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

Table of Contents for the Digital Edition of TAB Journal Fall 2013

TAB Journal Fall 2013

https://www.nxtbook.com/ygsreprints/AABC/G118248AABC_Spring2020
https://www.nxtbook.com/ygsreprints/AABC/G116302AABC_Winter2020
https://www.nxtbook.com/ygsreprints/AABC/G114573_AABC_Fall2019
https://www.nxtbook.com/ygsreprints/AABC/G111085_tab_summer2019
https://www.nxtbook.com/ygsreprints/AABC/g108931_tab_spring2019
https://www.nxtbook.com/ygsreprints/AABC/g104992_tab_winter2019
https://www.nxtbook.com/ygsreprints/AABC/g100221_tab_fall2018
https://www.nxtbook.com/ygsreprints/AABC/g97743_tab_summer2018
https://www.nxtbook.com/ygsreprints/AABC/g94804_tab_spring2018
https://www.nxtbook.com/ygsreprints/AABC/g91043_tab_winter2018
https://www.nxtbook.com/ygsreprints/AABC/g86727_tab_fall2017
https://www.nxtbook.com/ygsreprints/AABC/g81312_tab_summer17
https://www.nxtbook.com/ygsreprints/AABC/g78003_tab_spring17
https://www.nxtbook.com/ygsreprints/AABC/g73432_tab_winter17
https://www.nxtbook.com/ygsreprints/AABC/g7002_tba_fall2016
https://www.nxtbook.com/ygsreprints/AABC/g65897_tab_summer2016
https://www.nxtbook.com/ygsreprints/AABC/g62758_tab_spring16
https://www.nxtbook.com/ygsreprints/AABC/tab_winter2016
https://www.nxtbook.com/ygsreprints/AABC/tab_fall2015
https://www.nxtbook.com/ygsreprints/AABC/tab_summer2015
https://www.nxtbook.com/ygsreprints/AABC/g50853_aabc_spring2015
https://www.nxtbook.com/ygsreprints/AABC/g48663_aabc_winter2015
https://www.nxtbook.com/ygsreprints/AABC/g46741_aabc_fall2014
https://www.nxtbook.com/ygsreprints/AABC/g43965_aabc_summer2014
https://www.nxtbook.com/ygsreprints/AABC/g42323_aabc_spring2014
https://www.nxtbook.com/ygsreprints/AABC/g40447_aabc_winter2014
https://www.nxtbook.com/ygsreprints/AABC/g38793_aabc_fall2013
https://www.nxtbook.com/ygsreprints/AABC/g36202_aabc_summer2013
https://www.nxtbook.com/ygsreprints/AABC/g34728_aabc_tabspring2013
https://www.nxtbook.com/ygsreprints/AABC/g32884_aabc_tab-winter2013
https://www.nxtbook.com/ygsreprints/AABC/g30873_aabc_tabjournal_fall2012
https://www.nxtbook.com/ygsreprints/AABC/g28877_aabc_tabjournal_summer2012_2
https://www.nxtbook.com/ygsreprints/AABC/g25452aabc_tabspring12
https://www.nxtbook.com/ygsreprints/AABC/g23327aabc_fall_2011
https://www.nxtbook.com/ygsreprints/AABC/g21694aabc_summer11_final
https://www.nxtbook.com/ygsreprints/AABC/g19912nxtbk
https://www.nxtbook.com/ygsreprints/AABC/g19105_aabc_tabjournalwinter11
https://www.nxtbook.com/ygsreprints/AABC/AABC/p17871_aabc_fall10
https://www.nxtbook.com/ygsreprints/AABC/g16041aabc_tabjournal
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