Blowing smoke and shocking air particles
As the green building movement expands, so does the awareness of buildings' overall effects on human health and the environment. Although hospitals and their operations have always had a large footprint, only recently have designers, contractors, operators, and even ground-level staff been making decisions in light of this larger awareness, including product life spans, waste streams, occupants' means of transportation, and other far-reaching concepts. Another consideration is air quality, whose footprint covers anything from the insulation inside walls to adjacent properties and into the earth's atmosphere.
Of the different green and sustainable aspects of the Denver Health Pavilion for Women and Children (which has earned LEED-NC v2.1 Silver for new construction and major renovations), its approaches to air quality are perhaps the most innovative. As part of a first-responder hospital, as a facility dedicated to a vulnerable population segment, and as a publicly funded project, the Pavilion for Women and Children addition needed to be ever more aware of its health and environmental effects, including its air-quality footprint.
Aside from using low-emitting interior materials, installing cotton installation (avoiding the particulates from fiberglass fibers), running a strict construction indoor air-quality program, and installing a carbon dioxide monitoring system, the project team also took a close look at both the operating emissions that the building puts into the exterior air and how the building takes in that same air. The team chose to conduct tests on exterior exhaust streams in a wind tunnel and to install a state-of-the-art air filtration system that electrically charges and attracts particles. The combination of the air-filtration system and the design guidelines from the wind testing mitigate, if not totally eliminate, air-quality issues such as operating emissions from Denver Health buildings and surrounding buildings, general community pollution, bioterrorism, and pandemics.
In with the good
Manufactured by Louisville, Colorado-based StrionAir, the filtration system in the facility's air handler combines mechanical filtration, electrostatics, and ionization to achieve infection control along with energy efficiency and relatively low waste (figure 1). The code minimum for healthcare is Minimum Efficiency Reporting Value (MERV) 14, says Jim Lazzeri, principal of Cator, Ruma & Associates, the mechanical and electrical engineers on the project. Because the fibers in the filter media actually attract particles, the Strion filter has a MERV 15 with the same or lower pressure drop as a MERV 14. “You have the potential of catching more stuff without the penalty of using more power to push it through,” Lazzeri explains.
The filter rack in the Denver Health Pavilion for Women and Children's air handler. The filtration system combines mechanical filtration, electrostatics, and ionization. Particles passing through the air handler are charged and attracted to an oppositely charged filter media, which drops the pressure and saves energy in terms of blower motor efficiency. Image courtesy of StrionAir
The facility is a first-response hospital, which means it will have to handle an influx of patients in the event of a bioterrorism act or a disease pandemic. The filtration system can filter anthrax and has recently been shown in a joint Centers for Disease Control and Prevention (CDC) study to filter the avian influenza virus.1 According to the researchers' results, “weak electric fields used to enhance the efficiency of coarse low-pressure–drop filters may be an effective engineering technology for rapidly inactivating and/or destroying pathogenic viral agents in and on fiberglass and other polymeric media.”
The filtration system uses an ionization array to charge particles, which are then attracted to the oppositely charge filter media. “It is not a one-time charge, such as an electret filter,” says Douglas Powell, director of healthcare marketing at StrionAir. “The electret will eventually lose its charge. Our filter media is charged constantly.” Because of the electrostatic filter charge, the system can operate with a more permeable filter media while filtering at the same efficiency, which leads to a lower air-pressure drop. “The lower pressure drop and a more permeable filter enable the blower motor and fan to work less hard, therefore using less energy,” Powell says.
Denver Health decided to install the Strion system primarily because of the health benefits of the filter, but the efficiency and energy savings helped cost-justify it. “If you look at these air handlers with a total of 282,000 cfm, it's a pretty good size system,” says Robert Padgett, president of Engineered Mechanical Systems, LLC, who installed the system. “Our initial pressure drop was a .35" at altitude, and an equivalent filter would be in the order of .75". Strion saves almost a half an inch of pressure, and translating that into energy, that will save 35 brake horsepower constantly for the air handlers. That's a pretty big portion—about 10% of the air handler's internal pressure, so it saves 10% on fan energy right off the top.”
Not only does the electric charge throughout the filter media lower the pressure drop and hence save fan energy, it also kills living organisms, such as viruses, bacteria, and mold, by literally shocking and breaking down the organisms' cell walls to achieve a germicidal effect. As organisms are captured on the filter, they are trapped in the electrostatic field and are exposed to both the field and the ion current from the ionization array. The combination of electrical stresses on the microorganism has been shown to effectively inactivate the organism and in some cases rupture the cell membrane.
According to the CDC study, the electric and ionization field exposure resulted in the following inactivation responses: measles (vaccine strain), 99.99% in 90 minutes; vaccinia virus (MVAT7), 99.9% in 18 hours; wild-type human influenza virus, 99% in 200 minutes; wild-type avian influenza virus, 99.9% in 200 minutes.
