A Tutorial on Low Profile Roof Exhaust Mixed-Flow Impeller Technology

Where limitations on building height influence the design of the ventilation system, HVAC system designers might consider mixed-flow impeller exhaust fans to meet height restrictions and enhance roofline aesthetics.


Over the past decade or so, many municipalities have passed ordinances governing building height—including rooftop appurtenances. The challenge to architects and engineers is to provide adequate ventilation when exhaust stack height is restricted.

This is especially the case for designers of laboratories, controlled-environment spaces, pharmaceutical and chemical process areas, hospitals, etc. Research and production areas must be adequately ventilated to maintain acceptable indoor air quality (IAQ) for employees, and they must not emit hazardous air pollutants or generate objectionable odors in the neighborhood. Low-profile mixed-flow impeller exhaust systems are becoming increasingly popular for these applications.

Principle of operation

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 and bypass dampers. The 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 natural aspiration. 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.

Dilution in this manner also eliminates the perception of odors. In some cases, the concentration of an odorous compound present in the exhaust stream 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 will be detectable. This is true for some food-processing plants and restaurants, where the odor may even be relatively pleasant, yet not something the neighbors want to smell continuously.

Small, quiet package

Many ventilation applications employ centrifugal fans, which require tall exhaust stacks for efficient operation. They are generally expensive, complex, and heavy because of the associated mounting hardware, roof curbs, guy wires, and so forth. They generally use belt-driven motors that tend to be maintenance-intensive, and are frequently located in expensive rooftop penthouses to protect maintenance personnel working under adverse weather conditions. Furthermore, tall exhaust stacks on a building’s roof are unsightly and are often perceived as “pollution generators”—even if they are emitting only steam.

Exhaust acoustics are another aesthetic aspect of building design. Because mixed-flow fans operate more efficiently than centrifugal fans, they are inherently quieter.For particularly sound sensitive areas, unique in-line acoustical silencer nozzles may be fitted to mixed-flow fans—without adding stack height.

Reducing energy costs

When exhaust air cannot be returned back into the building, a facility requires 100% makeup air, which must be filtered, heated or cooled, humidified or dehumidified, depending upon circumstances. This significantly increases energy costs.

Mixed-flow impeller technology can help reduce these costs through energy recovery. The systems are designed to accommodate a heat exchanger containing coils filled with a glycol-water solution that extracts ambient energy from the exhaust before it is discharged. The glycol-water solution is transferred to the supply air handler to preheat the conditioned air entering the building during winter or pre-cool the air during the summer.

Energy recovery typically reduces heating costs about 3% for each 1°F of makeup air being preheated. A 10°F increase in intake air temperature translates directly into a 30% energy saving. Similar savings, although not quite as dramatic, may also be realized for cooling.

Final thoughts

Mixed-flow impeller exhaust systems can meet a building’s ventilation needs while complying with community height limitations and without creating an eyesore, as the photos illustrate. They have been used successfully in numerous university, hospital, industrial, and government laboratories, as well as manufacturing and pilot plants.

Mixed-flow impeller fans, on the other hand, are typically only about 15 ft high, with no need for structural reinforcements.Because they use direct-drive motors, there are no belts, pulleys, elbows, flexible connectors or spring vibration isolators to maintain, making their upkeep simpler and eliminating the need for penthouses. They can be mounted to the side or on top of a common mixing plenum, creating a smaller equipment footprint. Their low-profile design and the absence of auxiliary structures eliminates the smoke-stack look and the negative connotations associated with it. This allows mixed-flow impeller fans to be used where aesthetic considerations preclude the installation of tall stacks, such as in jurisdictions that restrict building height.

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