to fine particulate filters. They remove all solid objects that would otherwise be passed into process equipment.

In general, filters are staged from removing the largest contaminants to the smallest. Consider a municipal water supply used for making coffee. Large grates remove branches and rocks from the water intake, followed by finer-meshed nets and grates to remove leaves and small fish. From there, water enters the pump house, and is distributed to water towers and other storage facilities, with inline filters of finer meshes along the way. The building might contain yet another, finer particulate filter and there may even be another one on the coffee maker. Finally, the water passes through the coffee grounds, and then a coffee filter as a final means of removing particulates.

Filters are not only cascaded in size as the flow approaches its usage point, but additional filters, appropriately sized, are required at any other entrance point to the system. If part of the system is opened for routine maintenance and a worker kicks some large rocks into the input stream, there must be a way to catch those rocks before they damage finer, more fragile filters.

Scaling or plaque buildup in a PVC pipe. Source: Vjdeep/CC BY-SA 3.0

Chemistry

Natural sources, and even municipal sources of water may have a chemistry that is detrimental to process equipment. Quantities such as pH, total dissolved solids (TDS), ammonia levels, salinity and others may need to be monitored to promote longer lifetimes of equipment and reduce scaling on components that lead to inefficient heat transfer.

pH

Testing the pH and adding buffer solutions can reduce the amount of corrosion due to cooling water being too acidic or alkaline. These buffer solutions also help stabilize the pH, making it less subject to rapid changes. Typically, the solutions are added at a storage point that has some mixing capability.

TDS and connectivity

TDS can be tested several ways. The most common method is to test the electrical conductivity of the water. Neutral, clean water is actually a poor conductor of electricity. As dissolved solids, such as salts or iron enter the water, the electrical conductivity increases. Increased TDS can lead to increased corrosion rates removing material, and it can also lead to scaling, where more material builds up on the inside of equipment, reducing flow and heat transfer.

Calcium is one of the worst culprits, accelerating scale deposition. High calcium levels, sometimes referred to as “hard water,” is responsible for much of the scale that clogs filters, causes valves to seat improperly, restricts flow in heat exchangers and all sorts of other problems. High concentrations of magnesium will also lead to scale formation, but calcium is the driving factor most of the time.

A water softener can reduce the quantity of TDS. In particular, the softener removes the calcium and magnesium ions. Softened water systems use potassium chloride or sodium chloride as an ion exchange agent.

Biological agents

Besides the removal of particulates and dissolved solids, water should be free of biological agents, such as bacteria, fungi and algae. Most of these living contaminants will pass through filters untouched and end up in process fluid streams. While filtering does remove some of the energy sources (food) for the biofilms, additional methods of prevention and removal are necessary.

The first few cells find a spot of low flow, perhaps in an eddy, low spot, or downstream of a valve or other fixture. They will multiply and expand their coverage, becoming a biofilm that clings to the inside of process equipment and pipes. These biofilms have all the disadvantages of scaling in that they reduce flow and thermal conductivity, with the added problem of potentially causing sickness or product contamination downstream. This is a particular problem in the pharmaceutical, food and beverage industries.

Chemical additives, such as chlorine dioxide, biocides and even a few dispersants will limit biofilm growth. Alternative methods of cleaning include steaming, as the high temperature will kill biofilms, and exposure to UV light.

Phase change protection

For large industrial facilities, particularly those with outdoor installations, freeze protection is required. Freeze protection can take the form of heating tapes along pipes, forced air heaters at key locations, and other prevention mechanisms. Besides prevention, there may also be mitigation mechanisms, such as bellows or freeze plugs that expand and prevent ruptures elsewhere along pipes or in process equipment.

Equally as important, heat exchangers must be designed so that water is not allowed to boil at hot spots. This means tailoring the flow of the process fluids such that fluid does not sit in a heated zone for long enough to cause it to boil.

Final thoughts

Water quality and sourcing clean water will always be an issue. Even though the planet’s surface is 70% water, almost all of it must be cleaned before use in homes and in industry. The considerations listed above are only some of the problems and solutions used to make water safe to drink and efficient as a cooling fluid.