As opposed to a filter media with no electrostatic charge that collects particles by impaction on the side exposed to the air steam, the Strion system's polarized filter media fibers attract particles on all sides and disperses the debris throughout the filter. This in turn provides for a longer filter life, creating less waste compared to a filter with the same filtration efficiency. “It depends on what the filter has been handling, but generally speaking,” Powell says, “we can get a 9- to 12-month life out of the filters.”
“The life of the filter is a function of the dirt going through the air system,” Padgett adds, “so what Strion will do is typically increase the life of the filter by about 50% compared to one with the same filter efficiency.”
Out with the bad
On the other side of this air-quality footprint is the consideration of what a facility is putting into the air. With hospitals' wide range of exhausts—kitchen (odor), fume hood (chemical), nuclear medicine (radioisotopes), instrument sterilization (steam or chemical), diesel emergency generator, boilers, helicopters, and vehicles (odors and chemical pollutants)—analyzing their air-quality effects is more challenging than other types of facilities, says John Carter, senior associate at Cermak Peterka Petersen, Inc., (CPP), the wind engineer consultants on the Denver Health project.
On the Pavilion for Women and Children building, CPP specifically considered the isolation room, chemotherapy hood, and emergency generator exhausts by testing the paths of the air pollutants in a wind tunnel, which earned the facility a LEED Innovation in Design credit for risk management (see video at http://healthcaredesi.wpengine.com/DenverHealth).
Carter says hospitals have a number of distinct challenges: “As with many hospitals, Denver Health has several emergency generators located in close proximity to each other that operate simultaneously. Diesel exhaust is particularly complex and has a very distinct odor that is objectionable to a large percentage of the population. Finding reasonable solutions for these exhausts can be very challenging, especially in a hospital where you can't conduct testing when the building is unoccupied—it is never unoccupied.”
CPP visited the site before testing and identified the various nearby exhausts, which included boilers and emergency generators at the central plant (including planned additional units), existing nearby ground-level emergency generators, diesel vehicles idling at the planned loading dock, and a laboratory exhaust on the existing tower podium. The consultants also identified the areas that were potentially threatened by exhausts, which ranged from air intakes and operable windows to locations in the nearby neighborhood.
CPP then built a scale model of the building, campus, and neighborhood, installing the previously identified exhaust sources and sample ports, or receptors, at air intakes and other sensitive locations (
figure 2). CPP simulated the exhausts by sending a metered flow of tracer gas through the stacks to produce exhaust plumes identical to those in the real world (
figure 3). Then they measured the amount of the plume present at each receptor location for all wind directions and at a range of wind speeds to identify worst-case scenarios for each exhaust/receptor combination.
The wind tunnel has slightly simplified models of large buildings (the blue buildings) nearby the Pavilion for Women and Children and a typical roughness pattern representative of the suburban environment to the west (the green cubes). Upwind of the circular turntable model is the boundary region of the wind tunnel (about 75" long). This section is filled with a generic roughness pattern tuned to reproduce the atmospheric boundary conditions approaching the site. Image courtesy of CPP, Inc.
After building a scale model of the area, the wind engineers sent tracer gas through miniature exhaust stacks and tested a range of wind speeds and directions for each exhaust/receptor combination. Image courtesy of CPP, Inc.
Using the wind-testing guidelines established by CPP, project mechanical engineers determined the location, height, and diameter of the stacks. Some of the stacks were moved and resized from their original designs in the plans. “The stack diameters are sized to get a good velocity on the exhaust so that it is directed up and out,” says Jim Lazzeri, principal of project engineers Cator, Ruma & Associates. The velocity of the exhausts leaving the stacks is between 1,800 and 2,500 feet per minute. “If it had a larger diameter, the exhaust would be moving slower and have a tendency to fall back down onto the roof or spill over the sides of the roof. We were trying to get it up above any roof height and get it entrained in the wind so it can be diluted.” The object of the stack design is to push the pollutants out with enough speed to mix in with the ambient air and be diluted before they can reach any outdoor air intakes, open windows, or pedestrian areas.
Although altering the location, height, and size of the stacks does not remove pollutants, they do mitigate exhausts' direct effects on the surrounding environment, avoiding nuisance problems such as odors and potential health issues such as exposure to chemicals. “Exhaust dispersion is definitely a part of the footprint,” says Carter. “It is not possible to design an exhaust system that has zero effect, because as the emissions mix in the atmosphere they eventually come back down somewhere. Our priorities are to protect health first, then try to do that while consuming the least amount of energy. When we can maintain a healthy environment and allow the building exhaust systems to use the minimum amount of energy, we can feel pretty good about what we do for a living.” HD
For further information on the wind-tunnel testing, visit http://www.cppwind.com. For further information on the air-filtration system, visit http://www.strionair.com. Visit http://healthcaredesi.wpengine.com to view a video of the wind-tunnel test and to comment on this article.
- Raydel M, Rota PA, McKinney P, et al. Inactivation Potential of Filter Immobilized Airborne Mammalian and Avian Viruses in Weak Electric Fields. Centers for Disease Control and Prevention; Strion Air Corporation; Department of Infectious Diseases, College of Veterinary Medicine, University of Georgia; Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, 2007.