By Bob Ferrante and Patrick Williams, Aggreko LLC

When a fast-moving cold front passed through the Southeast in early spring 2008, it triggered tornadoes and severe thunderstorms throughout the region. One system north of Savannah, Ga., spawned a tornado that left a path of destruction several miles long.

A combined cycle power plant lay directly in the path of the storm, but the tornado lifted just before hitting one of the plant’s cooling towers. Even so, it damaged the fans and fan shroud structures at the top of that tower. It then touched down again, completely destroying the plant’s second, newly constructed cooling tower 300 feet away. The plant remained essentially undamaged other than the cooling towers, but with a total loss of condenser cooling capacity it was put out of business.

The company worked with a vendor to design a temporary cooling system and 10 days later the temporary system was online and the plant was generating power. The new cooling tower took almost four months to construct—four months of lost revenue and power production capacity without the temporary solution.

In addition to tornado, wind or ice damage, there is always the risk of fires, rot and structural degradation for even the most well-made or maintained structures, so temporary systems are used for emergency or planned repair.

There is a growing trend for plants to develop contingency plans to provide for partial or complete cooling water replacement in the event of an emergency, so that the planning and approval work is completed prior to any maintenance program. And as the nation’s fleet of crossflow, counterflow and hyperbolic cooling towers age, the temporary option gives plants the ability to plan partial tower outages, install a temporary system and continue generating at capacity.

Augmenting Cooling Systems

Many plants rely on river or lake water for their condenser and equipment cooling and must ensure that discharge water stays within certain temperature limits. Problems with reduced river flows and lake levels and increased intake temperatures mean that plants are having to reduce load to meet discharge temperature requirements. The alternative is to use temporary cooling towers.

Houston, Texas-based Aggreko LLC operated five thermal discharge projects at power plants during the summer of 2008. Discharge water from the plant is passed through the cooling towers for cooling and is then released into the river or lake. Depending on the specifications of the permits, the water may be pumped from the discharge area of the body of water, cooled and returned downstream or away from the temporary cooling system intake. With modern data recording systems, it is relatively easy to document system performance, Btu removal and system run time, which helps to verify cost-benefit models as well as satisfy regulating bodies or inspectors.

A thermal discharge installation used because of structural failure of the permanent tower. Photo, Aggreko.Click here to enlarge image

During summer months and periods of increased temperatures, compressor back pressure issues can occur. Plants are finding financial benefits in using temporary cooling towers to augment installed cooling water systems to reduce compressor back pressure. The advantages of this type of system include continued generation, reduced number (or severity) of de-rates and reduced fuel usage costs.

For a myriad of reasons, many plants are finding that they desire reduced cooling water temperatures or additional cooling water for equipment and lube oil cooling. Increases in reliability and lifecycle tracking have led to the desire to reduce lube oil temperatures in turbines and cooling water temperatures for other equipment. Temporary cooling tower systems can be used to augment existing cooling towers or to provide cooling water to systems that are isolated from the existing cooling towers. By isolating the cooling water systems, the temporary system provides exclusive equipment and lube oil cooling and the existing cooling towers see reduced flow, which lowers the supply water temperature.

Industry Trends

The temporary cooling tower industry started in 1985. Today, towers are modular, allowing for almost limitless system capability. Typical system flows range from 80,000 gpm to 250,000 gpm. The elevated basin design accommodates gravity draining, which eliminates the need for additional pumping requirements.

A variety of cooling tower manufacturers and rental companies can provide temporary systems. Choices range from companies with full engineering staffs and project managers to companies that simply provide equipment.

Rental cooling tower companies can undergo the same certification process permanent towers do by using a Cooling Technology Institute recognized testing company. A third party will test in accordance with CT-105 specification. CTI certification can make engineering management and plant manager approval for a proposed system easier because of the verification of performance by an independent source. CTI certification should be a minimum requirement for any temporary cooling tower system provider.

There have been many variations in the design of temporary cooling towers and the fill used to provide the optimum performance in the greatest range of temporary uses. Many power plants use river, lake or otherwise untreated water. Because of the quality of the cooling water, the fill in a system like this must be designed for extended use, minimal fouling, high surface-to-volume ratio and high thermal performance with untreated water. Without fill material designed for these uses, temporary systems with untreated water will experience reduced cooling efficiency and higher system fouling rates and cleaning costs as sediment collects in the towers. Understanding the risks associated with fouling and identifying the responsible party during the bid process will help to ensure a successful project.