When it comes to improving air flow in HVAC duct systems to optimize efficiencies the mantra “less is more” rings true.
Taking The 'Less Is More' Approach
Energy conservation is always a goal, and is even more paramount in construction today. When it comes to improving air flow in HVAC duct systems to optimize efficiencies the mantra “less is more” rings true.
Over the years, there has been some controversy with mechanical contractors over the use of single thickness versus double thickness turning vanes in square throat elbow applications. However, research conducted by the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) and ASHRAE Fundamentals Handbook puts the controversy to rest. SMACNA test data clearly indicates that using double thickness turning vanes instead of single thickness vanes increases the pressure loss of square throat elbows which, in turn, decreases HVAC performance, air flow and energy efficiency.
Single thickness vanes have a recommended installation length of 36 inches as outlined in the SMACNA Duct Construction Standards, Metal and Flexible Third Edition-2005. If a square throat elbow must exceed that length, it is recommended that the single thickness be installed in sections or a tie rod be fastened to the face of the vane for reinforcement. This method will reduce additional pressure loss.
To further maximize air flow efficiency, the SMACNA duct design committee also suggests turning vanes be installed in all rectangular square throat elbows and that contractors use vanes without trailing edges, because trailing edge turning vanes can be installed backwards due to careless workmanship. For many years mechanical contractors, often with the system designer’s approval, eliminated every other turning vane from the vane runners installed in rectangular square throat elbows. This practice is not recommended because it more than doubles elbow pressure losses.
This is proven by a SMACNA sponsored research performed by ETL Laboratories of Cortland, New York, to test single thickness turning vanes with a radius of 4 inches. The distance between the vanes varied from 3 inches to 6 inches. The testing used standard turning vane rail runners, with airflow velocities varying from 1,000 to 2,500 fpm in the 24 x 24 inch test elbow. The loss coefficient of 0.18 for the standard spacing was compared with the loss coefficient of 0.46 inch at a 6 inch spacing with every other vane missing.
After analysis, the test data clearly indicated that the pressure loss of the elbow with missing vanes was more than two and a half times the pressure loss of a properly fabricated square throat elbow containing all the single thickness vanes with 3 inch spacing. a 4-inch single thickness turning vane is more effective than both 2 inch and 4 inch double thickness vanes. According to test results a 4-inch single wall turning vane is two times more effective than 2 inch double thickness turning vanes at all tested velocity fpm.
A variety of single vane products are available on the market for square throat elbow applications. Sheet Metal Connectors, Inc., for example, offers the E-Z Rail II single-wall turning vane for improving HVAC system air flow through square throat elbows. In addition to enhancing the HVAC performance, use of the E-Z Rail II reduces energy usage because the system uses less fan motor energy to divert air flow.
The E-Z Rail II is a single-wall turning vane manufactured in 22 Gauge G-60 Galvanized Steel with a vane radius of 4 inches, vane spacing of 3 inches, and extra long tabs with self-locking buttons to hold the vane securely in place. The vane and rail are packaged separately in 10 foot lengths and 10 pieces per bundle. Contractors may also order single-wall vane and rail cut to length. ETL Testing Laboratories has successfully tested the E-Z Rail II single-wall vane up to 3000 fpm (test reports available upon request). The single-wall turning vane should be inserted at a 45-degree angle in the point heel of the elbow to the point throat at the elbow.
There are a number of ways that mechanical contractors can improve air flow in duct systems and decrease HVAC energy usage. Significant savings and efficiencies are accomplished by utilizing single-wall turning vanes in square throat elbow applications, installing vanes that are tangent to air flow in square throat elbows, and eliminating trailing edges in rectangular mitered duct elbows.
via The Chief Engineer