Cooling Water Treatment vs. Cooling Systems, Part 4

Paul Puckorius continues his series on cooling water treatment vs. cooling systems with a look at the heat transfer equipment in a process cooling system.

As I have noted throughout this series, a thorough knowledge of the cooling water equipment and the cooling system operation is required prior to determining or selecting a water treatment program.

In my last column, I looked at heat exchangers as a part of my discussion of the heat transfer equipment in your cooling system. In addition to heat exchangers, other equipment is specific to certain cooling water systems.

The heat transfer equipment is considered to be the most critical because it is the heart of the cooling system and performs the main objective of the intent of the cooling system: to cool. Thus, the heat transfer equipment is the most important and is listed as the top priority.

The next most critical equipment varies with different types of cooling water systems and in different industries and often depends on the different types of other equipment within the cooling system. For example, a cooling water system utilizing a galvanized steel cooling tower would likely make the cooling tower the next priority to be protected. But, if the cooling tower was fabricated with plastic, wood or even concrete, it would be a much lower priority. As another example, in a closed cooling water system, the heat rejection coils or radiator would be a high priority.

The piping often is quite high in the priority rating, particularly if fabricated with mild steel and if of small diameter (3" or less). This is due to restrictions by corrosion product or scale deposits.

The pump and its components usually are very low in priority.

Each cooling water system owner must identify and prioritize his list of cooling water equipment that must be protected.

Materials of Construction

Perhaps I am getting ahead of myself. Once the design of the cooling water equipment is identified, the materials of construction of that equipment must be identified. Mild steel corrodes rapidly and thus needs to be located within the cooling water system to determine how critical severe corrosion (i.e., leaks and deposits of any corrosion products) would impact the integrity of the cooling water system. Heat exchangers with mild steel tubing are a good example of equipment requiring excellent corrosion control. Even mild steel piping requires very good corrosion control, particularly with inner diameters of 3" or less. Copper alloys can corrode but usually at a much lower rate; galvanized steel cooling towers and/or piping can corrode quite rapidly.

Stainless steel also can corrode rapidly in cooling water under certain water qualities, but under protective water qualities, it can last a life time.

Aluminum is very susceptible to corrosion, again depending upon the water quality.

Nonmetallic materials of construction generally do not deteriorate rapidly, but cooling tower wood can decay, concrete can be attacked, certain plastics can soften or harden and fail. Even fiberglass-reinforced plastic can gradually deteriorate under specific water qualities.

To know your system, you must identify the metallurgy and nonmetallic materials of construction and in which specific cooling water equipment they are located. You are now two-thirds of the way to knowing your system.

Cooling System Piping. Identifying the piping's materials of construction will help determine the water treatment requirements. Most common in all cooling water systems is mild steel piping. Copper is common in smaller diameter (2" ID or less) piping. Stainless steel is utilized in some facilities; galvanized piping is seldom used. Occasionally, aluminum piping appears, usually in closed cooling water systems. Rarely, titanium and specialty molybdenum-containing steels are used due to unusually severe conditions. Fiberglass-reinforced plastic as well as PVC piping are being used more today to reduce corrosion potential. Concrete often is used in large cooling water systems.

Heat Rejection Equipment. The heat rejection equipment of a cooling water system also must be identified relative to materials of construction as well as design. These units, whether cooling towers, evaporative condensers, evaporative coolers, or even radiators or coils, can influence the selection of the water treatment program.

Cooling towers can be constructed of preservative-treated wood, galvanized steel, stainless steel, PVC, and concrete. Identifying the materials of construction will determine if the water treatment should include products to protect the cooling tower. Galvanized steel certainly does; wood towers require special consideration for water quality; and concrete also may require some considerations.

The cooling tower internal design also is a consideration that can influence the cooling water treatment program.

Cooling towers utilize several types of internal fill designs to provide maximum water evaporation. Boards, slats or bars have been used to break up the water droplets in the past. Though these methods still are utilized, in some applications they have been replaced by a new design fill of closely packed plastic sheets to maximize water evaporation. This design is prone to deposit buildup, even plugging, reducing cooling tower efficiency. Severe cases of fill plugging have resulted in its collapse into the basin of the cooling tower.

If used, plastic fill greatly influences the water treatment program and must be identified and included in the information on the cooling water system.

Cooling System Operation

The next most important piece of information about your cooling water system is its operation (table 1). Yes, this means if it is continuous 24 hours a day, 7 days per week, or if it shuts down periodically. This also includes individual chillers, heat exchangers and even cooling towers. Continuous adequate cooling water flow is desirable and is easiest to develop an economical, effective water treatment program. Periodic shut down, if not considered in selecting and implementing a water treatment program, can lead to premature failures and reduction in efficiency as well as higher costs.