Cooling Towers: Efficiency Waiting to Happen

Cooling towers serve the vital role of cooling water for power plant heat exchange equipment. Sustaining excellent system performance is important because a one-degree increase in water temperature can cause a 2% increase in energy usage. Proper maintenance and a few upgrades could improve a cooling tower's efficiency, while also saving water in the process.

The smallest inefficacy in a power plant can cost a generator greatly, and many energy generators need to look no further than their aging cooling towers to find a hotbed of potential inefficacies, slowly leaching their profits. As the efficiency of a cooling tower stumbles, the temperature in the tower rises, increasing energy consumption in the unit by as much as 2% per one-degree increase. The good news is, from the fans to the nozzles, to the fill media, and even the water itself, opportunities to ramp up the efficiency of a cooling tower abound.

Of course, nobody intends to build an inefficient system, but many of today's cooling towers are relics of the past, and while they may have once been state-of-the-art, they fall far behind today's standards. One of the main areas that older plants lag behind in is fill material. Some decades-old cooling towers still in use continue to use plastic, fiberglass, or wood splash fill. This system positions splash bars to break falling water into droplets. Splash fill is better suited for crossflow towers, according to SPX Cooling Technologies Inc., because the air can easily flow horizontally through the vertical full-height fill. Newer counter-flow towers can integrate splash fill or newer film fill systems.

Around the 1980s, cooling tower efficiency efforts got a boost with the development of better performing fill media. "A breakthrough for the counterflow fills of the 1980s was the introduction of PVC film fill packs," Terry Dwyer, director of field erected products for SPX, said. These new fill types "increase the surface area of the fill in a given cubic foot of the cooling tower, so the PVC—the plastic fill packs—increase the surface area, which increases the heat transfer."

In fact, according to Dwyer, the easiest way to increase a cooling tower's efficiency quickly is to change out the fill media. "We have other, higher performing fills today than we had many years ago," Dwyer said. "It's kind of like with a car, the fuel economy of a car today is better than it was 20 years ago. A lot of that is the engine design and technology."

Today's high-performance film fills boast nearly twice the thermal performance of splash fill. Film fill consists of stacks of labyrinth-like material through which the water and air flow in opposite directions. Film fill increased greatly the exposure of the water surface to the air, resulting in a much more efficient transfer of heat.

However, film fill isn't without its disadvantages. As noted, while splash fill works to break water into droplets by—as the name suggests—making it splash off relatively wide-set bars within the tower, film fill directs the water through small funnels. Unfortunately, and unsurprisingly, most cooling towers aren't running with filtered water. The water being used in these towers is generally pretty dirty, full of mineral and biological sediment that can easily clog the fill with microbiological growth, fouling, or scaling.

While splash fill is at a disadvantage to more modern fill styles in cooling efficiency, it's not without its upsides. Splash fill is less dense, and thus more forgiving of difficult water. Water heavy in sediment can clog film fill far more easily than splash fill, but in general, the tradeoff is not worth it as chemical treatments to help deal with difficult water are advancing quickly.

The four main water quality issues power generators face in keeping their towers running in top shape are corrosion, scaling, fouling, and microbiological activity.

Corrosion occurs when chemicals in the water eat away at the components of the cooling tower. This can result in a loss of heat transfer, and in turn decreased efficiency. Corrosion can also lead to equipment failure, which in turn can lead to plant downtime and equipment replacement costs.

Scaling is the buildup of dissolved minerals on equipment. This too can result in the reduction of the heat exchange ability of the system as the scale can act as insulation, making it much more difficult for the system to effectively cool.

Similar to scaling, fouling is the buildup of suspended particles. While scaling is limited to minerals, however, fouling applies to anything from organic matter to oils. At best, fouling inhibits heat transfer the same way scaling does, by acting as an insulator. At worst, fouling can completely plug fill, reducing the evaporative areas of the system, dealing a blow to the efficiency of the system.

Finally, microbiological activity refers to the impact that any microorganism living in the system has on the plant. The microorganism can be suspended in the water or may grow on the surfaces of the cooling tower equipment, again resulting in reduced heat transfer due to the insulation and blockage of the fill material.

