In a building already under construction, a children’s health care facility in Dallas changed the design of one of its critical air handlers. That change provides greatly increased reliability in a limited equipment space, as well as reducing the acoustic signature. The change, the installation of a multiple fan array product within an air handler, was initiated by the facility owner at the suggestion of trusted advisers.

CHILDREN'S FACILITY

The facility, Our Children’s House at Baylor in Dallas, is the newest of several similar facilities operated by the Baylor Health Care System in the Dallas-Ft. Worth region. The facilities are dedicated to improving the health and quality of life of children with special needs.

The new facility is designed to allow medical specialists to evaluate and treat patients in a warm, comfortable environment created with children in mind.

The two-story facility was designed by architectural firm Page Southerland Page LLP, and built by MEDCO Construction, to look and feel like a home, while providing a tightly controlled physical environment, especially in the treatment areas.

A gabled, residential-type roof façade at the front of the building is a visual barrier for the flat roof, where the air handlers and other building utilities are mounted.

CRITICAL LIMITATIONS

Charles Eversole is the HVAC manager with Trammell Crow Healthcare Services, the contractor that has operated Baylor’s facilities since 2001. He was involved through the design and construction phases of the two-story, 57,000-square-foot building, and explained some of the challenges: “Baylor wanted to create an attractive residential home look to the facility for the children it served.”

This created some limitations on the space available for HVAC equipment, especially on the roof. Eversole worked on the mechanical systems with the engineer of record Fred Henke, of Piazza Engineering.

Piazza designed a system that uses three Temtrol roof-mounted air handlers served by air-cooled chillers located on the ground level in the rear of the building. Also involved on this project was Rusty Vaughn from Texas AirSystems, which represents the manufacturer’s equipment.

Construction of the building was well underway when Eversole asked Vaughn and Henke about alternatives to the design for a critical application: an air handler serving the three surgical suites and related treatment areas. The original air handler, a single-fan design, had been used due to those rooftop space limitations. “While we were in the final design stages, the lack of redundancy - being equipped with only one fan - increasingly concerned Charles,” Henke said.

“This really bothered us from a reliability standpoint,” Eversole said. “I was concerned that we were relying on a single fan for supply air for that location. With the other air handlers I was OK with the single-fan systems, but temperature control and ventilation in the surgical and treatment areas are very critical.”

MULTIPLE FAN ARRAY

“At that point, the air handlers were already ordered, and the roof curbs had been installed,” Eversole continued. “We were limited in what we could do.” He noted that Vaughn’s firm, Texas AirSystems, is experienced in meeting specialized needs. “Rusty suggested using a patented multiple-fan system in the Temtrol unit.”

The air handler manufacturer, Temtrol Inc., is a CES Group company. Another CES Group company, Huntair, has developed and patented a system called Fanwall Technology™. This product features multiple, smaller-diameter, direct-drive fans that can be mounted in a narrow-profile array.

The multiple fan array features a very low acoustic signature, minimal airway tunnel turbulence, and high reliability through redundant fans and motors. Vaughn began investigating the possibility of replacing the single plug fan in the critical air handler with this solution.

MULTIPLE BENEFITS

Vaughn and Henke presented the idea to Eversole, who was enthusiastic. Henke recalls that Vaughn had a demonstration unit set up to show the advantages of the multifan approach. “That really helped persuade Charles and me that this was a viable solution,” he said. Henke and Vaughn were able to modify the design for the third air handler to replace the plug fan with a multiple-fan array.

“We think there are a lot of applications where this technology makes sense,” Vaughn commented. “In this case, fan redundancy is the primary benefit. In other cases, it is the space-saving aspect of this system, or the very low noise levels that it produces.”

He explained that the mounting of multiple smaller-diameter fans means the entire air handler box can be much shorter than the equivalent air handler with a single fan. “That’s a major benefit for some owners as it frees up floor space,” he said.

“Another aspect is the uniform velocity profile of the air immediately downstream and upstream from the fan array as compared to a single-fan system of the same capacity.” This pattern results in more uniform velocity profiles through coils and filters. Lower airway tunnel turbulence leads to more efficient filtration and outstanding heat transfer conditions in unit-mounted components.

According to Henke, another benefit is the lower acoustic signature of the multiple fan system. “I was very impressed at the low noise level of this product. I believe that’s a result of the laminar airflow, the smaller motor sizes, and the direct-drive design. It really is remarkably quiet considering the volume of air it is moving.”

STANDBY CAPACITY

In the case of Our Children’s House, the revised air handler has an array with six individual fans, two wide and stacked three high. One of the six fans is held in standby mode, with a damper system to prevent backflow.

Eversole pointed out that the system can meet the specified airflow with only four of the fans operating. In fact, it could do a reasonably good job with just three. “We’ve actually got double redundancy,” he said, “so I feel quite secure that we can always meet the required airflows and temperatures in this critical area of the building.”

The individual fans have aluminum fan wheels turned by heavy-duty, three-phase, direct-drive motors operating at 208 vac. “The direct-drive feature means fewer bearings that by design require minimal maintenance,” Vaughn pointed out. “Because the fans are direct drive, there are no belts to maintain. This reduces maintenance costs.”

VARIABLE FREQUENCY

The fan rotors are precision balanced and there is no need for spring isolation for the array, Vaughn said.

The fans are equipped with two variable-frequency drives (VFDs), either one of which can operate all six motors. The system is designed to alternate the two VFDs to give them equal hours and ensure availability of a backup. Again, in the worst case, the six fans could all be operated effectively on 60-Hz line voltage.

The air handler serves three surgical suites, providing 13,615 cfm at a total static pressure of 7.25 inches. The air is filtered through 2-inch, 30 percent efficient pleated flat filters; a 45 percent efficient bag filter upstream of the fan system; and finally through a 95 percent efficient bag filter and final ceiling-mounted HEPA filters mounted external to the air handler.

Eversole pointed out that temperatures of 60°F or even lower often are requested in the surgical suites. The temperature requirements in the spaces can change quickly with different surgeons and varying procedures. “With 20 air changes an hour, we can respond to changing temperature requirements quickly.

“This was a very timely solution,” he said. “It gives us redundancy, better airflow, and acoustic performance in a small package. Now that I know about this solution, we will definitely be considering it for additional projects.”

Publication Date: 06/18/2007