Stay Cool in a Crisis

When water-based process coolers and chillers fail or underperform, temporary cooling equipment can avert a plant shutdown

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Prevailing winds. The temporary tower likely will be lower in profile and could potentially send its hot discharge air into the air inlet of the primary tower. It should be positioned cross- or downwind when possible.

Tower elevation. In some cases, the temporary cooling tower inlet might be higher than that of the primary tower and might require the plant to pinch back the valves to each cell of the primary tower to ensure adequate water to the temporary cooling tower. It is unlikely that the elevation difference will impact pump performance significantly; however, the difference should be considered.

Basin elevation. Some temporary towers can be elevated to allow the cooled water to be gravity fed from the temporary tower basin into the permanent cooling tower basin. If the primary tower basin is too high and pumps are required to pump the water from the temporary tower basin to the primary tower basin, the plant should consider special automated valves and a level controller.

Fig. 2 shows a typical cooling tower augmentation.

 

Figure 2. Typical Augmentation

Isolating exchangers

Regardless of how well the primary cooling tower is performing, one or more exchangers might not have adequate water flow or might be more sensitive to temperature than others. When an exchanger is partially fouled, additional cooling water and colder temperatures can compensate enough to meet production demands until the plant can schedule an outage to correct the problem.

In these cases, the affected exch-anger(s) can be isolated from the primary tower and placed on a designated temporary cooling tower. The temporary cooling tower and pump can be sized to meet the new or desired requirements. This setup not only will relieve the limitations of the affected heat exchangers, but also will remove part of the heat load of the primary tower. When a portion of the heat load is removed from the primary tower, the tower will provide more and colder water to the exchangers still being served.

Fig. 3 illustrates how the limited equipment can be separated from the primary cooling tower.

 

Figure 3. Isolating Exchangers

Once-through cooling systems

At manufacturing facilities with large cooling needs, the use of once-through cooling systems is common. Many of these plants once were served reliably by once-through cooling, but now find they have severe limitations. Temporary cooling towers can be an effective tool in relieving these limitations.

Thermal discharge (environmental cooling).

Almost all once-through cooling systems have very strict environmental permits. These permits establish guidelines that limit the water volume that can be removed from a water source and the temperature at which the water can be returned to that source.

In some cases, a plant that depends on once-through cooling becomes limited in the hot summer months by the maximum discharge temperature limit. The restrictions often are magnified during low river/lake levels or drought conditions.

Temporary cooling towers ," meeting discharge, intake or total replacement cooling needs ," can serve as a quick, cost-effective solution to once-through cooling problems.

The most common and least intrusive solution is the installation of a temporary cooling system at the plant discharge area. A portion of the hot discharge water is pumped from the discharge canal to the temporary cooling towers, where it then is cooled and redeposited downstream with the remaining discharge water. The thermal requirements for most of these applications can usually be met by treating approximately 10 percent of the total water flow.

When a river is being used for the cooling source and the plant is limited by reduced water availability or excessively high inlet temperatures, intake cooling becomes attractive.

The temporary solution is installed in much the same way as the discharge cooling system. A portion of the discharge water is pumped from the discharge canal to the temporary cooling towers, where it is cooled and then redeposited into the plant intake area. This solution solves both the limited intake water supply and the excessively high intake temperatures. However, this option is usually more expensive than the discharge cooling option because the water temperatures required for intake normally are cooler than those required for the thermal discharge limit.

A third alternative ," the most radical choice ," is to completely remove the plant from its once-through cooling source and place it on a temporary cooling tower until normal water levels or temperatures return.

Preparing for disruptions

To better prepare for temporary cooling system disruptions, plant personnel should:

Have a contingency plan in place.

Know the cooling system, including its water flows, operating temperature ranges, wet-bulb temperatures, power requirements, heating or cooling demands and water sources.

Properly maintain the cooling tower and process water, and keep good records. Monitor the condition of parts that degrade (such as the wooden structure of the tower), and check the chemistry of the process water (fouling, scale, dissolved solids) and, if used, the water treatment system.

The cooling tower's manufacturer should be able to provide much of this information, especially that concerning maintenance. A water-treatment consultant can provide information on water-quality maintenance. In addition, a vendor of temporary cooling systems can provide information about contingency planning or process optimization.

Creating a contingency plan

A temporary cooling system is rarely a drop-in replacement for the installed unit, especially if the process conditions have changed since the cooling tower installation. Thus, chemical facilities can save time and headaches when they need a temporary system by creating a contingency plan.

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