Good Neighbors—Preventing Generator Exhaust Re-entrainment

Two major problems with emergency diesel generators are the possibility of re-entraining their exhaust into the hospital's ventilation system or adjacent buildings and disseminating unpleasant diesel exhaust odors in the neighborhood. To prevent these problems at the Meridian Health System for Brick Hospital in Brick Township, N.


Two major problems with emergency diesel generators are the possibility of re-entraining their exhaust into the hospital's ventilation system or adjacent buildings and disseminating unpleasant diesel exhaust odors in the neighborhood. To prevent these problems at the Meridian Health System for Brick Hospital in Brick Township, N.J., Bill Transue, facilities manager for the hospital, sought a practical and cost-effective solution.

Diesel generator exhaust re-entrainment and odor control at hospitals have become sensitive issues over the past few years. Occasionally, employee complaints have culminated in lawsuits for damages claiming illnesses caused by re-entrained exhaust air from rooftop exhaust fans. Until recently, the most common method for handling diesel generator exhaust was through individual centrifugal-type fans with tall, dedicated exhaust stacks on the building's roof. For the most part this technology did not eliminate re-entrainment or odor problems, because the exhaust stream was subject to constantly variable atmospheric conditions. Depending upon wind speed and direction, noxious diesel exhaust gas—and unpleasant odors—went right back into the hospital or adjacent buildings.

When the hospital's west wing was completed, says Transue, its emergency diesel generator exhaust was being re-entrained into the fresh air intake ventilation system. In an attempt to correct the problem, new ducting was constructed leading to the roof of the existing south wing. "Even this didn't solve our problem all the time," Transue adds, since during some weather conditions the atmosphere could not properly absorb or dissipate the exhaust. As a result, it was also re-entrained into the south wing's ventilation air intakes. "As a matter of fact, when it was relocated, it actually created more of a problem than if it had been left where it was," Transue says. "We were living with that problem for about seven years."

During the past few decades, another, more practical alternative to centrifugal fans has been gaining in popularity: mixed-flow impeller systems that send a vertical jet plume of exhaust well above the roofline. While mainly used for exhausting laboratory workstation fume hoods, these systems are now installed at a number of hospitals around the country for this very purpose. The mixed flow impeller system's jet plume disburses diesel generator exhaust pollutants—and odors—high enough into the atmosphere to eliminate re-entrainment possibilities, comply with applicable pollution abatement laws and prevent neighborhood odor. These systems work by mixing outside, ambient air with the exhaust gases to effectively dilute the diesel exhaust stream, thus preventing re-entrainment into the building's intake air ventilation system, doors and windows.

There are two diesel-powered generators at Brick Hospital, one of which is rated at 900 kW and the other, at 635kW for the south wing. Transue contacted Steven J. DiFlora, P.E., at Kallen & Lemelson, New York. DiFlora and his associates recommended that mixed flow impeller fans replace the existing centrifugal fans on the roof to serve the two generators. The new fans are rated at 20 hp and 15 hp respectively, with the 20-hp fan operating at about 7,600 cfm and with flue gases at 840°F, which mixed with the ambient air at a 560% dilution ratio, effectively provides about 43,000 cfm total at 186° F. Even with a 10-mph crosswind, the exhaust stream is projected at a nozzle velocity of over 46,000 fpm rising to a height of approximately 65 ft. above the roofline, thus providing effective dissipation and preventing possibilities for re-entrainment.

In configuring this system, Kallen & Lemelson terminated the existing flue pipes on the roofs of the south and west wings, intercepting them and forming new transition sections to connect to the fans' intake plenums. A minimal amount of reinforcing steel was added to the building's roof framing structure. Each fan was bolted to its curb, and each curb was bolted through the roof to the supplementary support steel. Because the fans are precisely balanced and use direct drive motors, there was no need for additional vibration isolation. "The fans operated smoothly with no sensation of vibration below in the occupied spaces," says Transue. "And from an aesthetic standpoint, these fans are barely noticeable from the surrounding area. Thanks to their capabilities, there was no need for any tall flues or stacks."

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