Cooling towers use massive amounts of water, so it's no surprise that municipalities and states have in many cases delegated the lowest quality of the resources for use by power generators. In 2010, water withdrawals for thermoelectric power accounted for 45% of total withdrawals in the United States, according to the U.S. Geologic Survey's most recent national water use report. However, water usage for thermoelectric-power generation has been on the decline (Figure 1), the report notes. Between 2005 and 2010 water withdraws in that category decreased 20%.

To preserve freshwaters, some areas of the country require cooling towers to use municipal gray water or to cycle their towers more (see "Reclaimed Water Reduces Stress on Freshwater Supplies" in this issue of POWER). This was the case for a client of GE Water & Process Technologies.

"Our customers were asked to use municipal gray water. This is the wastewater from a municipal plant. When they were zoned and started up they could not use fresh water from the river or the lake, and these are very tough to treat waters," Peter Macios, executive product manager for the company, said.

GE Water & Process Technologies offers a wide range of customizable chemical and equipment treatment options for generators dealing with difficult waters. "We were able to develop and design programs that allowed our customers to—number one—operate as efficiently [as possible] with the use of that water and negate all the effects on … microbiological controls, disposition, and corrosion," Macios said.

Chemical treatment of cooling tower water is a complex balancing act. Unfortunately, many chemical treatments to address one issue negatively impact chemical treatments to address another. Fortunately, GE believes it has found the right balance, developing chemical treatments that do not adversely interact.

"With traditional programs, when you set a biocide or microbiological control, it always degraded the corrosion inhibition and deposit control," Macios said. "In other words, the programs are always at odds with one another. At GE, we’re able to keep both microbiological deposit control and corrosion in check without affecting any of the programs."

GE's GenGard water treatment technology works across the pH spectrum to inhibit corrosion, while GE's Spectrus microbiological control agent keeps microbial species, including bacteria, algae, yeast, and fungi, under control. Unlike other chemical treatment options, the two technologies can be used together. "The beauty of the program is they’re synergistic. The oxidizer [Spectrus] doesn't degrade the GenGard program. Spectrus doesn't have an effect on the GenGard," Macios said.

Even when a cooling tower's fill is clean and operating efficiently, inefficiencies may still be lurking. One efficiency issue that often receives little attention is the performance of the spray nozzles at the top of the tower.

It has generally been accepted that a water spray is going to take the shape of an umbrella, resulting in a circular spray pattern. The problem with that is that it is difficult to line up circles to evenly distribute water over a surface. If the circles are lined up to just touch, entire areas of the fill media are left dry.

To fix that problem, tower designers move the nozzles closer, overlapping the spray. However, that action plan still results in an uneven water distribution, flooding the fill media in the areas of overlap. When a part of the fill becomes flooded, air cannot travel through, which reduces the efficiency of the plant.

"The quality of the air over water mixture determines the efficiency of that heat transfer. Of course, I think the industry has known that forever," Howard Curtis, inventor of Curtis Technologies’ variable flow nozzle (VFN, Figure 2), said.

Curtis's VFN allows for an even distribution of water and in turn, better cooling efficiency. "We developed a nozzle that produces a square water pattern, and it's hydraulically balanced, so we get better water balance over the fill media, which means you’re going to get better air to water contact," Curtis said. "The most economical thing you can do is to change out your nozzles to bring sometimes 10 percent or more thermal efficiency."

Another piece of equipment that often gets overlooked in the search for low-hanging fruit is the cooling tower fan, Dwyer said. "The important part of the heat transfer is the air movement through the fill media," he said. "If you can increase your airflow through the media it really very directly impacts the heat transfer. So, the more air, the more cooling, the more cooling, the more efficient. If you can take an old fan that isn't very efficient and put a more modern design in there, you should be able to increase your airflow and with your increased airflow have better cooling and better efficiency."

As with most things in life, the first step to addressing a cooling tower efficiency problem is to identify where the problem is. When a power generator is feeling the effects of an inefficient cooling tower, pinpointing the source of the problem quickly and accurately is vital to developing an effective game plan.

That's where companies like Quantum Technical Services come in. The company offers a cooling tower efficiency study performed during peak operation to determine the thermal operating condition of the cooling tower. Such an audit studies the air and water flow of the unit, an evaluation of the heat transfer area, and infrared thermal scans of the tower to identify any potential problems.

