Last summer, the University of Notre Dame (UND) conducted a major renovation project on its Galvin Laboratory complex in South Bend, IN. A chief priority was upgrading the building's laboratory fume hood exhaust system.
The building is a 30-year-old structure containing four floors and 138,000 sq ft of space. Two additions have been made over the years, each with its own system, placing strain on the building's collective ventilation operation. In fact, no major ventilation renovation had been done for the past 30 years, other than minor repairs and alterations.

The original structure and subsequent extensions house 60 laboratories, 78 lab workstations with fume hoods, and a few biological safety cabinets. Exhaust stacks that are 6 in. above the roofline are mounted on the roofs of the second and third levels. In addition to housing animal, biological, and aquatic research, it also contains classroom and office space.

Centrifugal fans had been used to exhaust the workstations, either individually or with manifolds connecting two or three workstations into a common exhaust. There were 27 individual fans on the three-story portion of the facility and 14 on the two-story section.

The main chemicals used at the laboratory workstations are solvents, bases, aromatics, carbonyl compounds, and even shortlife radioisotopes. Because it is used so intensively, the structure is a 100% fresh-air building. The intake air vents are located on the first floor.

While a third extension was being planned, the architectural-engineering firm Ellerbe-Becket (Minneapolis) was called in, and it hired Rowan Williams Davies & Irwin (Guelph, Ont., Canada) to perform a computer simulation of the building's laboratory fume hood exhaust system.

The firm found that serious re-entrainment was taking place. Exhaust from the roof was reaching the ground level of the building, thus being drawn back into the air inlet vent. Although the re-entrainment was not detectable by sight or smell, it was decided that a complete overhaul of the exhaust system was justified.

Daniel Dickinson, P.E., a project manager for Ellerbe-Becket, was responsible for the work completed at the UND site. Dickinson notes that "The initial and greatest concern was about the effect this facility had on itself. Also of concern was what effect this facility might be having on other buildings adjacent to it. The former was known to be a problem."

EXHAUSTING TIPS
An effective exhaust system transports laboratory fume hood vapors into manifolded ductwork on the building's roof. Exhaust stacks are given sufficient height to ensure that gases are evenly dispersed into the atmosphere. ASHRAE recommends a stack height of 2.5 times the buildingws height. If the stack exhaust is not high enough, exhaust will re-enter the building via the outdoor intake air vent.

To prevent health hazards, facilities management must ensure that exhaust gases do not:

Pressure balancing is an important feature in the design of a building's ventilating system. The pressure of the laboratories should be lower relative to the rest of the building. Air should flow from offfices to the laboratory and then to the workstation fume hood, not vice versa.

DESIGN BASIS
In order to design a proper exhaust system for the Galvin Complex, the UND team had to first define a design basis.

In this case, a scenario analysis was done, and then a worst-case design was selected. The system was sized for venting fumes from a spill where the largest vessel in the facility would potentially be turned over and the fluid with the fastest evaporating rate would be spilled. The size of the exhaust system was then designed so that the full flow of the evaporating liquid would be vented and so concentration levels in the air would not be a threat to people.

A second stipulation on this project design basis was the use of a variable air volume (vav) system. Such systems are complex, requiring intensive control and monitoring systems. The control system constantly checks temperature, pressure, and humidity of the laboratory air.

In contrast to constant-volume systems, vav systems regulate the increase or decrease of the fresh air intake that is necessary to maintain the system at its setpoint. This has great value in that excess air is not conditioned needlessly; only the necessary amount of air required is conditioned. Vav systems normally have a short payback time, too.

The third stipulation was aesthetics. Systems like this normally require huge stacks, which are not aesthetically desirable. This particular building is easily visible from the main road on the UND campus. Moreover, the building is next to the football field. Large industrial stacks do not really project the scholarly environment that the alumni, visitors, and especially the TV cameras, have come to expect.

Finally, the stacks could not be noisy. The mechanical sound of the fans must not be heard widhin the building, from the sidewalk below, in adjacent buildings, or across campus.

Exhaust acoustics is not an issue to be overlooked, since it is an element of dhe building's aesthetics. Dickenson cites the fact that UAcoustical analysis early on, prior to installation, can minimize dhe acoustical impact on the surrounding areas. This was particularly true at UND, where the campus is primarily pedestrian...."

For this reason, Ellerbe-Becket retained the architectural acoustics firm of Kvernstoen, Kehl and Associates (Minneapolis). "We identified what the noise levels were currently on the site," recalls Steve Kvernstoen, a principal at the firm, "because it seemed important to know what we were up against. If it was going to be a very quiet space, then a small amount of noise would probably be perceived as quite intrusive.

"And yet, if it was a space where there was a lot more traffic, or something else was already making noise on the site, then our noise might very much blend into what was already there."

THE SOLUTION
UND decided to install a Tri-Stack laboratory fume hood exhaust system made by Strobic Air Corp. (Harleysville, PA). The system uses a specially designed fan blade that the company says gives exhaust gases extra propulsion and lift. Moreover, the exhaust stack has a nozzle shape that is designed to enhance the exit velocity of the gas.

All parties believe the system is an excellent choice for operating in conjunction with a vav air conditioning system, because air inlet vents are located within the exhaust stack itself. While the lab is operating under vav conditions, the inlet vents in the stack supply makeup air to the fan blades, thus keeping constant flow to the fans.

Furthermore, the high-velocity exhaust jet ensures dilution and eliminates the need for a tall, unsightly vent stack. The dilution air is taken from the rooftop via nozzle inlet vents and mixed with exhaust gas. Also, the high-velocity jet is so powerful that the plume is propelled to elevations equivalent to a traditional exhaust stack height (up to 350 ft). This system is typically 60% shorter than conventional systems.

According to the manufacturer, the new exhaust system offers a variety of other benefits:

25% less energy consumption relative to conventional centrifugal fan systems;
Effective noise abatement;
Virtually maintenance-free operation; and
50% footprint reduction.
Dickenson adds that "Steve Kvernstoen suggested the inclusion of acoustical attenuation as much as possible at each fan [the source]. In addition, an acoustical screen wall louver was recommended. Attenuated silencers were incorporated at the [stack] outside air inlets as well as at the fan discharge."

THE RESULTS
The new exhaust system has been in place for nearly a year. To date, the laboratories have reported no problems with re-entrainment or fan noise.

Previously, the building employed 41 fans for ventilation; the new system has reduced the number to 16 while providing nearly 300,000 cfm of ventilation.

Mark Hummel, P.E., UND's maintenance manager, says, "That's the benefit. Rather than building a 30- or 40-ft chimney and discharging it high to get into the atmosphere, you can discharge at a velocity that ejects the plume to an equivalent height."

Ron Erichsen, UND project manager, adds, "We're in the process of converting the entire building to a vav system. When we get all diversification on the vav system, we should be able to save approximately 30% of the energy in heating and air conditioning consumption."

Hummel also commented on the speed with which the vav-laboratory workstation exhaust system was installed. "As far as downtime to the laboratory usability, never was any laboratory down more than four days. We did this in a fully occupied building."

Dickenson believes that the investment in a complete acoustical analysis was well worth it. "The result has been that most people have not noticed any change in the noise level. UND seems happy with the fans, as well as their acoustical performance."