Retiring old cooling towers
“The resort’s cooling towers had reached their expected lifecycle,” said Jason Dawkins, Project Manager at Nashville Machine Company, Inc. “They’d been considering replacement of the towers for almost a decade. The resort suffered a severe flood in 2010, and replacing the cooling towers was discussed at the time. Ultimately, they were reconstructed, but after eight years, maintenance costs to battle leaks and fan failures was rising. We’ve done many projects at the resort, so we were familiar with the cooling system long before the retrofit began.”
Nashville Machine Co. was founded in 1887 as a machine shop. As the city grew and expanded, so did the firm, branching into commercial HVAC, plumbing and fabrication. The company provides pre-fabrication, pipefitting, welding and sheet metal for other mechanical shops in addition to their own projects. As a union shop, Nashville Machine Co. typically employs between 500 and 1,000 tradesmen and women.
Their task was to remove and replace the resort’s four original cooling towers, with a total of 19 cells. The units are adjacent to a mechanical/service building east of the resort proper. The system includes seven, 1,200-ton water-cooled chillers to provide cooling capacity to fan coil units and air handlers throughout the huge resort.
These components, along with large pumps that serve the cooling towers, were still serviceable. Chilled water is piped roughly 1,000 feet from the mechanical room to terminal units within the resort.
The design phase for the retrofit included input from Nashville Machine Co., Beech Construction Services (the resort’s chosen general contractor), Mechanical Resources Group (MRG), a Nashville-based commercial manufacturer’s rep firm, and I.C. Thomasson & Associates, an employee-owned engineering consulting firm based in Nashville.
“Our in-house CAD department worked with MRG to complete the 3D modelling for the retrofit,” said Nashville Machine Co. Project Manager, Miller Orr. “Throughout two phases, we replaced the original towers with new stainless steel EVAPCO models for the same cooling capacity.”
Counterflow solutions
The original spec included crossflow cooling towers, but given the goals set by the resort – ease of maintenance, aesthetics, and ideal discharge air movement – MRG’s design team worked with EVAPCO to assemble a new spec using counterflow models.
“In conventional crossflow cooling towers, air is drawn across the tower’s heat transfer media (fill) perpendicular to the direction of the cascading cooling water,” explained Brett Alexander, Applications Engineer at EVAPCO. “In a counterflow tower, air is induced up through the fill in the opposite direction of the falling water. Our counterflow models also feature sun-tight louvers, helping stem the growth of algae in the basin of the tower. That can’t be accomplished with a crossflow tower.”
“After learning what the resort was hoping to accomplish, it became apparent that a counterflow tower design would better serve the property,” said Phil Collison, Director of Replacement at EVAPCO. “At first, there was resistance from the director of physical plant, simply because his earlier experience with cooling towers involved crossflow designs.”
MRG organized a trip with several of the resort maintenance personnel to visit EVAPCO’s manufacturing headquarters in Maryland, allowing them to see the potential advantages of counterflow towers in person.
One of the benefits that counterflow units provided was to simplify the maintenance process. The original crossflow towers included 19 cells lined up in a row with no space between. Components could only be accessed from either end of the 192-foot row of towers, requiring shutdown of each tower that personnel passed through.
“The size and layout of the new counter-flow towers provides 360-degree access to the water basins of each tower via access corridors between the towers,” said Matt Fruetel, Business Development VP at MRG, which has carried the EVAPCO line for 12 years. “Maintenance personnel no longer have to duck-walk through water basins to reach any of the cells.”
Tower aesthetics was a significant factor, too. Because the cooling towers are installed just above ground level, they’re easily seen from the street. Scale buildup or icicles on the fill material of the original towers were impossible to hide. In a counterflow tower design, the heat transfer sections are fully enclosed.
“Another design consideration was avoiding air recirculation,” said Fruetel. “During the winter, visible plume from the original towers showed that the units were recirculating discharge air.”
This challenge came from the fact that the old towers sat below the roof level of the mechanical building, and because the cross-flow towers only drew air from two sides, one of which was against the side of the building.
“The new towers, which were designed to have a smaller footprint and greater height, clear the top of the building,” said Collison. “Pulling air from four sides is a big advantage, too. To match the original capacity, the resort could have selected smaller box sizes and larger fan motors. That would have reduced the initial cost. But they opted for larger boxes, focusing on energy efficiency.”
Phased replacement
“The largest upfront challenge was tower sequencing with new steel, while keeping the plant online to provide enough heat rejection capacity to support the hotel loads,” explained Dawkins. “Good planning with the general contractor and execution by field personnel including but not limited to Wolfe and Travis, Inc. for power requirements, and MRG for product support and tower start up.”
The original tower cells were taken offline and replaced in two phases. Temporary piping was used to maintain service to the resort’s chiller plant. New dunnage was fabricated and the original tower basin steam heat system was modified to serve the new cells.
“We replaced everything from the strainer on,” said Orr. “Prefabricating components in our shop helped reduce downtime. A total of 9,000 tons of cooling capacity is now provided by stainless steel EVAPCO AT towers.
“The footprint of these units needed to fit very specific dimensions on top of a raised platform behind the mechanical building,” said Fruetel. “We selected EVAPCO’s AT line because of the huge range of models and box sizes available, including the largest counterflow towers on the market.”
The counterflow design of the AT towers saves water and increases efficiency, and with Cooling Tower Institute certification, designers could rely on the provided cooling performance data. MRG’s knowledge of the line streamlined both the design phase and start-up.
From start to finish, the system was operational in about six months. The investment made by the resort will pay dividends by reducing energy, water and sewer expenses, and all of the resort’s expectations were met or exceeded.
“Today, the new system looks infinitely better from the street, maintenance has been simplified, and there are no air recirculation issues,” said Orr. “The client is thrilled with the result.”
