Computational Fluid Dynamics Analysis of Flexible Duct Junction Box Design

R. Beach, D. Prahl, R. Lange

Research output: NRELSubcontract Report


IBACOS explored the relationships between pressure and physical configurations of flexible duct junction boxes by using computational fluid dynamics (CFD) simulations to predict individual box parameters and total system pressure, thereby ensuring improved HVAC performance. Current Air Conditioning Contractors of America (ACCA) guidance (Group 11, Appendix 3, ACCA Manual D, Rutkowski 2009) allowsfor unconstrained variation in the number of takeoffs, box sizes, and takeoff locations. The only variables currently used in selecting an equivalent length (EL) are velocity of air in the duct and friction rate, given the first takeoff is located at least twice its diameter away from the inlet. This condition does not account for other factors impacting pressure loss across these types offittings. For each simulation, the IBACOS team converted pressure loss within a box to an EL to compare variation in ACCA Manual D guidance to the simulated variation. IBACOS chose cases to represent flows reasonably correlating to flows typically encountered in the field and analyzed differences in total pressure due to increases in number and location of takeoffs, box dimensions, and velocityof air, and whether an entrance fitting is included. The team also calculated additional balancing losses for all cases due to discrepancies between intended outlet flows and natural flow splits created by the fitting. In certain asymmetrical cases, the balancing losses were significantly higher than symmetrical cases where the natural splits were close to the targets. Thus, IBACOS has shownadditional design constraints that can ensure better system performance.
Original languageAmerican English
Number of pages73
StatePublished - 2013

Bibliographical note

Work performed by IBACOS, Inc., Pittsburgh, Pennsylvania, and Applied Science Consultants, LLC, Pittsburgh, Pennsylvania

NREL Publication Number

  • NREL/SR-5500-60710

Other Report Number

  • DOE/GO-102013-4297


  • ACCA Manual D
  • ACCA Manual D group 11 fittings
  • box plenums
  • CFD
  • computational fluid dynamics
  • duct fitting design
  • duct fittings
  • ducted mini split systems
  • ducts
  • fitting pressure drops
  • junction boxes
  • k-factors in duct fittings
  • small-diameter ducts
  • wire-helix duct


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