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In most cases, the methods for duct construction are dependent on the type of duct being installed and the constraints of the physical space. The overarching process is usually about the same for all ducts; the biggest differences usually center on the materials used and the specifics of the building’s layout. In general, there are three main types of duct systems: low-pressure, medium-pressure, and high-pressure. Identifying the duct system at issue will help determine the type of work needed for the job and will also frame the methods needed for optimal construction. Different tools and components may be required for different projects, too. Components such as diffusers, registers, flues, vents, roof penetrations and filters may or may not be required, and knowing when and how to use these and other similar pieces is a big part of construction in most cases.
Ducts are basically large passageways built into the walls or ceilings of various structures in order to funnel and direct airflow. They’re normally connected to heating, ventilation, and air conditioning (HVAC) systems. Homes and offices built with central air almost always have ducts planned from the drafting of the structural blueprint; in a sense these are the easiest to install since they’re built with the structure and can be integrated seamlessly. It’s common for older structures to be retrofitted with ventilation systems later, though duct construction in these cases may have to be more carefully planned to fit within a structure that may not easily be able to accommodate all systems.
Most duct systems, whether new or existing, fall into one of three categories based on how much pressure they’re designed to withstand or support. All look similar in terms of their physical qualities, but things like how much support they’ll need on the inside and how much clearance they’ll need within the walls or ceilings often takes some careful calculation. The precise method a construction engineer will need to use also depends a lot on the contours of the structure, its size, and features like whether air needs to circulate between floors. Variables including the number of vent openings and their size also factor in in most cases.
All duct construction involves components that maximize ventilation efficiency. Major elements include corner closures, elbows, hangers, “T” connections, fire dampers, access doors, and air intakes. Duct fittings include caps, offsets, single-branch take-off, square-to-round transitions, register box, and round flexible duct reinforced with spiral wound wire. The galvanized sheet metal used should have a galvanized coating on both sides of the sheet equal to at least 1.25 ounces (35.4 g) of zinc per square foot (0.09 square meters) and should generally be of lock-forming quality.
The best duct design is one that utilizes these construction components to eliminate vibration, which is accomplished by tightly installing rigid, constructed metal ducts. Rectangular ducts should be made with standing seams and braced with angle iron to stiffen the sheet and avoid vibration. The velocity in the main duct and the remote branches should be lowered gradually to distribute the air uniformly. This is done by increasing the size of the duct, and it usually cuts down the friction in the smaller ducts.
Reducing “leakage” is another important aspect in duct construction. Air pressure loss within the duct system can be avoided by soldering, welding or otherwise sealing all joints and seams to make them tight. They also can be riveted or spot welded. The lap joints should be made so that the outlet of one length of ductwork fits into the inlet end of the next length of ductwork in the direction of airflow.
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