These existing fans were older centrifugal belt driven types that required costly regular maintenance and parts replacement. They were part of a 1960 design where a central HV unit pumped tempered make-up air, produced by an auxiliary air system, directly into each lab hood. Access to this temperature regulated air was controlled by the use of a sash positioned above each researcher, which could be opened to allow air to enter.

According to Dave Hastings, Senior Staff Engineer for Hercules, "there was poor containment of toxic fumes in the hoods, due to the inability of the older fan systems to provide proper fume exhaust discharge velocities. When a sash was opened to allow tempered air to enter the hood through a perforated plate, the lack of exhaust pull resulted in an intense roll over effect. This created unstable air turbulence around the sash." The older system did not properly exhaust the hoods, and, according to Hastings, "this led to dangerous reentrainment of hood fumes from the roof. They were gettinq back into the tempered make-up air and going back into other hoods."

The company's Facilities Management staff was obviously dissatisfied with the effectiveness of the nearly 100 space-consuming individual fans to provide the required exhaust flow. Hastings was also concerned that the existing system did not have stack heights that he wanted for proper plume dispersion. He added that, "we had another primary air unit where re-entrainment might have taken place and introduced some of the fumes from the roof back into the building. This is a problem with individual exhaust fans spread all over a roof, since fumes dissipate in all directions."

Tri-Stacks Chosen Over Alternatives

The company had originally considered solving the re-entrainment problem with replacement centrifugal fans, but this would have required the additional time and expense needed to install 30' high exhaust stacks. It would have also necessitated the construction of costly support structures and additional mounting equipment. A staff design engineer at Hercules learned about the Tri-Stack Lab Fume Hood Exhaust System and brought it to the attention of Facilities Management. They realized that it would provide a radical departure from conventional lab fume hood exhaust systems and offer superior performance in merging outside air with plenum exhaust air to effectively dilute lab hood fumes. The staff was also attracted to the Tri-Stack system's economical and quick installation, that it conforms to the 1992 revised ANSI/AIHA Z9.5 standard for laboratory ventilation, and that it's UL listed and 100% factory tested.

According to Rick Biddle, Project Manager for Hercules, once Facilities Management learned more about the Strobic Air Tri-Stack exhaust systems, "we felt there was a good opportunity for energy savings, less maintenance potential, and more effective plume dispersion."

Concern Over Risky Tall Stack Installation

The original plan to install replacement centrifugal fans posed another costly dilemma for Hercules. Hastings recalled that, "we were worried about wind loading, which could possibly cause the tall stacks to collapse in high winds, and there was a problem in designing a stack for this." The company was faced with the expensive procedure of building a system where, according to Hastings, "we had to go all the way down to the ground to get support for the stack. We didn't have to do that with the Tri-Stack design, which made it easier for our structural people. They just extended the buildings' steel columns through the roof, which provided the necessary support for the horizontal steel platform they built for the Tri-Stack system." Biddle remarked that, "once the decision was made to install the Strobic Air Tri-Stack system in each building, the existing 92 self-supporting 10' centrifugal fan stacks were disposed of as scrap metal."

Increased Exhaust Capabilities

Once installed, the Tri-Stack fan system's performance advantages in eliminating re-entrainment were evident. The previous centrifugal fans exhausted a combined total of 98,000 cfm throughout the facility. Only two Tri-Stack exhaust fans, each handling close to 50 lab fume hoods, now provide a total of 108,000 cfm. Even though the number of workstations was not increased after the Tri-Stack installation, Facilities Management could rely on it to increase the air volume in the older of the two buildings. This was a separate application that allowed the addition of scroungers or "elephant trunks" designed to remove excess non-toxic fumes and odors from counters and areas adjacent to workstations.

The Tri-Stack system made it possible for Hastings to design in approximately 100 cfm in the older building. He utilized new air handling equipment to produce the additional exhaust air necessary for the scroungers to function properly, and to provide the make-up air needed to deal with this extra exhaust. This could not be done in the newer of the IWO buildings since the 1970 design there does not produce sufficient exhaust air volume. Facilities Management could not change the building's air handling system to generate the required make-up air.

Based on the ability of the Tri-Stack system to accommodate the scrounger installation, a retrofitting of existing air handling units for this purpose is planned for the newer building within the next five years. Hercules has since successfully installed a second Tri-Stack lab fume hood exhaust system in their newer Savannah, GA. plant. This second facility was originally designed to incorporate two Tri-Stack systems, so installation was accomplished during actual construction. This application afforded Hercules the same effective protection against re-entrainment the company now receives at their Wilmington R & D site.