Imagine attending a symphony performance and right in the middle of a violin solo, you hear the sound of rushing air.

It penetrates the ceiling above the orchestra and overwhelms the concert hall’s painstakingly engineered acoustics.

Or pretend you are a surgeon, performing surgery in a medical center suite. You or your patient wouldn’t want to have your concentration broken by such noise. What if you were giving a business presentation in a top-floor conference room of an office building and that same rushing noise, generated by the facility’s HVAC cooling tower, infiltrated the room, leaving attendees distracted and annoyed.

Not a good impression.

That rushing or ringing sound is composed of HVAC construction airflow noise generated by air turbulence at cooling tower fans and water flowing through the cooling tower into the collection basin. It is the bane of many occupants and operators of office, residential and institutional buildings. And finding a solution can be difficult and expensive unless a more advanced cooling tower technology is applied.

Out of tune

In the case of the Woodruff Arts Center’s symphony hall, home to the Atlanta Symphony Orchestra, Alliance Theatre and High Museum of Art, the noise was coming from cooling towers located on the roof directly above the concert facility.

Steve Trott, director of engineering at the arts center, said the sound was similar to that of a jet aircraft passing over the Memorial Arts Building, where the Atlanta Symphony performs on the top floor.

New cooling towers were installed on the building a little over a year ago.

“Our old, metal cooling towers were ‘end of life,’ and so we knew that we had to replace them soon,” Trott said. “We considered typical metal towers, stainless steel, partial stainless steel … we looked at cross-flow and counter-flow designs. Then we looked at the HDPE-based cooling towers, which were recommended by the Atlanta office of DLB Associates Consulting Engineers. We found those cooling towers were very quiet, and would provide the sound insulation needed without having to go to a special quiet tower package, which would really increase the cost.”

The HDPE (high-density polyethylene) design that Trott’s team learned about was pioneered by Delta Cooling Towers, the company says, primarily to solve corrosion problems. Such difficulties have plagued many metal towers due to problems such as soft water and ambient factors such as salt air or caustic industrial gas.

While the dramatically improved lifespan of these towers have been a major factor in purchasing decisions for years, more recently the quieter operation has taken on significant importance for many building owners.

Noise pollution

The noise from commercial HVAC market equipment has become an increasingly unwanted problem for communities across the nation in recent years.

Martin M. Previtera, a regional manager with Delta Cooling Towers, said that the close proximity of commercial and industrial buildings to residential neighborhoods in many urban areas have caused heightened noise concerns among building owners and municipal governments.

For example, the Franklin Institute building in Philadelphia recently replaced its cooling towers, and by city ordinance was mandated to restrict the sound output to less than 50 decibels, which is less than the normal noise output of a traditional cooling tower.

“They decided on HDPE cooling towers for a combination of factors,” Previtera said. “They were able to achieve better pricing while also meeting the ‘dBA’ rating of less than 50. This is a four-cell cooling tower, they installed the same type of cooling tower at Woodruff Arts Center in Atlanta.”

To appreciate the difference between decibel levels, Temple University’s Department of Civil/Environmental Engineering describes the difference of just 10 dBAs (A-weighted decibels, the relative loudness of sounds in air as perceived by the human ear) as very substantial, with 70 dBAs being the equivalent to a vacuum cleaner, and 50 dBA being the equivalent of a quiet suburban home.

“In many cities businesses are encroaching on residential areas, and some cities require that new office buildings incorporate residential spaces in the top portions of the buildings,” said Trott. “Certainly the occupants of those expensive spaces don’t want to hear the noise of cooling towers.”

The cooling towers that Woodruff Arts Center selected for the symphony hall building were two sets of twin-cell Delta TM Series 500-ton models, each featuring a 20-year warranty on corrosion or other physical damage.

Reliability

Trott said these HDPE towers are providing other important benefits as well.

“One of the other reasons we chose these cooling towers was that they are available with direct-drive motors,” he said. “That means no gearbox and no belt, which greatly improves operating reliability and reduces maintenance requirements. At the same time it provides significant energy savings.”

The new cooling towers feature variable-speed fan drives, which provide very smooth operation and very consistent water temperature. During the first summer of operation, most of the time the fans were running at 50 percent speed or less, which was more economical, Trott added.

He noted that the Delta cooling towers are look different compared with traditional ones.

 “Ours are a nice tan color that blends in with the roof; so that is not something that attracts the eye in a way that would notice that there is machinery on the roof,” he said. “We feel that the aesthetic features of the cooling tower design are beneficial in a high-visibility location — since all of our neighbors are in taller buildings that look down on our roofs, it is nice that they are less conspicuous than traditional towers.”

This article and its images were supplied by Delta Cooling Towers Inc.