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Giving Vent PUBLIC ACCESS

New Ideas Offer to Breathe Some Fresh Air Into the Conventions of Indoor Climate Control.

[+] Author Notes

Associate Editor.

Mechanical Engineering 125(04), 49-51 (Apr 01, 2003) (3 pages) doi:10.1115/1.2003-APR-5

This article focuses on new ideas that offer to breathe some fresh air into the conventions of indoor climate control. Since most of the air drawn out of a room gets returned, contamination becomes hard to get rid of. Even without recycling, variations in the amount of air delivered to different parts of the building create pressure differences that force air through interior spaces. Chemical fumes, mold spores, and viruses get circulated from room to room and floor to floor. This is bad enough during cold and flu season, but it is a design feature that could become a complication should the American offices again become the target of biological terror like the anthrax attacks in the fall of 200l. Variable air volume (VAV) systems have other flaws, including one that Bernard Bloom addresses with a passion: difficulty controlling moisture in cool, damp conditions. VAV systems often use air conditioning as the main control for humidity. VAV systems dominate the commercial ventilation and cooling market. About 95 percent of major office buildings use them in one form or another. They cost less to build than competing systems and have the reputation for being cheaper to run.

There was a time in the 1980S when sick buildings were all the rage. Office workers and high-rise apartment residents realized their hard-to-define maladies— everything from red eyes and sore throats to fatigue and nausea—were the result of fumes from carpet adhesives, cleaning fluids, fake leather upholstery, or mold growing in the construction materials. Those chemicals would linger in interior spaces because modern buildings were so airtight and were drawing in so little fresh air from outdoors that the fumes were never completely vented.

Even the headquarters of the Environmental Protection Agency was affected.

Sometimes, ventilation systems weren’t simply passive elements in the problem. Bacteria growing in wet duct insulation or standing water could create widespread illnesses or, in the case of the Legionnaire’s disease outbreak in 1976, even death.

But by the early 1990s, ventilation standards were revised: More fresh air was to be brought in, water would be better drained. But sick buildings haven’t gone away. Bernard Bloom, a senior environmental scientist at Ver-sar Inc. in Springfield, Va., is passionate about this point.

“Little kids are still getting sick from being exposed to poorly ventilated school buildings, from the way the ventilation systems spread germs and contaminants around,” Bloom said. “There’s been a lot of hand waving about indoor air quality, but few people have wanted to do the unsexy work of doing the engineering. We have more knowledge than we use.”

What Bloom and others are calling for is a whole new way to cool and ventilate buildings. And in University Park, Pa., a demonstration project running in an architecture lab may be pointing to the future. By separating temperature control from ventilation and humidity control, this system may not only make for healthier buildings, but more economical ones as well.

It you work in a modern office building or school, you spend your day blotting out the background noise from the air being forced through the ducts in the ceiling. That flow of air must do double duty. Not only does it circulate fresh air through the workspace and pull out contaminants and odors, but it is asked to help heat, cool, and dehumidify the space as well. In a typical office, the heat from workers (about 250 Btu an hour per head), equipment, and lighting is enough to require cooling all year around.

One of the more common ways to do this involves mixing recycled air taken from the exhaust stream with fresh air drawn from outdoors. (Fresh air typically constitutes less than 20 percent of the total airflow.) When these two streams are mixed, the resultant air is somewhat closer to the room temperature, reducing the energy needed to condition the fresh air before introducing it into the work areas. This air is delivered at roughly the same temperature throughout the building and throughout the year; to accommodate different cooling demands, the rate of airflow varies.

But that bit of recycling has unintended consequences. Since most of the air drawn out of a room gets returned, contamination becomes hard to get rid of. Even without recycling, variations in the amount of air delivered to different parts of the building create pressure differences that force air through interior spaces. Chemical fumes, mold spores, and viruses get circulated from room to room and floor to floor. This is bad enough during cold and flu season, but it’s a design feature that could become a complication should the American offices again become the target of biological terror like the anthrax attacks in the fall of 2001.

