HVAC Duct Design

HVAC Duct Design

  • Air distribution systems shall be designed in accordance with ASHRAE Fundamentals Handbook with the preference for equal friction method.
  • Air duct material selection criteria is to consider the environmental conditions of the location and the quality of air passing thru the ductwork. In spaces/rooms such as laboratories, battery rooms ..etc where air is corrosive stainless steel duct shall be selected for use. As general duct material used is galvanized steel conforming to ASTM A653/A653M with G-60 zinc coating rate.
  • Ductwork fabrication, construction and installation shall be in accordance with SMACNA Duct.
  • Construction Standards Metal and Flexible, based on the design static pressure of the system.
  • Supply air duct shall be designed for a maximum pressure drop of 0.82 Pa/m ( 0.1 inch w.g. per 100 ft). Return, general exhaust and outside air ducts shall be sized for maximum pressures drop of 0.65 Pa/m (0.08 inch w.g. per 100 ft) where space conditions allow.
  • Flexible duct shall not be used in return or exhaust systems. Duct reinforcement and joining method shall be in accordance with SMACNA Duct Construction Standards. Flexible duct used in supply air, the duct length shall not exceed 2400 mm (8 ft).
  • Volume dampers shall be installed in all branches from mains and risers, and in the branches to outlets and inlet grille openings. Volume dampers shall be installed ahead of flexible duct serving supply air grilles, registers and diffusers and at duct take-off or at other places shall be two gages heavier than the duct.
  • When return plenum are used instead of ducted return air, the material and equipment installed inside the plenum shall confirm to NFPA 90A and NFPA 70 requirements.
  • Ductwork penetration exterior walls or roof for blast resistant buildings shall be protected by required blast dampers and shall  be designed to withstand the rated blast contours. Duct penetration of fire rated wall or roof shall install fire/smoke or combination fire/smoke dampers wherever the code calls for.
  • Ductwork passing thru electrical rooms shall confirm to building/electrical code requirements.
  • Mechanical rooms shall not be used as a return air or mixing plenums.
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    Ductwork shall be coded on HVAC drawings in accordance with the schedule in Table III. Should ductwork be fabricated of materials other than those listed in Table III, a new duct code shall be developed and included on the drawings.

 

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