Calculation of Pressure Drop in Divided Flow Branches
Calculation of Pressure Drop in Divided Flow Branches
(OP)
I am calculating a pressure drop in a return duct system and have come across a fitting that does not have a Loss Coefficient listed in the SMACNA "HVAC Systems Duct Design". Refer to Figure 2-5 in the SMACNA "HVAC Duct Construction Standards Metal and Flexible" on page 2.7, titled "Divided Flow Branches", for an illustration of a similar fitting.
Two branch ducts (54"x36" and 42"x36") end with a 90° mitered elbow, which then converge to form one main duct (96"x36"). Assuming that the air flows through the 54”x36” duct and based on experience I would normally calculate:
1. the loss in the 54”x36” as a straight duct run (with the Branch Airflow).
2. the Loss Coefficient for a Mitered Elbow with Diverging Flow at the elbow.
3. the loss in the 96”x36” as a straight duct run (with the Total Airflow).
In the grand scheme of things the pressure loss may be minimal as compared to the total system pressure drop; however I am curious – “Would anyone calculate this differently?”
Two branch ducts (54"x36" and 42"x36") end with a 90° mitered elbow, which then converge to form one main duct (96"x36"). Assuming that the air flows through the 54”x36” duct and based on experience I would normally calculate:
1. the loss in the 54”x36” as a straight duct run (with the Branch Airflow).
2. the Loss Coefficient for a Mitered Elbow with Diverging Flow at the elbow.
3. the loss in the 96”x36” as a straight duct run (with the Total Airflow).
In the grand scheme of things the pressure loss may be minimal as compared to the total system pressure drop; however I am curious – “Would anyone calculate this differently?”





RE: Calculation of Pressure Drop in Divided Flow Branches
RE: Calculation of Pressure Drop in Divided Flow Branches
RE: Calculation of Pressure Drop in Divided Flow Branches
The calculation was performed on the longest run/highest pressure drop in the system. I agree with your point that the smaller duct may in fact have a higher-pressure drop, however the largest pressure drop in the system is through the larger branch. Refer to SMACNA "HVAC Systems Duct Design - Equal Friction Method", for further details.
Additionally, since posting this question I have discovered the fitting in question, refer to the "ASHRAE Duct Fitting Database - ER5-5 Bullhead Tee without Vanes, Converging." Unfortunately however, I have reviewed past copies of 1989, 1993, 1997,and 2001 ASHRAE Fundamentals Handbooks, the "Bullhead Tee without Vanes, Converging" is not listed in any of them.
RE: Calculation of Pressure Drop in Divided Flow Branches
For W1/W = 54/54 = 1.0
and H/W = 36/54 = 0.667
Therefore loss coefficient C = 1.22
& inches wg. loss through fitting is = 1.22 x Vp
Where Vp = in. wg velocity pressure upstream of the fitting.
RE: Calculation of Pressure Drop in Divided Flow Branches
I attempted the calculation using the fitting you suggested "14.20 Table 14-10E. Elbow, Rectangular, Mitered w/ Converging or Diverging Flow"; the results did not match the bullhead tee. I believe the difference between the two fittings would be due to the airflow through a single branch (Mitered Elbow) vs. the airflow through two branches (Bullhead Tee).
In a return/exhaust bullhead tee you have two branch flows converging to one large tee (Hb1 = Hb2 = Hc, Ab1 + Ab2 = Ac, Qb1 + Qb2 = Qc). The calculation using "ASHRAE Duct Fitting Database - ER5-5 Bullhead Tee without Vanes, Converging" yields a loss coefficient Cb1 = 1.94.
Thanks for the help.