Calculation of Pressure Drop in Divided Flow Branches 1

[1HVACEngineer]

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?”

 

[lilliput1]

Yes I agree with your calculation so long as you use the corresponding CFM going through each duct section. Designwize I recommend requiring turning vanes on rectangular mitered elbows. Note loss coefficient for single thickness is less than that for double thickness. However our standard spec requires double thickness vanes because they are more rigid, less prone to pulsation.

 

[ChasBean1]

Just curious - why didn't you account for the smaller duct?

 

[1HVACEngineer]

ChasBean1,

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.

 

[lilliput1]

Refer to SMACNA HVAC System Duct Design 1990 page 14.20 Table 14-10E. Elbow, Rectangular, Mitered w/ Converging or Diverging Flow.
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.

 

[1HVACEngineer]

lilliput1,

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.