Laboratories, manufacturing plants, hospitals, restaurants and other processing facilities require effective means of controlling odoriferous emissions. Common odor control technologies can include chemical scrubbing, bio-scrubbing, thermal or catalytic oxidation, charcoal filtration and precipitation. All of these processes eliminate odors by capturing or destroying odor-causing substances before the exhaust stream leaves the facility.

Diluting odoriferous exhaust, however, eliminates or reduces the perception of odor. In some cases, the concentration of an odoriferous compound may be well below the substance’s safe exposure limit but above its odor threshold. Thus, even though the odor does not represent a health hazard, the compound remains objectionable.

In these situations, dilution of the odoriferous exhaust with ambient air and discharge of the resulting stream high into the atmosphere may be an efficient, cost-effective odor management strategy. This often is true at food-processing facilities and restaurants, where the odor may even be relatively pleasant, yet not something the neighbors want to smell continuously.

Such was the case at the Cadbury-Schweppes Science Technology Center in Whippany, N.J. The 148,000-sq. foot research center contains 12 laboratories that perform flavor analysis and other analytical procedures for the company’s confectionary development efforts. Housed in a compact building, it has a small roof area that must accommodate the HVAC system’s air handlers, leaving little room for exhaust equipment.

Engineers determined that the use of conventional centrifugal roof exhaust fans could result in re-entrainment of exhaust into the building’s air intakes – allowing odors to go right back into the laboratories and offices.

There also was concern about odors in the community. The research center is located in an industrial area, but also has two residential neighbors whose houses predate the current zoning laws. “We make a lot of sweet stuff – candies and goodies – but the last thing we want is to have strawberry odors surrounding the facility,” said Steve Wehner, the director of facility operations. Centrifugal fans were ruled out, he said, because “they are not robust enough to handle the requirements of some of the laboratory waste streams.”

To address these issues, the confectionary company installed an exhaust system that consisted of eight independent mixed-flow impeller fans, each serving one laboratory. “Because of laboratory requirements, we didn’t want to mix the exhaust from different workstations, so we used individually dedicated exhaust systems,” said Wehner.

The above photo is copyrighted by Thomas H. Kieren – customcorpphotog@earthlink.net.

Mixed-flow impeller systems (see sidebar) dilute odoriferous and/or contaminated exhaust with up to 170 percent additional volumes of fresh outside air, and eject the mixed discharge stream into the atmosphere at high velocity. The combination of added mass and high velocity minimizes the risk of exhaust being re-entrained into fresh air intakes, doors, windows or other openings. It also ensures the safe dilution/dispersion of the exhaust to prevent odors from pervading the surrounding community.

As an example, a mixed-flow fan moving 80,000 CFM of combined building and bypass air at an exit velocity of 6,300 ft/min can send an exhaust-air jet plume up to 120 feet high in a 10-mph crosswind. This high velocity is more than double the minimum of 3,000 ft/min recommended by ANSI Z9.5 standards. Because nearly twice as much free outside air is induced into the exhaust stream, a substantially greater airflow is possible for a given amount of exhaust – improving dilution capabilities and higher effective stack heights over centrifugal fans without additional horsepower.

Centrifugal fans require tall exhaust stacks for efficient operation. Tall stacks can require roof curbs and guy wires and could be perceived by the community as pollution generators. The mixed flow impeller system, in contrast, required stacks only about 15 feet high, with no need for structural reinforcements. Their low-profile design and the absence of any auxiliary structures eliminated the so-called “smokestack” look and the negative connotations associated with it.

Stack height was a special consideration at the facility, according to Wehner. “There is a ‘visualization’ line, as community ordinances restrict height of rooftop equipment for aesthetics reasons. They do not want people to drive by and see equipment on the roof,” he said. “The exhaust fans are hidden.”

Since the building opened in November 2005, according to Wehner, the mixed flow impeller fans have proven to be “a very robust system, requiring a minimal amount of preventive maintenance. We haven’t experienced any issues with the units at all. They came online beautifully – a very smooth operation,” he said.  

SIDEBAR: Characteristics of mixed flow impeller technology systems

Direct-drive, mixed-flow impeller systems operate on a unique principle of diluting contaminated exhaust air with unconditioned, outside ambient air via a bypass mixing plenum. The resultant diluted process air is accelerated through an optimized discharge nozzle/windband where nearly twice as much additional fresh air is entrained into the exhaust plume before leaving the fan assembly. Additional fresh air is entrained into the exhaust plume after it leaves the fan assembly through a natural aspiration effect. The combination of added mass and high discharge velocity minimizes the risk of contaminated exhaust being re-entrained into building fresh air intakes, doors, windows or other openings.

Mixed-flow impeller systems also reduce noise, and use less energy. A typical reduction of $0.44 per CFM at $0.10/kilowatt-hour provides an approximate two year ROI. Energy consumption for mixed-flow fans is about 25 percent lower than conventional centrifugal fans designs, with substantially reduced noise levels, particularly in the lower octave bands. They also conform to all applicable laboratory ventilation standards of ANSI/AIHA Z9.5 as well as ASHRAE 110 and NFPA 45, and are listed with Underwriters Laboratory under UL 705.

Mixed-flow systems are designed to operate continuously with a minimum amount of required maintenance. Direct-drive motor bearings have lifetimes of minimum L10 100,000 hours. (This refers to a sample of 100 motors in which the bearings in ten motors would fail within a 100,000-hour timeframe. It is a baseline for comparison of motor bearing lifetimes.) Non-stall characteristics of the system’s mixed flow wheel make it ideally suited for constant or variable air volume applications, along with built-in redundancy, and design flexibility. Variable air volume capabilities are achieved via the bypass mixing plenum or by using variable frequency drives to provide optimum energy savings.