Enlarge this picture Figure 1. In once-through cooling systems, the water passes through heat exchangers, where it cools the process, and then is discharged, never to return.

Continuing my discussion of cooling water treatment vs. cooling systems, in this column I will look at the third basic type of cooling systems -- once-through cooling water systems -- and offer some guidelines of selecting the type of cooling system to use for your process.

Remember, the objective of all cooling water treatments is to protect the cooling-water-contacted equipment. The cooling water must carry those protective inhibitors to those surfaces requiring protection. Treating the water is not the main objective.

Once-Through Cooling Water Systems. As shown in figure 1, these systems are similar to drinking water systems: The water comes in, is used and discharged. Once-through cooling water systems generally utilize river, lake or ocean water where plentiful supplies are available. The water passes through heat exchangers, where it cools the pro-cess and then is discharged, never to return. These systems do not aerate the water and no evaporation or concentration of water minerals occurs, so scale is seldom a problem. However, foulant deposits can occur due to suspended solids, organics, etc. Also, microbiological deposits and/or corrosion can occur.

Because water treatment of these systems can be cost prohibitive, the systems are designed with high velocities to minimize deposits and with corrosion-resistant alloys or coatings to minimize corrosion. But microbiological corrosion or deposits can occur, and at times, deposit and microbiological treatments are needed. Treatments of these systems will be discussed in future issues.

Selecting Cooling Water Treatments Accurately

As discussed, each cooling water system is considerably different; thus, the specific cooling water treatments needed to protect the cooling-water-contacted equipment will vary. It is important to note that treating the cooling water may not control the problems. The cooling water must bring those inhibitors to the water-contacted surfaces.

Specific cooling water treatment needs depend upon the design of the cooling system, including the different metallurgy and the type of cooling equipment such as heat exchangers with water on the shell- or tube-side or jacketed vessels (table 1). It requires a thorough knowledge of the quality of water contacting the water-using equipment. This means identifying scale, fouling, corrosion and microbiological contaminants. How consistent is the water quality or, more importantly, how much variation in water quality occurs? Design concerns and impacts on water treatment requirements have been identified in previous columns.

Also, the operation of the cooling water system is critical relative to the water treatment requirements. Does the cooling system operate continuously, or is it shut down periodically? This will greatly influence the levels of water treatment needed as well as the selection of specific inhibitors.

If velocities through heat exchangers are low (less than 1 ft/sec), this likely will result in deposits. Likewise, if water and skin temperatures are high --water temperature above 120^oF (44^oC) or skin temperature of 160^oF (65^oC), it could require better or higher levels of deposit inhibitors.

If restrictions on water discharge contaminants occur, treatment could be limited or prevented.

What Type of Cooling Water Equipment Protection Is Needed?

What are the requirements for the life expectancy and/or the operating efficiency of your equipment? This will greatly impact the water treatment requirements. Should you or your water treatment supplier dictate these requirements? I believe that the end user must establish these requirements. Therefore, you need to monitor the effectiveness of your cooling water treatment program and specifically the degree of equipment protection required for your facility. In future issues, I will discuss the monitoring technique available to ensure the desired cooling water equipment protection.

Remember! Use the cooling water to carry the protective inhibitors to the cooling-water-contacted surfaces. Treating the water likely will not protect your equipment. Understanding the needs, treatment limitations and objectives of cooling water treatment is critical to successfully protecting your cooling water equipment. Our next several articles will provide practical guidance.

Here is to better equipment protection, better understanding of water treatments and lower overall costs.