Elbow Grease

The engineers for an automobile plant designed an air-supply system with the top horizontal discharge blower in the penthouse located as high as possible to provide space for a 2-in.-diameter-centerline-radius elbow directly onto the fan discharge. Ducts connected to the outlet of the elbow connected to the plant's distribution system. In calculating the required static pressure in the total distribution system, AMCA'S system-effect manual was followed closely.

The system was run at the factory and found to meet specifications. After the system was installed, airflow measurements disclosed the air volume was considerably below specification. When checked, the blower was running in the right direction and at the speed measured at the factory. The fan outlet connection to the distribution system was, however, a 90-degree square-throat elbow with pre-manufactured turning blades, rather than the specified 2-in.-diameter centerline radius elbow. When the installation contractor was questioned, he stated that the sales representative who sold him the turning blades assured him that with the blades installed, the efficiency of the elbow would be as good as the elbow specified.

In spite of this assurance, the contractor was told to replace the square-throat elbow with the elbow specified and detailed on the drawings. After this was done, the air volume handled by the system met the specifications.

Perhaps the square-throat elbow, with turning blades, would have performed satisfactorily if the velocity of the air entering the elbow had been uniform across the duct. However, the air velocity at the blower discharge was not uniform. In fact, very high air velocity always is found in the upper part of a top, horizontal blower discharge. This results in much of the air volume trying to flow between the upper turning blades. This restriction to airflow causes high turbulence and reduced air volume.
Kenneth E. Robinson
Apex, N.C.

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