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Air Distribution Grills, Registers, Bonnets and Take-offs

Air Distribution Grills, Registers, Bonnets and Take-offs. North Seattle Community College HVAC Program Instructor – Mark T. Weber, M.Ed. Duct Systems. Lesson Objectives. Become familiar with register and grille types. Become familiar with fitting types. Typical Duct System Components.

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Air Distribution Grills, Registers, Bonnets and Take-offs

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  1. Air Distribution Grills, Registers, Bonnets and Take-offs North Seattle Community College HVAC Program Instructor – Mark T. Weber, M.Ed.

  2. Duct Systems Lesson Objectives • Become familiar with register and grille types. • Become familiar with fitting types.

  3. Typical Duct System Components

  4. Velocity Bonnets and Static Take Offs • Velocity bonnets change airflow direction with least amount of friction loss. • Static take offs can be used when main ducts must run at right angles.

  5. Curve, Radius, and Aspect Ratios • Standard elbows: • Curve ratio of 0.5. • Radius ratio of 1.5. • Aspect ratio 0.75 to 3.0.

  6. Splitters and Turning Vanes • Required where standard elbows cannot be used. • Reduce resistance. • If throat radius is less than 4 in., a square elbow with turning vanes is preferable.

  7. Splitters and Turning Vanes In some installations, the available space for ductwork may be so limited that there is no room to use a standard elbow. If so, elbows with shorter turns or even square turns may have to be used. In order to keep fan horsepower down, splitters or turning vanes must be used to lower the resistance of these high-loss elbows. With the proper application of splitters, the friction loss of the elbow will approach that of a standard elbow. From a construction standpoint, if the throat radius is less than 4 in., it is better to use an elbow with a square turn and appropriate turning vanes.

  8. Splitters and Turning Vane Application • Note the distance of take-off from elbow with and without a splitter. • Note the distance of the take-off from elbow with formed and solid turning vanes.

  9. Velocity Take-offs • Reduce velocity pressure. • Diverts air to branch duct. • As pictured, friction loss is the about the same as a standard elbow.

  10. Static Take-offs • Type “A” is preferable for grille connections. • Type “B” has lower friction loss, but results in poor air distribution to grilles. • Type “C” causes turbulence and is questionable if the grille is closer than 10 duct diameters.

  11. Vertical Stacks • Stack performance increases left to right in the images below.

  12. Side Outlets in Horizontal Ducts • Devices are needed at the outlet to direct airflow. • Airflow should be perpendicular to the outlet. • Multiple curved deflectors are the best option.

  13. Ceiling Outlets in Horizontal Ducts • Air must make 90° turn before entering the outlet. • The shorter the connection, the greater the need for directive devices. • Vertical guide strips are effective.

  14. Duct Through Unconditioned Space • May cause conditioned air to lose desired qualities: • Cooled air picks up heat; and • Heated air becomes cooled. • Insulation is required when a temperature difference of greater than 15°F exists between the outside and the inside of the duct: • Supply duct installed in an attic must be insulated. • Flex duct and fiberglass have built-in insulation. • Metal duct may be insulated inside or outside: • Internal insulation is applied during the manufacturing process, and adds additional resistance to air movement.

  15. Fire Dampers • NPFA standards are important. • Fire dampers are found in large duct systems. • Restricts passage of heat (fire). • Several varieties: • Single blade: • Connected multiple blade: and • Interlocked multiple blade.

  16. Supply Register Factors • Comfort: Body-heat loss equals body-heat production. • Draft: Slight movement of air that results in “coolness.” • Air velocities of less than 40-80 ft/min (Fpm). • No more than 4°F difference between floor to shoulder height. • Reduced stratification improves comfortability.

  17. Supply Register Categories

  18. Supply Air Outlets: Group B • In or near the floor with non-spreading vertical discharge. • In cooling, total air reaches the ceiling and spreads out: • Stagnant zone on opposite side of room. • In heating, total air spreads all the way across ceiling and down opposite wall: • Stagnant zone near the floor.

  19. Supply Air Outlets: Group C • Outlets in or near the floor with a vertical spreading discharge. • This group includes floor diffusers, baseboards and low sidewall units that have wide-spreading vertical jets. • Well suited for heating applications.

  20. Return Air Intakes • Sized to return proper air to air handler while meeting minimum static pressure and noise level requirements. • Commercial return grilles have 45 %–55% free areas: • “No see through;” • Face velocity should not exceed 500 Fpm; and • Sized and selected according to pressure and noise requirements.

  21. For more information please contact Mark T. Weber At North Seattle College WWW.NorthSeattle.edu Mark.weber@seattlecolleges.edu

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