Process cooling is integral to the operation of power plants, process reactors, low-temperature processes, pharmaceutical operations and many other applications. The larger-duty setups tend to be operated with water, which can pick up heat in one section and transfer it to another. Waste heat can be disposed of with cooling towers, which transfer the heat from the water to open air, or with chillers, which run various types of refrigeration processes and vent unwanted heat.
Modern cooling systems require relatively little maintenance and are dependable, provided they are sized correctly for the actual process conditions. But a host of situations ," ranging from out-of-normal process conditions to catastrophic breakdowns ," can give the plant operators few choices other than shutting down. Only rarely does a plant have a backup cooling system in place. If the plant must rebuild the cooling system, the downtime could translate into months.
Those providers serving the chemical processing industry have responded to this situation by developing portable, temporary cooling equipment. The systems can be operated from existing plant power or from additional temporary power generators.
A bank of temporary cooling towers processes 64,000 gpm of refinery cooling water. The refinery's existing cooling tower had collapsed.
Rebuilding a typical industrial-scale cooling tower of, for example, a 15,000-gallon-per-minute (gpm) capacity can take six months or more. Even if a contractor is readily available to cut that time to weeks instead of months, temporary cooling equipment provides a desirable option during the rebuild period. In other cases such as routine plant turnarounds, temporary equipment provides a means to keep a plant running while maintenance is being performed.
Next to emergency outages, the most common known use of temporary cooling towers is for supplying cooling water during the repair or replacement of a permanent cooling tower. Temporary cooling towers allow this work on the fly, permitting the plant to maintain full production.
This scenario not only brings about positive financial consequences for the plant, it also provides a safer work environment for the repair crew. Workers do not have to service an active tower, where they likely would have to negotiate their way along a poorly lit, slippery and potentially rotten structure.
When basin repairs or structural members below the water line are required, a temporary cooling tower might be the best solution. A temporary cooling tower can be installed complete with pumps, piping and other required equipment to allow workers complete, safe and unrestricted access to all areas of the primary tower. At the same time, the plant can enjoy uninterrupted cooling tower performance.
For example, if a three-cell tower ," designed to cool 20,000 gpm from 110F to 90F with a 78F wet bulb ," required one of its cells to be taken out of service for repairs, the cold-water temperature would rise from 90F to 98F, and the hot water temperature would rise from 110F to 118F. In a temperature-sensitive process, this could result in as much as a 40 percent reduction in production. In other less-temperature-sensitive processes such as steam turbines, a combination of increased fuel costs and reduced production would result.
Both cases more than justify the cost of the temporary cooling towers. To take one of the three cells out of service without any reduction in plant operations or efficiency, it would be necessary for the wet bulb to drop from 78F to approximately 59F.
Fig. 1 shows a three-cell cooling tower with a temporary cooling tower. The repair crew can work on each cell of the permanent tower, in turn, with the temporary unit providing the necessary cooling.
Figure 1. Repair Application
Temporary cooling towers also offer a quick fix to underperforming or overworked cooling towers. In some cases, the addition of temporary cooling capacity is a straight calculation of added operating costs for higher efficiency; justification of a capital cost does not enter into the equation.
Plants can augment current systems by diverting a portion of the hot water from the return line before it reaches the primary cooling tower. The diverted water then is sent to a temporary cooling tower, where it is cooled and then fed back into the primary cooling tower basin.
Augmentation is most effective when the primary and temporary cooling towers are working together. In other words, the plant first must establish the performance of the existing tower before it can accurately determine the amount of augmentation required.
For example, if a cooling tower is to cool 20,000 gpm from 110F to 90F at a 78F wet bulb, the plant would determine the impact of augmentation by reducing the water flow to the primary tower and maintaining the same temperature range. The cooling tower manufacturer should be able to provide information detailing the expected tower performance at varying water flow rates. The information will be in the form of curves or a printed table such as that shown below.
If the plant determines the optimum temperature reduction would be 3F, it would need to size the temporary cooling tower to cool 20 percent or 4,000 gpm from 107F to 87F at a 78F wet bulb. The primary tower and the temporary tower should be engineered to work as one. Any variation of this approach would require one of the towers to achieve a very close approach to the wet bulb temperature and will likely require more equipment and produce less desirable results.
With any augmentation, plants should consider:
How the reduced water flow will impact the water distribution of the primary tower. If too much water is diverted from the primary tower, the water distribution will be affected and might reduce part of the benefit provided by the temporary cooling towers. Rarely is it possible to augment more than 25 percent of the design water flow.