Wide Open Spaces

People congregate for many reasons. Mirroring this diversity of purpose are the varied types of assembly facilities. But they all face similar engineering challenges. And in fact, many of these facilities are multipurpose, used for worship, lectures, concerts or other types of gatherings.

With respect to mechanical systems, the issue is how to keep audiences—and the performers and presenters—comfortable and attentive. In terms of electrical, communications and lighting, it’s how to provide a cost-effective yet state-of-the-art audio-visual experience for the audience.

Whether it’s a small and modest high school auditorium or an enormous convocation space such as the 21,000-seat main auditorium at the Mormon Church’s new Conference Center in Salt Lake City, mechanical systems designers are faced with keeping the air temperature and humidity at a pleasant level. At the same time, the uses that these facilities are put to make sound control critical. After all, what could be worse for an audience than to have a speaker or musician battle with the noise from the ventilation system? Moreover, HVAC is not the only culprit. Electrical systems can also contribute to noise pollution in these venues.

But before discussing sound attenuation, let’s consider the traditional and new types of HVAC systems that have been successfully deployed in large assembly spaces.

Air strategies

There are three primary ways to provide air for these spaces: downflow air from above, displacement ventilation from below or natural ventilation. “Theater and concert hall auditoriums have traditionally relied on downflow air,” explains Byron Stigge, Buro Happold Consulting Engineers, New York. “This is especially the case with retrofits, because the only way to install ductwork diffusers is through a roof void.”

Natural ventilation for these facilities is relatively new and appears best suited to smaller facilities of 300 seats or less.

Underfloor displacement ventilation, however, has definitely begun to catch the attention of system designers. One recent example is Seattle’s McCaw Hall, which replaced the city’s Opera House. Norman Brown, P.E., of CDi Engineers, Lynnwood, Wash., explains that this type of facility is well suited to the use of an underfloor plenum for ventilation. “Performing halls have a built-in structure for them,” he says. “The way the seating areas are raked toward the stage make them ideal for this.”

When he began to research the possibility of underfloor displacement, Brown discovered that it wasn’t widely used. “We could find only one similar facility with this type of system,” says Brown. “We visited them, talked to their staff and did a mockup in an airflow laboratory.”

Brown explains that the traditional system for large occupancy facilities is to dump air on the occupants from overhead, with return-air plenums under the seats. Traditional overhead systems drop air from the ceiling at 57

“We just reversed that concept,” says Brown. “In an overhead system, you’re conditioning the entire volume of space to the comfort level.” By conditioning only the lower five feet of space, air from a displacement underfloor system can be 63

“We don’t have to cool the air down as much, and we can use the economizer cycle to bring in outside air for cooling as opposed to having to mechanically cool the air,” says Brown. “And another energy benefit: We can use slightly less airflow, because we are distributing air right where the people are.”

One additional benefit is better indoor air quality. IAQ is improved by bringing fresh air right to the seated audience, reducing the need for mixed air.

But there is a side note to the McCaw Hall story. Orchestra pits have traditionally been difficult to ventilate. “There’s usually not a lot of space there for mechanical ductwork and grilles,” says Brown. Further complicating the issue at McCaw is the fact that the orchestra pit moves up and down.

The solution was to use the movable platform as a plenum space. A series of small holes in the floor of the platform, says Brown, make it like “a giant air hockey table. You could not put a puck on it, because the air velocity is kept pretty low for comfort reasons. Musicians are pretty particular. They don’t want any air drafts.”

But no matter what type of system is used, one thing is always certain: The ideal is considered, but the real world intervenes.

Ideal encounters reality

This is the word from Louis Trama, P.E., president and CEO of Detroit-based DiClemente Siegel, which has successfully completed many large assembly-space projects. Trama says that his firm takes a variety of approaches depending on the specific facility. “ASHRAE suggests that air be supplied high and returned low, but that doesn’t always work out in reality,” he says.

For example, in some of these spaces, air is supplied on the outside walls, with return under the seating. “Even though we don’t use supply air from below, we’re still able to maintain stratified air above like underfloor displacement does. Air still sits at the top and stratifies,” he explains.

Related to air delivery is the matter of humidity. “Humidity is definitely a major factor,” says Trama. “There are basically two strategies here. We like to go with chilled water for finite control over the range, with a valve you can just open, modulate and close. But with a DX system in an assembly area, it will be more of a challenge. Second, we do a dehumidification cycle—subcool and then reheat supply air to the space. And, we look at CO ₂ levels.”

But once temperature and humidity are right for occupants, they won’t enjoy a lecture or performance if the building systems are making a racket.

Air can be noisy

Sound attenuation of HVAC systems is critical in these facilities. “There are two issues that I can think of right away,” says Trama. “Location of mechanical rooms, and second, handling of duct distribution. With the first, you want the mechanical room to be as far away as possible. But it doesn’t always happen because putting the mechanical room out further can mean additional cost.”

Trama goes on to describe some of the techniques for dampening HVAC noise: low-rpm equipment with proper piping isolation; vibration isolators and hangars; and tight seals. “As for the second item,” says Trama, “you want to have sound attenuators in the distribution system, with double-wall ductwork and a maximum airflow of 500 fpm.”

Noise in ductwork is generally caused by turbulent airflow, especially where there are branches and fittings such as elbows or Ts.

Finally, Trama points to another source of noise pollution: electrical and lighting systems. “Designers must be careful with placement of dimmer and control panels,” he says. “Dimming equipment especially can be very noisy.”

And speaking of lighting systems in assembly spaces—both theatrical and primary lighting systems—advances in control technologies in these venues is a story in itself. One facility that showcases state-of-the-art lighting for assembly spaces is the newly expanded and renovated Minneapolis Convention Center, which reopened its doors in 2002. For a detailed discussion of the lighting control systems in this facility, go to www.csemag.com and click on the yellow Lighting button at left.

Secondhand praise

Any assembly area with significant swing in occupancy will need to take a good hard look at all these issues,” says Trama. One thing is for certain: The building systems designer is likely to be a behind-the-scenes player and that’s good. People probably aren’t going to be thinking about the systems when they are sitting in a large assembly hall—unless they’re uncomfortable with the temperature and humidity or can’t hear the speaker. But one never knows. As Trama observes, “You want people to say, ‘Who did this project?'”

Upgrading Sacred Ground

Cathedral of the Most Blessed Virgin, the Mother Church of the Archdiocese of Detroit, was in need of modernization. The project is typical of the renovations of this type of facility. Significant upgrades to the sanctuary were typical of those needed for a 21st century assembly space: lighting for televising and recording the service, a revamped sound system and improved lighting. Fourteen custom designed ceiling-mounted lamps dramatically brightened the nave. In addition, Detroit-based Diclemente Siegel designed new mechanical and electrical systems, including panels, a transformer and wiring.

And as is the case with the retrofit of any large occupancy space these days, upgrading plumbing meant meeting current codes and Americans with Disabilities Act (ADA) requirements.

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