Variable air volume, or VAV, systems have other flaws, including one that Bloom addresses with a passion: difficulty controlling moisture in cool, damp conditions. Ideally, you want to add moisture when it’s dry and dehumidify when it’s wet, but VAV systems often use air conditioning as the main control for humidity. It’s not a bad bet, since the weather is often dry during the winter and soggy in the summer. Except when it’s not, as in the Pacific Northwest. Or during the spring in other parts of the country.

Or when there’s a field trip, Bloom said, and a classroom doesn’t have enough warm bodies for the dehumidification benefits of air conditioning to work. And once humidity runs out of control, water can condense and infuse into interior spaces, creating an environment where mold can spread.

“Little kids sit on the floor and get read to,” Bloom said. “Water goes into carpets and fosters the growth of microbes and dust mites while the kids are sitting there.”

Maybe the solution is to get rid of the carpets, but then you develop problems with noise. The noise that office workers spend all day blotting out gets much worse when it is bouncing off tile floors. (This is especially true in offices and classrooms where heating and cooling comes from fan-driven units nestled in the wall below the window.)

And, yet, variable air volume systems dominate the commercial ventilation and cooling market: About 95 percent of major office buildings use them in one form or another. They cost less to build than competing systems and have the reputation for being cheaper to run.

The Dedicated Outdoor Air System installed in a Penn State architecture studio combines small ducts delivering fresh air for ventilation (in background) with panels hung from the ceiling providing cooling.

Grahic Jump LocationThe Dedicated Outdoor Air System installed in a Penn State architecture studio combines small ducts delivering fresh air for ventilation (in background) with panels hung from the ceiling providing cooling.

When Stanley Mumma began looking into VAV, he was only interested in improving energy efficiency while meeting improved ventilation requirements. This led Mumma, an engineering professor at Pennsylvania State University, into studying control strategies and, eventually, trying to keep track of all the paths that air could flow through a building, a requirement of national ventilation standards. “This told me that no one could verify that you’re properly distributing the ventilation air,” Mumma said. “There are too many unknowns.”

By the late 1990s, Mumma had thrown in the towel. Instead of trying to tie up all the loose ends entailed by variable flows, he thought it might be more efficient to use a constant flow of outdoor air. His concept, which he calls the Dedicated Outdoor Air System, is to provide a steady stream of outdoor air that has been cooled to draw out humidity. A constant flow of air makes it easier to balance the relative pressures between rooms, pretty much eliminating the infiltration from one space to another. And unlike VAV systems, which must have ductwork large enough to accommodate the peak amount of airflow, the air ducts for Mumma’s ventilation system can be much, much smaller since they have to supply only enough air to keep the room from feeling stuffy.

That means, however, that you can’t rely on the ventilation system to provide more than a fraction of the cooling needed to control the temperature. So Mumma calls for a second cooling system. Taking advantage of the overhead plumbing that’s already in place in most offices as part of the sprinkler system, Mumma wants to pipe cool water through the ceiling cavity and into metal panels. These panels cool the room through a combination of radiation and convection, much the way old-fashioned steam radiators heat rooms. Water absorbs much more heat than does air, so circulating a little bit of chilled water can do the job of lots and lots of blown-in air.

Radiant cooling itself is not a new idea: Hospitals and surgical theaters have used radiant cooling systems for some time. But hospitals have particularly low tolerances for blowing large volumes of air around and they are willing to foot the extra cost involved. Of late, chilled ceilings are found in hundreds of buildings in Europe, Mumma said, because decisions there are not so much driven by initial costs.

Europeans are also willing to pay more for gasoline and more for housing, he said. “But I’m convinced that for this to have a ghost of a chance of making it in the U.S. marketplace, it’s got to compete first in cost with VAV,” Mumma said.

Mumma also knew that before he’d get a fair hearing on his system, he’d have to get someone to demonstrate that it was, in fact, cheaper. He needed a proof-of-concept project. He was able to convince the authorities at Penn State to let him install an experimental system in a 3,200-square-foot architecture studio for some 40 students and their computers. The space is in a nearly century-old engineering building on the University Park campus, and hadn’t been cooled at all.

The system has been up and running since August 2002 and reviews so far have been positive. “We got through the end of last summer with no complaints from the occupants,” said the Department of Engineering facilities administrative officer, Clark Colborn. The real test will come this summer, when the temperatures and humidity in central Pennsylvania begin to rise.

