Vaneaxial fans boast increased efficiency

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USA: A custom air handling unit with vaneaxial fans installed on the roof of a building in Chicago is said to have offered lower energy and installation costs.

The air handling unit was acquired by a major US high tech firm at its new Chicago headquarters in the Fulton Street area of the city. The building, which was formerly a meat freezer locker, was gutted and completely renovated. When selecting the rooftop air handling unit, engineers were looking for a design that minimised noise, because the rooftop area contains an outside garden and employee snack bar.

The original design for the air handling unit included a fan wall for both supply and return. The fan wall concept has been the favoured approach for high rise building ventilation for the past 10-15 years in the US. With the fan wall idea the ventilation unit can still operate if one fan goes down. Also, the fan unit length is shorter, reducing the overall length of the air handling unit. The smaller footprint can save on the total air handling unit cost.

BTU Company, Inc, a professional manufacturer’s representative firm in the HVAC and refrigeration industries based in Westmont, Illinois, and BBM Engineering, worked with the project engineering design firm WMA Consulting Engineers to instead consider the use of JM Evase Vaneaxial fans by Howden American Fan Company.

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Comparison between the vaneaxial design and the original fan wall design. The vaneaxial design is said to have resulted in 371.91hp in operating load savings

Calculations suggested that vaneaxial fans would perform better and save on energy costs. The static pressure on the fan wall was shown to be higher than a vaneaxial fan, due to losses on the outlet of the fan. This increased the overall total motor horsepower of the fan wall compared to the vaneaxial fans.

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The evase vaneaxial fan design provides a static regain and reduces the overall brake horsepower

The team also designed the inlet and outlet of the vaneaxial fan in the air handling unit to be more efficient and quieter using an evase casing, a piece of the fan casing that increases in size as the air moves through the fan. This option makes the fan more compact. The design provides a static regain and reduces the overall brake horsepower. It reduces turbulence through the fan on the downstream and lowers noise.

Howden maintains that fans designed with an evase can save energy over the installed life of the system by recovering the kinetic energy contained in the high velocity air stream’s mass. These fans recover energy by converting the velocity pressure in the system into static pressure using a built-in discharge cone, which allows the diffusion of the high velocity airstream to slow to a lower rate. This causes an effective conversion of velocity pressure into static pressure. When applied to the installation, the resulting reduced static pressure allows the fan system to perform more efficiently at reduced static pressures, with resulting lower energy consumption.

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The evase design, with the duct increasing in size as the air moves through the air handling unit. The fans start out at the inlet at 50in id and then expand to 55in id at the outlet around the motor

Since you don’t have to specify a discharge cone for the system, the fan’s overall footprint is reduced. Air velocity is low enough for outlet damper and shutters to operate without fear that these relatively sensitive components will be destroyed by high velocity flutter.

The evase fan design incorporated integral stationary anti-rotation guide vanes to enhance the regain effect by slowing the rotation imparted to the airstream by the fan impeller and recovering rotational energy losses, while reducing fan noise generation. A separately formed impeller casing was used to ensure uniform minimal tip clearance for full pressure development.

The design for this custom air handling unit also includes an inlet trap with a special bullet (pod) inside to direct the air. With vaneaxial fans there is a hub and blades are attached to the hub, the bullet is designed to be the same size as the hub, so there are no concerns about vortices around the hub. A specially made sound trap with turning vanes designed by BBM Engineering helps reduce the sound as it goes down into the rest of the building.


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