In addition to the hvac system designer working in colder climates, the thought of snow conjures up images of very wet roofs and ponding as the snow melts. That may be why systems 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, an engineering consulting firm based in Lakewood, CO, decided to place ductwork on the rooftop of a Colorado prison, it departed from the prevalent engineering culture of the region.
RHM Group, the project’s engineer of record, was charged with designing a replacement hvac system for the Arkansas Valley Correctional Facility (AVCF) in Crowley, 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 in order 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 this setting.
Design PreferenceOnce the decision was made to place the duct on the roof, the next consideration was whether to install rectangular or spiral ducts.
“We preferred spiral duct because of its appearance and its airflow performance,” said Hugh Dang, RMH’s project manager for the AVCF job. “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.”
Boyd had heard of a lightweight spiral duct liner that installs in metal duct systems at a lower price than prefabricated double wall spiral ductwork. “I read about it in a trade publication and wanted to give it a try,” he said. After much deliberation, approval from the Colorado Department of Corrections was received, and work began.
Minimum Distruption to the PopulationThe first step was to fill in the space where the underground duct system had been located with a concrete-like material, which was pumped into the cavity. With that done, the project proceeded in four phases in order 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, the workers from Cobb Mechanical had free access to all locations in the building, allowing the project to proceed 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 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 multi-zone units placed on the roof of each building, with each unit serving a point of delivery below. There are 100 linear ft of return and 200 linear ft of supply ductwork serving the housing units.
The ductwork is spiral for each building, with duct sizes ranging from 10- to 28-in. in diameter.
According to Boyd, the decision was made to line the duct with two applications of 1-in. ToughGard Ultra*Round, making the thickness 2 inches. The lightweight liner snaps into place within the spiral duct without the need for pins or glue.
Cost Considerations“I must say the lower cost involved with the use of the high-efficiency ToughGard, instead of a double wall spiral duct system, was a big factor in making our choice,” said Dang.
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.
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,” said Boyd. “We think everyone benefited on this project.”
Publication date: 10/09/2000