About the same time that Mumma decided to abandon variable air volume cooling as hopelessly baroque, others in the field were drawing the same conclusion. William Coad, the former president of the American Society of Heating, Refrigerating, and Air Conditioning Engineers, published an article in 1999 questioning the wisdom of VAV systems.

“There is an overt failure in what is now the state of the art,” Coad told us in an interview. The complexity of VAV leads to breakdowns in air quality, Coad said, and to “sick” buildings.

“There’s no reason why ventilation and temperature control have to be in one system,” Coad said. “And if you can separate them, then you can handle both in a simple, straightforward fashion. When you keep them simple, there’s a good chance you can keep them working. If you don’t, then there’s a good likelihood of malfunction.”

Coad, in fact, has been designing ventilation systems that include elements of what Mumma is calling for since the 1960s. But Coad is a practicing engineer and looks for solutions one job at a time. Rather than laying out a revolutionary course of action, Coad has been quietly making things work.

For example, one place where Coad and Mumma diverge is in the importance of radiant cooling. Mumma sees it as generally the best way to take care of the cooling load that the constant air stream can’t handle. But Coad thinks there’s more than one way to skin that cat. “Stan has committed to radiant cooling in the rooms,” Coad said. “I think that’s only one of many options. You can do whatever you have to.”

Coad also pointed out that the original concourses of the terminal at O’Hare International Airport in Chicago used a system similar to Mumma’s in the late 1950s. But with the constantly open doors at the gates, they could never get the humidity under control, and the chilled ceiling panels often collected condensation. “The whole concept got the reputation as a disaster, since everyone in the world was changing planes at O’Hare and getting dripped on.”

Mumma knows this is an ongoing issue with radiant cooling. Unless the temperature of the panels is kept above a rooms dew point, water droplets will form on the ceiling the way a glass of iced tea sweats on a summer’s day. The demonstration system Mumma has installed controls the flow of cool water to the panels in such a way that the flow slows if the humidity of the ventilation air increases. Things will warm up a bit, but the panels won’t collect water.

Bloom, for one, is enthusiastic about Mumma’s work. "The separation between cooling and ventilation has to come if we want to make school buildings healthier,” Bloom said. “Mumma is definitely pushing things in the right direction.”

With air recycling and blowing from space to space and floor to floor, airtight office buildings resemble immense airliners. And, just as a few sneezes can infect an entire planeload of passengers, the ventilation systems in office buildings can turn a couple of ill employees into an office-wide epidemic.

The Dedicated Outdoor Air System should, in theory, cut down on this: The air comes in at a lower speed and is vented back outside without any recirculation, and there should be no pressure gradients blowing bugs from office to office. Mumma is somewhat embarrassed by the attention another aspect of the system has received: The lack of pressure gradients also has the benefit of thwarting some kinds of terrorism.

Because water can absorb a lot of heat, these chilled ceiling panels hooked up to the sprinkler plumbing system can cool a room quite efficiently.

Grahic Jump LocationBecause water can absorb a lot of heat, these chilled ceiling panels hooked up to the sprinkler plumbing system can cool a room quite efficiently.

“Since this is a constant volume system, and I’m not moving much air to begin with, you don’t have constantly varying pressures between spaces like you do with VAV,” Mumma said. “You’re not going to have much transfer between zones. If somebody releases a vial of anthrax spores in my office, I’m in trouble—but it won’t get carried through the whole building by recirculation.” Terrorism aside, Coad thinks the industry is a long way from abandoning VAV. “There’s far from universal agreement on the concepts that Stan Mumma and I have,” Coad said. “A lot of guys think they can just tweak these systems so they will work better.”

And, realistically, it will be concrete savings—up front as well as in remedying hard-to-define indoor air quality problems—that will win over the engineers who design building ventilation. To show that, Mumma needs more demonstration projects like the one in the architecture studio. He hopes to convince Penn State to install the system in a new campus building set for construction next year. Even while Mumma grapples with the laws of thermodynamics, his biggest foe will probably remain garden-variety inertia.

Copyright © 2003 by ASME
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