When possible the company also tries to take into account the original design of the tower in determining if it is operating as it was intended. "I believe this is the best thing to do," Paul Chila, an engineering consultant with Quantum Technical Services, said. "When [the tower] was built it was designed to run under certain percent efficiency. What we do is, wherever I can, try to dig up those old design sheets, and once we get our picture in place, compare the measured efficiency versus what the design was to let them know where they’re at and we compare everything on the spec sheets."

Taking the original design specifications of a tower into account allows the company to first determine if the plant is operating as it should be, before determining how well it could be operating. "We had one fairly recently where… they were so far off spec on the flow rate being supplied to the tower that we told them, you need to fix this problem first, and then we can come back and reevaluate the tower," Chila said. "It can be a process where you need to say, you need to address this, this is a big factor."

For counterflow towers, the company's GamaScan technology can offer a helpful picture of what is happening inside the fill material. "GamaScan takes a look literally through the fill, and it gives us a density profile," Chila said. "From that, we can tell if the film fill is being fouled and if it is fouling fairly uniform across the cell. That alone, just by the amount of fouling that we have the ability to measure, that's obviously going to affect the efficiency of how that tower is performing."

Regularly assessing cooling tower performance, no matter how old or new equipment may be, allows the operator to determine if their treatment programs are working. "I’ve got quite a few that are the newer style—with the film fill—that were built within the last two to three years, and they just brought us out and said, ‘Come and get the baseline,’ "Chila said. Establishing such a baseline makes it easier for an operator to determine if their tower is starting to drag.

GE also offers regular maintenance and monitoring of their treatment programs. Using a water monitoring system called InSight, GE can track in real time what is happening inside a customer's cooling tower. The product allows customers to quickly react to any problems that may arise. Before such real-time tracking technology became available, operators were forced to be reactive instead of proactive when issues threatened their tower efficiency. "If you think about it, when you do it manually, it's always a rearview mirror look. You get the data, you assess it, and it's already passed," Macios said. "The idea is: Can I pull the data, run the analytic, and know what's going on in my system now?"

While there are many options for upgrading cooling towers to improve efficiency, at some point it might be time to trade your 1975 crossflow in for a shiny new counterflow model. Replacing cooling towers is not a decision taken lightly of course, but should a tower become so degraded or inefficient that the generator risks being taken offline or worse, suffering a collapse, the pros may outweigh the cons.

While cooling tower technology and design has improved significantly in the last several years, many generators have opted to upgrade their older towers instead of starting from scratch. "There are thousands of cooling tower cells that were built in the 60s and 70s that were an older style cooling tower, primarily crossflow cooling tower with splash fill," Dwyer said.

Replacing a cooling tower doesn't always mean replacing the whole thing from the ground up. It is possible to gut a crossflow tower and convert it to a counterflow unit. "That can be a bit of a challenge, because sometimes it's hard to fit the counterflow cooling tower into the same footprint, and you have to do a bunch of piping modifications, but even if you had an old counterflow cooling tower, you can replace the fill in it … with a more modern heat transfer media," Dwyer said.

However, if it becomes clear to a generator that their old, inefficient cooling tower has to go, options for replacement are not as burdensome as they once were. SPX offers a line of factory assembled cooling towers that can be shipped largely complete to a site. The smaller line, the NC Everest (Figure 3), can be shipped anywhere in the country in just a few pieces. Once the tower arrives at the site, it is bolted together with minimum field assembly required. "Those six large pieces bolt together in the field in hours. What we’ve basically done is moved the long duration of field assembly into a factory environment," Dwyer explained.

SPX also offers a larger version of their factory assembled cooling tower, the F400 (Figure 4). Because of the larger size of the tower, however, shipping of the factory-assembled pieces becomes expensive quickly.

The decision to upgrade or replace usually boils down to dollars and cents. "If it costs $10,000 to fix your car and it's only going to last two more years, you’ll probably just decide to go ahead and buy a new car for $15,000," Dwyer said. The same methodology can often be used when cooling tower choices are evaluated. ■

—Abby L. Harvey is a POWER reporter.

SHARE this article

More O&M News