TAB Journal Fall 2013 - (Page 10)

When a Right Angle is Wrong Gar Conaway, TBE, Engineered Air Balance Co., Inc. S ome who have worked in the sheet metal industry for many years claim that the duct fitting with a 90˚ elbow and 90˚ throat with a radius heel was first used on residential jobs, when the use of plasma cutters in the sheet metal industry became a standard. It was used as an 3.1 end fitting for a floor register to come up under a floor joist and inversely for ceiling register over a ceiling joist. This might have worked at a very low velocity with a register mounted to the end of it. In medium or high velocity systems, however, they tend to waste large amounts of energy. Q For example,˚ four of these fittings are put in a series to get over A r another duct. ASHRAE and SMACNA rate fitting losses with laminar flow entering the fitting. With the setup below, only the first one has good entering conditions-after that it is all WxH turbulence. Moreover, let's say the duct measures 36"x12", with medium pressure, designed for 7,500 CFM And the AHU fan is designed for 3.5" external s.p. Airflow entering the first fitting is 3.1" s.p. but leaves the last 90˚ elbow at 1.1" s.p. That is a 2.0" loss in static pressure in less than five feet! Q A˚ r WxH 3.1 Here are calculations for a sharp throat, 1.1 radius heel: INPUTS 1.1 OUTPUTS Width (W, in.) 12.0 Velocity (V0, fpm) 2500 Height (H, in.) 36.0 Vel Press at Vo (Pv, in. wg) 0.39 Loss Coefficient (Co) 1.00 Flow Rate (Q, CFM) 7500 Pressure Loss (in. wg) 3.1 0.39 (CR3-2) Elbow, Sharp Throat (r/W=0.5), Radius Heel, 90 Degree (Idelchik 1986, Diagram 6-1) Q A˚ C =KCp ˚ where K= angle factor ASHRAE duct fitting data rates the loss at 0.39" s.p. X 4 = 1.56" static loss plus the turbulence entering each fitting after the first. r WxH Q A˚ 10 r=2.0 (50),s = 2.125 (60)in. (mm) WxH TAB Journal Fall 2013

TAB Journal Fall 2013

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