What is the maximum velocity of the air in square throat, sheet metal elbows without turning vanes?

• A. 500 fpm
• B. 1500 fpm
• C. 1000 fpm
• D. 2000 fpm

Answer: (C)

When air must bend sharply in a square throat elbow with no turning vanes, the flow can separate at the inner corner, creating turbulence and a high pressure drop. Without vanes to guide and straighten the stream, keeping the velocity moderate helps the flow stay attached to the walls, reducing noise and losses through the elbow. About 1,000 feet per minute is the practical upper limit in this case, which is why 1,000 fpm is the correct choice. Velocities like 500 fpm are lower and not the maximum for this configuration, while 1,500 or 2,000 fpm would worsen turbulence and losses without the aid of turning vanes.