Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Mechanical Engineering

First Advisor

Kurt Bassett


The distance between supply air diffusers and return air grilles is an important design parameter that has not been properly investigated. Sometimes, design and installation constraints force the engineer or the contractor to place the return duct close to the supply duct which can have a huge impact on the overall efficiency of air distribution in the zone. When the return grille is too close to the supply diffuser, air by-passes the room and flows directly back into the return duct. By-passing of the air in a zone of a building will prevent the heat transfer and the circulation that are needed for the HVAC system to work properly. This has an impact on the system’s overall efficiency and results in wasted energy. The work presented here studies the effect of the distance between supply and return air openings at various exit velocities on the overall efficiency of an HVAC system. The study modeled air distribution in a typical office space that was assumed to have negligible leakage and was adiabatic. The density of air was assumed to be constant. Also, for the scope of this study, temperature was assumed to be constant since the interest is only in proper duct placement and thermodynamic effects were not being considered. The study investigated both square and round diffusers. The diffusers and return grilles were placed equidistant from the center of the ceiling at lengths of 12, 24, 36, 48 and 60 inches away from each other. They were coincident within the same x plane. Each placement configuration was modeled at three different flow rates: 200, 350 and 500 cfm that represented low, medium and high air flow for a typical VAV system. The analysis showed that round diffusers are more efficient than square diffusers when they are placed less than 60 inches away from the return grille. At the designed flow rate of 350 cfm and at 12, 24, 36, 48 and 60 inches away from the return grille, the square diffuser showed short-circuiting percentages of 23.4, 21.9, 17.2, 10.9 and 5.6 percent respectively. On the other hand, at the same flow rate of 350, the round diffuser showed short-circuiting percentages of 10.9, 9.4, 7.5, 6.6 and 5.8 percent when the supply and the return openings were placed 12, 24, 36, 48 and 60 inches apart respectively.

Library of Congress Subject Headings

Offices -- Heating and ventilation


Air flow

Architecture and energy conservation -- Heating and ventilation


Includes bibliographical references (pages 118-120)



Number of Pages



South Dakota State University


Copyright © 2016 Khaled Abdel Razzaq Saadeddin