Mark Hallman, P.E., LEED GA, Rowan Williams Davies & Irwin Inc.
As laboratory buildings age, energy use and occupant safety can become concerns for owners and users alike. In many cases, old buildings have outdated exhaust designs that include numerous individual rooftop fume hood exhaust stacks, each discharging a small volume of fume hood exhaust. These exhausts are high energy consumers due to less flexibility in reducing discharge flow during times of low building occupancy and less efficient fans compared to today's models. Additionally, these low-flow stacks have low discharge momentum, resulting in poorer dispersion due to the increased likelihood of the exhaust being heavily influenced by local wind flow patterns and wakes. Please refer to Figure 1 for an illustration. Poor exhaust dispersion can pose occupant safety concerns due to the re-entrainment of hazardous chemical vapors at fresh air intakes.
Individual exhaust system
Whenever feasible, retrofitting existing exhausts into fewer and larger manifolded exhausts is an excellent strategy to realize improvements in energy consumption, operational flexibility, and occupant safety. A successful retrofit of this nature was completed at Georgia State University's Natural Science Center in 2009, which involved converting an existing exhaust system of over 100 individual exhaust stacks into a manifolded exhaust system of 10 fans. Key goals of the retrofit project were to modernize the building exhaust system, realize energy savings via fan turndown, maintain low noise levels within the building, and maintain or improve overall building comfort and safety for occupants. Rowan Williams Davies & Irwin Inc. (RWDI) was retained to conduct a dispersion modeling study of this new system to assess its performance and safety for building occupants.
The 10 manifolded fans were divided into three groups on the roof, with each fan discharging between 22,000 and 27,000 cubic feet per minute (cfm). Exit velocities for the fans ranged between 4,000 and 5,000 feet per minute (fpm), an approximate four-fold increase over the previous stack exit velocities. Other key components of the retrofit included reducing laboratory room ventilation rates from eight air changes per hour (ACH) during occupied periods to four ACH during reduced occupancies. Manual setback switches were installed in each room to control the exhaust flow rate throughout occupied and unoccupied periods. Each fume hood exhaust duct was equipped with manually operated control valves to enable fume hood exhaust flow turn down. Additionally, a negative pressure difference set point was enabled for all laboratory rooms, set at a minimum 100 cfm difference between room supply and exhaust.
The dispersion modeling assessment demonstrated that the manifolded exhausts met RWDI's recommended safety criterion for both full-flow (between 22,000 and 27,000 cfm per fan) and reduced-flow (between 15,000 and 18,000 cfm per fan) conditions. This criterion was developed by RWDI to address health and odor targets for numerous commonly used laboratory chemicals. Therefore, if building activities allow it, the building exhaust airflow rate can be decreased during times of low occupancy, reducing overall fan power and energy use while still not compromising safety. Please refer to Figure 2 for an example of the improved dispersion.
Manifold exhaust system
From an energy use perspective, savings that were anticipated with the retrofit have not been fully realized since the building has resumed operation. This is due to the fact that the extensive research activities that occur in the building hinder the ability to turn down the ventilation rate, given that many researchers remain in the building at all hours. Automatic ventilation controls are difficult to implement for this reason, as it is difficult to anticipate when the building will be unoccupied. This underlies a key point: for any retrofit project, it is important that new measures work in harmony with existing building activities in order for all project objectives to be met.
Mark Hallman is a technical coordinator providing exhaust dispersion consulting services with RWDI. He has provided consulting services on exhaust and intake designs for a variety of new and existing facilities in the university laboratory and healthcare sectors, including the Georgia State University Natural Science Building laboratory exhaust retrofit project.
When consulting on projects, Mr. Hallman looks to provide advice on exhaust designs that can maximize energy efficiency without compromising the safety or comfort of building occupants. Mr. Hallman has a background in environmental engineering and has recently earned his LEED Green Associate accreditation.