The first step in designing an efficient ductwork layout is to determine the volume of air required in each space. The next consideration is to determine the type, number, and location of each outlet for the space, and the air volume required for each outlet.
Duct routing is often limited by building constraints. Most restrictive is the clearance between the top of the ceiling system and the underside of the roof (or next floor above). A ceiling system, including light fixtures, is seldom more than 125 mm (5") in total thickness. The minimum clearance between the bottom of the duct and a finished ceiling should be at least 150 mm (6"). Because ductwork is installed prior to ceiling installation, normal practice is to determine the height of the duct's lowest feature in relation to the finished floor in order to maintain adequate clearances. This height needs to be specified whenever it is critical to a layout.
The underside of the roof (or floor) above presents parameters to be considered. If this space is restricted, some ducts may need to be installed between joists and beams or threaded through an open-web beam. Vertical ducts are used to transport air from equipment to a horizontal distribution system. Shafts may be used in multistory laboratory buildings. Other considerations for duct routing include access requirements for damper adjustment or equipment servicing. These requirements must be coordinated with architectural and ceiling configurations.
Once the routing has been established, definitive design can start. The height dimension limitations determined by the routing can help in the selection of the shape of the ductwork. The most efficient duct shape is round. It is the least expensive and most energy efficient, particularly in sizes up to a diameter of 500 mm (20") diameter. Round ductwork should be used wherever it will conveniently fit. Rectangular and sometimes flat oval ducts are used when airflow rates are greater than can be carried by a round duct in the space provided.
The arrangement and location of system components has a significant effect on duct leakage. In one typical example, a laboratory exhaust system with fan static pressure of -1,500 Pa (-6 in. WG) suffered a leakage rate of 15.2 l/s per m2 (3.0 cfm per sq.ft) of duct area. Moving high- loss filters closer to the fan and replacing elbows and tee connections with smoother parts lowered the static pressure just beyond the filter by 69% and dropped the leakage rate by 45%.
Laboratory Duct Construction
Laboratory-Specific Duct Layout
Duct Noise Considerations
Animal room distribution system: case study