HVACR416 - Design Air Distribution Part 2

Air Ducts Air carriers that deliver air to conditioned space. Originally hot air ducts were thin, tinned sheet steel. Later, galvanized sheet steel, sheet aluminum, and insulated ducts were developed. Passageways formed by studs or joists are sometimes used for return air. (panning) Ducts work on the principle of air pressure differences. If a pressure difference exists, air moves from the higher-pressure area to lower-pressure area. The greater the pressure difference, the faster the air will flow. 23.4

Air Ducts Three common classifications of ducts: o Conditioned air ducts. o Recirculating air ducts. o Fresh air ducts. Ducts may be round, square, or rectangular. 23.4

Air Ducts Round ducts are the most efficient. o Less material is needed for the same capacity as a square or rectangular duct. o Resistance to airflow is less. o Round ducts have less surface area, so the heat loss or gain is lower. Square or rectangular duct conforms best to building construction. They are easier to install between joists and studs. 23.4

Air Ducts The quantity of sheet metal used for rectangular duct may be the deciding factor. o Perimeter, or circumference, of a circle is: Circumference = Diameter X  o Perimeter of a rectangle or square is: Perimeter = (2 X Width) + (2 X Length) 2

Types of Duct Systems Ducts deliver air to the rooms and return air to the heating or cooling system.

Types of Duct Systems Several types of supply duct systems: o Individual round pipe system. o Extended plenum system. o Reducing trunk system. Return air system types: o Single return system. o Multiple return system. o Combination or single and multiple system.

Types of Duct Systems

Types of Duct Systems Typical branch duct design.

Types of Duct Systems Overhead duct system.

Types of Duct Systems In basements: o The main duct is run perpendicular to and just under the floor joists. o Branch ducts run between the joists to the grille or diffuser openings. o Return ducts usually use the joists and floorboards for three sides of the duct. ( No longer allowed by Code) o The main return duct runs alongside the conditioned air duct. Ducts are usually insulated in a crawl space or attic to prevent heat loss/gain

Types of Duct Systems Ducts installed in a concrete slab: o Usually made of metal, plastic, or ceramic. o Branch ducts usually connect to a perimeter duct. o Grilles or diffusers are connected to the perimeter duct at intervals along the floor. o A downflow furnace is used.

Types of Duct Systems

Types of Duct Systems Some buildings are designed with unit ventilators. False (Drop) ceilings are often used to conceal piping, wiring, heat exchangers, and ducts. Diffusers may be used. The space between the false and real ceiling may be used as a fresh air plenum chamber. Some systems use heated panels to provide radiant heat or radiant cooling.

Types of Duct Systems