When you say "Colorado," most people think of snow, skiing, and white-capped mountains. To the hvac engineer, these images also bring to mind concerns about reducing heat loss, improving thermal efficiency, and reducing the potential for condensation on air duct surfaces.

In addition, to the hvac system designer working in colder climates like Colorado, the thought of snow conjures up images of very wet roofs and ponding as the snow melts. That may be why system ductwork is normally not installed on rooftops in these regions, as it is in warmer states like California, Texas, and Nevada. So when The RMH Group (Lakewood, CO), a consulting engineering firm, decided to place ductwork on the rooftop of a Colorado prison, it departed from the prevalent engineering culture of the region.

RMH Group, the project's engineer of record, was charged with designing a replacement hvac system for the Arkansas Valley Correctional Facility (AVCF) in Cowley, CO. The 13-year-old facility houses 1,007 medium custody inmates near Pueblo, in the southeast corner of the Rocky Mountain State. The original wrapped duct system ran underground and had been affected by groundwater problems. The new plan was to move all new ductwork to the roof areas of the six housing units for the prison population.

According to Steve Boyd of Cobb Mechanical Contractors (Colorado Springs, CO), the mechanical contractor who worked closely with RMH Group, the decision to go upward was made to avoid air quality problems and ensure easy access for periodic maintenance. Being able to perform maintenance with a minimum of disruption to the inmate population is important in a correctional facility setting.

Strong Preference For Spiral Design

Once a decision was made to place the duct on the roof, the next thing to consider was whether to install rectangular or spiral duct. Initially, spiral duct was considered. "We preferred spiral duct because of its appearance and its airflow performance. We knew the duct would have to be insulated because of its location on the rooftop, but we didn't know of the existence of a spiral duct liner we could use that would meet our budget limitations," said Hung Dang, RMH's project manager for the AVCF job. "So we decided to go with rectangular duct."

Still, Cobb Mechanical's Boyd decided to look further into other options. "In my career, I've seen how rectangular duct performs on roofs. There are often big problems with snow and water accumulating on top of the duct, especially the larger size duct."

Boyd had heard of a lightweight spiral duct liner that installs in metal duct systems at a lower price than pre-fabricated double wall spiral ductwork. "I read about it in a trade publication and wanted to give it a try," he said. Boyd approached Dang about converting the duct system back to the preferred spiral design and using CertainTeed ToughGard™ Ultra*Round fiber glass for spiral and oval metal duct systems. "Dang approved that change after consulting with the Colorado Department of Corrections, so we went from there."

Minimum Of Disruption To Population

The first step was to fill in the space where the underground duct system had been located with a concrete-like material, pumped into the cavity. With that done, the project proceeded in four phases to handle the movement of the prison population. At the start of Phase I, Building I was vacated over a 24-hour period, with the inmates and personnel moving temporarily to Building II. With the building vacant, it enabled the workers for Cobb Mechanical to have free access to all locations in the buildings so the project could move as efficiently and safely as possible. The movement of people continued in the same way as work was completed on each building.

The total time for the AVCF facility job was 300 calendar days. Building I was completed in 94 days; Buildings II, III, and IV took approximately 70 days each.

The project involved the replacement of the entire mechanical system. The system had three multizone units placed on the roof of each building, with each unit serving a point-of-delivery below. The 100 linear ft of return and 200 linear ft of supply ductwork serving the cells is spiral for each building. Duct sizes range from 10 in. to 28 in. in diameter.

Cobb Mechanical fabricated the spiral duct with its new Spiral Helix machine at its shops. According to Boyd, the company decided to line the duct with two applications of 1-in. ToughGard Ultra*Round, making the thickness 2 in. The lightweight fiber glass "snaps" into place within the spiral duct without the need of pins or glue as fasteners and is designed to require less labor than methods previously used to line spiral duct, resulting in a lower installed cost than double wall spiral duct liner.

'Lower Cost Big Factor'

"I must say that the lower cost involved with the use of the high-efficiency ToughGard Ultra*Round instead of a double wall, spiral duct system was a big factor in making our choice," noted RMH's Dang. RMH had previous experience with another correctional facility project, the Federal Correctional Facility, in Florence, CO.

The fabrication took place on a "just-in-time" basis for each building. "This avoided the need for storage on site and the possibility of weather exposure," explained Boyd. Included in the fabrication process was the use of Paint-Lok metal, chosen to withstand the tough Colorado clime. Cobb Mechanical's hvac distributor for the AVCF project was Southwestern Insulation in Denver, a division of Milwaukee Insulation. "This was the first time we supplied ToughGard Ultra*Round to Cobb Mechanical, but we anticipate more requests for it in the future, considering the success of this project," said Barbara Warren of Southwestern Insulation.

Success For Everyone

Now in place and operating well, the new rooftop system has proven to be a good choice. "We were able to see the Department of Corrections get a superior product without any increased spending, while enabling us to implement a more intelligent hvac system design, Boyd said. "We think everyone benefited on this project."ES