Custom designing and manufacturing heat exchanger coils for wind tunnels was certainly nothing new for one heat exchanger maker. Super Radiator Coils had already produced coils for 15 such projects, including NASA’s Ames Research Center in Mountain View, Calif. But the company’s latest effort - for the nation’s premiere facility to test aircraft components - was different. And, it was bigger than anything like it that the heat exchanger maker had manufactured in the past.
The project involved replacing the centerpiece of NASA’s Icing Research Tunnel (IRT), namely its massive heat exchanger coils. The retrofit would enable the facility to reach -40°F (-40°C), or 15°F (8°C) colder than previously possible. The IRT is an integral part of NASA’s Glenn Research Center, a 300-acre complex with 150 buildings and 31 research facilities in Cleveland. Developed at the end of WWII, the IRT has helped improve air travel safety by determining how to prevent ice from forming on aircraft. The testing area itself measures 6 by 9 by 20' and can accommodate full-sized aircraft components.The retrofit project began in late 2009 and concluded in late 2011 when the icing tunnel was recommissioned and put back online.
According to Rob Holt, president and CEO of Super Radiator Coils, the project involved challenges beyond the sheer size and scope. Highly efficient, cascade-style heat exchanger technology was developed through a collaborative effort between Super Radiator Coils and Jacobs Technology, Tullahoma, Tenn., the prime contractor for the $20 million project to replace the aging equipment in the IRT. Key project objectives included:
- Achieve a new, lower temperature (-40°F) to allow more precise control over aero-thermal testing and broaden the understanding of icing and its effects on aircraft components.
- Minimize downtime of the research tunnel. The IRT averages 20 to 25 unique testing programs annually, totaling 1,600 to 2,400 operational hours over 100 to 150 test days. Customers include NASA, defense agencies and contractors, and commercial aircraft designers.
A New Slant to Coil DesignJacobs’ engineers designed a slanted coil configuration that created more coil-face area to cool down air passing over them (figure 1).
“They [SRC] took the engineering requirements for the IRT coils and ran with them, which is exactly what we need from our subcontractors,” says Chris Porter, project manager for Jacobs Technology.
NASA also wanted two major operational system changes with the new heat exchanger coils. First, wind speed needed to be increased to 400 mph at a -40°F - without increasing the power of the IRT’s 5,000-hp propeller fan. The fan has 14 blades that are 25' in diameter and, before the retrofit, was able to create wind speeds up to 186 mph.
“We met that requirement by reducing the air-side pressure drop across the heat exchanger coils,” says Jim DeWitt, vice president of Super Radiator Coils. “Our design engineers changed the orientation of the new coils to reduce air pressure drop. We then verified air pressure drop predictions from the new orientation with empirical tests,” using the company's own wind tunnel testing laboratory in Richmond, Va.
The second system change was to reduce or eliminate ice particles blown from the heat exchanger (shedding), which can ruin a test. Inside the IRT wind tunnel, just downstream from the coils, are misters that inject tiny water droplets into the air that freeze on contact with equipment being tested.
DeWitt said slanting the coils also reduced the velocity of air going through the coils, which in turn eliminated the shedding issue. Another major design element that helped eliminate shedding was locating the headers (or manifolds) outside of the airstream at the ends of the 50' long modules. Headers distribute the refrigerant mixture through the copper tubes in each module. The headers on the previous coils were located in the airstream.
Each of Super Radiator Coils’ new coil modules contain thousands of aluminum cooling fins and more than 500 copper tubes, each 50' long with the ends brazed to a header. Assembling the six modules required a total of 12,000 joints to be brazed together. “If all the tubes in the six modules were placed end-to-end, they would stretch 30 miles,” DeWitt says. Each module went through multiple stages of testing, including checking for leaks by pressurizing the coils to 200 psi and submerging each one into a specially built tank that was filled with 16,000 gallons of deionized water.
“Every step of fabrication through final testing was a challenge, especially holding precise dimensional tolerances,” DeWitt says. “These coils were square within one-sixteenth of an inch, which is remarkable. Another example involved the thousands of aluminum fins, which had to be aligned perfectly, so the holes would allow the 50' tubes to be pushed through them without binding.”
“Getting the tubes pushed through the first coil and expanding the tubes were huge milestones. We actually got five weeks behind with the first coil just figuring out how to push the tubes through, including taking the unit apart and re-doing it, then making up lost time,” DeWitt says. “The other five modules proceeded without incident. We made up the lost time and shipped on schedule.”
Holding Down DowntimeA key objective from NASA was to complete the entire project with the least amount of downtime. Although the whole project spanned 20 months, the facility itself was out of service only five months in 2011.
Jacobs Technology’s engineers accomplished this by designing and constructing an 8,400 ft2 refrigeration equipment building next to the IRT to house a 1,800 ton primary refrigeration system. The building also was designed to contain a secondary coolant system with an improved temperature control loop for delivering a brine solution to the new heat exchanger coils once they were installed inside the IRT. That strategy allowed the existing compressors and other equipment inside the IRT building to remain operational until the switchover in August 2011.
Jacobs also developed a secondary coolant loop configuration using brine and R507 refrigerant to replace the previous R134a fluid that was used to cool the exisiting coils. “The R507 refrigerant cools down the brine solution, which then goes into the heat exchangers, all of which provides better control and temperature distribution,” Porter says. PC