Take a "Less is More" Approach to Improve Air Flow in Duct Systems
November 6, 2008
Energy conservation is always a goal, and is even more paramount in construction today. When it comes to improving airflow 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.
Other Steps to Improve Pressure Flow
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 systems 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 4½ inches. The distance between the vanes varied from 3 inches to 6½ inches in increments of ¼ 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 of 3¼ inch was compared with the loss coefficient of 0.46 inch at a 6½-inch spacing with every other vane missing. (Figure 1) 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 3¼-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.
There are a number of ways that mechanical contractors can improve airflow 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.