Keeping quarantined patients away from hospital personnel is important, of course, but these special rooms must also be isolated from the hospital's general supply ventilation system, with the room exhaust contaminants diverted away from the hospital intakes.

To solve this problem, Newcomb & Boyd specified room pressure controlling systems connected to a dedicated air handling system that provides 100% outside air to the isolation rooms, which in turn are vented directly outside by use of the Strobic Air Tri-Stack fans.

This unique project was supervised by Ed Murphy at Newcomb & Boyd in Atlanta. Piedmont facilities management people selected a unique central system which also incorporates variable volume on the supply side and tracking exhaust boxes on the exhaust side. Murphy commented: "That enables us, by using room pressure controllers, to convert the rooms from negative to positive pressure. "Murphy also specified the Strobic Tri-Stack fans on the exhaust side. The fans have HEPA filters at their inlets so all of the exhaust air is filtered before it is dispersed to the atmosphere.

Four Tri-Stack fans were used in this installation. In the unlikely event of failure of one of the fans (or an intentional shut down), the other three fans the system provide full redundancy so that all the rooms are always ventilated.

As with virtually all projects of this kind, the hospital and the engineer considered practical alternatives prior to making a buying decision. Murphy said, "We looked into putting filter banks in a horizontal duct that ran above the roof." This configuration would require a different type of fan with an integral discharge stack. To seek alternatives, he contacted Roger Cone of Ecotech Consultants, the local representative for Strobic Air, who then worked closely with him on the project. Due to the low vibration levels of the Tri-Stack fans..."We were able to put the HEPA filter system directly under the fan so it really eliminated a lot of the duct work that is above the roof. It is a lot cleaner installation," Murphy added.

"We were able to achieve a lot higher discharge velocity because of the Tri-Stack design..."

A system like this would have required at least four stacks for conventional centrifugal fans, one for each fan, to provide performance comparable to the Tri-Stack fans and it would have cost substantially more as well.

Once it was determined that the Tri-Stack fans would meet the hospital's requirements for isolation room exhaust, considerations such as initial and operating costs, installation costs, maintenance efforts and costs, and performance comparisons with other types of systems were carefully reviewed.

According to Murphy, "The Tri-Stack system offered clear cut advantages over other approaches in all of these areas." He commented on the ability of the Tri-Stack system to provide variable air flow and reduced energy consumption. "The fact that Tri-Stack fans are virtually maintenance free, and do not require guy wires or other expensive mounting attachments was also a major consideration," he said. "No maintenance man wants to change belts or bearings on a TB isolation room fan." he added.

Operating efficiency also played a major role in the decision, according to Murphy. He said, "We were able to achieve a lot higher discharge velocity because of the Tri-Stack fan design than any other system. This enabled us to get more dispersion into the atmosphere.

TB guidelines from the Center for Disease Control (CDC)

The Center for Disease Control (CDC), in its "Guidelines for preventing the transmission of mycobacterium tuberculosis in health-care facilities" (1994), defines specific requirements with regard to HEPA Filtered Devices (HFDs). These Guidelines encompass subjects such as local exhaust ventilation with specific performance specifications and, in general, how "HEPA-Filtered Devices can be used by health care facilities to meet CDC Guidelines and help prevent the spread of airborne TB droplet nuclei."

Among the specific subjects addressed are "General ventilation: dilution and removal, airflow patterns within rooms, achieving negative pressure in a room; HEPA filtration with regard to air cleaning before exhaust and prevention of exhaust re-entrance; and, recirculation of HEPA-filtered air either within other areas or a discrete room." Other sections of the Guidelines are concerned with installing, maintaining, and monitoring HEPA filters, preventing escape of droplet nuclei as well as reducing concentration of droplet nuclei.

Strobic Air Tri-Stack roof exhaust fans are designed to conform to the intent of all applicable fan requirements of the current CDC TB Guidelines. The Piedmont Hospital installation described here is an example of Tri-Stack conformance to those Guidelines.