Duct materials
Like modern steel food cans, at one time air ducts were often made of tin, like 'tin cans' were made for food. Tin is more corrosion resistant than plain steel, but is also more expensive. With improvements in mild steel production, and its galvanization to resist rust, steel 'sheet metal' has replaced tin in ducts as well as food cans. However, the term tin-knocker or tin-basher is still used in practice to describe sheet metal duct installers.
Today, if we consider all the materials which allow us to construct ducts in rectangular, round, or oval cross-sectional shapes, we will find that the four main ones are:
Galvanized steel
Ducts are still most often made of galvanized steel. Various fittings allow transitioning between the various shapes and sizes. A "tee" connection, for example, is where the air flow can be divided into two or more downstream branches. Many factory-made shapes and sizes are available but galvanized steel can easily be cut and bent to form additional shapes when required. Steel ducts are commonly wrapped or lined with fiberglass thermal insulation, both to reduce heat loss or gain through the duct walls and water vapor from condensing on the exterior of the duct when the duct is carrying cooled air. Insulation, particularly duct liner, also reduces duct-borne noise. Both types of insulation reduce 'breakout' noise through the ducts' sidewalls.
Polyurethane and Phenolic insulation panels (preinsulated aluminum ducts)
While as mentioned above, galvanized steel is still very common, more and more rectangular ducts are being manufactured from rigid insulation panels, thanks to the fact that custom or special shapes and sizes of ducts can easily be shop or field fabricated. In addition to the fact that ducts made with rigid insulation panels do not need any further insulation. Among the various types of rigid polyurethane foam panels available, a new water formulated panel stands out. In this particular panel, the foaming process is obtained through the use of water instead of the CFC, HCFC, HFC and HC gasses. The foam panels are then coated with aluminum sheets on either side, with thicknesses that can vary from 80 micrometres for indoor use to 200 micrometres for external use in order to guarantee the high mechanical characteristics of the duct. The ducts construction starts with the plotting of the single pieces on the panel. The pieces are then cut from the panel (with a 45° cut as explained below), bent if necessary in order to obtain the different fittings, and finally closed through an operation of gluing, pressing and taping. Having obtained the various duct sections, they can easily be installed by using an invisible aluminum flange system.
Fiberglass duct board (preinsulated non metallic ductwork)
Also the fiberglass panels provide built-in thermal insulation and the interior surface absorbs sound, helping to provide quiet operation of the HVAC system. The duct board is formed by sliding a specially-designed knife along the board using a straightedge as a guide; the knife automatically trims out a "valley" with 45° sides; the valley does not quite penetrate the entire depth of the duct board, providing a thin section that acts as a hinge. The duct board can then be folded along the valleys to produce 90° folds, making the rectangular duct shape in the fabricator's desired size. The duct is then closed with staples and special aluminum or similar 'metal-backed' tape. Commonly available duct tape should not be used on air ducts, metal, fiberglass, or otherwise, that are intended for long-term use; the adhesive on so called 'duct tape' dries and releases with time.
Flexible tubing
Flexible ducts, known as flex, have a variety of configurations, but for HVAC applications, they are typically flexible plastic over a metal wire coil to make round, flexible duct. Most often a layer of fiberglass insulation covers the duct, and then a thin plastic layer protects the insulation. Flexible duct is very convenient for attaching supply air outlets to the rigid ductwork. However, the pressure loss through flex is higher than for most other types of ducts. As such, designers and installers attempt to keep their installed lengths (runs) short, e.g., less than 15 feet or so, and to minimize turns. Kinks in flex must be avoided. Flexible duct is normally not used on the negative pressure portions of HVAC duct systems.
Fabric Ducts
Fabric ducts, also known as air socks, duct socks or textile ducts, are designed for even air distribution throughout the entire length. Usually made of special permeable polyester material, fabric ducts act like a conventional system with much more diffusers. Fabric ducts are normally used where even air distribution is essential. Due to the nature or the air distribution, textile ducts are not usually concealed within false ceilings, because of this, they are manufactured in various colours to coordinate with the interior. Fabric ducts fitted above a ceiling will need to be pvc coated so that no maintenance is required to maintain performance. Because air passes through the body of a textile duct, it is impossible for condensation to form on the ducts surface, they can therefore be used where air is to be supplied below the dew point, without needing to be insulated. Also see air sock
Duct system components
Besides the ducts themselves, complete ducting systems contain many other components.
Vibration isolators
A duct system often begins at an air handler. The blowers in the air handlers can create substantial vibration and the large area of the duct system would transmit this noise and vibration to the inhabitants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler. The rubberized canvas-like material of these sections allow the air handler to vibrate without transmitting much vibration to the attached ducts.
Take-offs
Downstream of the air handler, the supply air trunk duct will commonly fork, providing air to many individual air outlets such as diffusers, grilles, and registers. When the system is designed with a main duct branching into many subsidiary branch ducts, fittings called take-offs allow a small portion of the flow in the main duct to be diverted into each branch duct. Take-offs may be fitted into round or rectangular openings cut into the wall of the main duct. The take-off commonly has many small metal tabs that are then bent to retain the take-off on the main duct; round versions are called spin-in fittings. Other take-off designs use a snap-in attachment method, sometimes coupled with an adhesive foam gasket to provide improved sealing. The outlet of the take-off then connects to the rectangular, oval, or round branch duct.
Stacks, boots, and heads
Ducts, especially in homes, must often allow air to travel vertically within relatively thin walls. These vertical ducts are called stacks and are formed with either very wide and relatively thin rectangular sections or oval sections. At the bottom of the stack, a stack boot provides a transition from an ordinary large round or rectangular duct to the thin wall-mounted duct. At the top, a stack head can provide a transition back to ordinary ducting while a register head allows the transition to a wall-mounted air register.
Volume Control Dampers
Ducting systems must often provide a method of adjusting the volume of air flow to various parts of the system. VCD's (Volume Control Dampers - Not To Be confused with Smoke/Fire Dampers) provide this function. Besides the regulation provided at the registers or diffusers that spread air into individual rooms, dampers can be fitted within the ducts themselves. These dampers may be manual or automatic. Zone dampers provide automatic control in simple systems while VAVs allow control in sophisticated systems.
Smoke/Fire Dampers
Smoke and Fire dampers are found in ductwork, where the duct passes through a firewall or firecurtain. Smoke dampers are automated with the use of a mechanical motor often reffered to as an Actuator. A probe connected to the motor is installed in the run of duct, and detects smoke within the duct system which has been extracted from a room, or which is being supplied from the AHU (Air Handling Unit) or elsewhere within the run. Once smoke is detected within the duct, the Actuator triggers the motor release and the smoke damper will automatically close until manually re-opened.
You will also find Fire dampers in the same places as smoke dampers, depending on the application of the area after the firewall. Unlike smoke dampers, they are not mechanical nor are they triggered by any electrical system, which is perfect in the event of an electrical failure where the Smoke dampers would fail to close. A fire damper is held open by a bar crossing the corrigated screen, which will break and allow the damper to close when air in the duct is above a certain temprature. This again will then have to be manually re-opened.
Terminal units
While single-zone constant air volume systems typically don't have them, other types of air distribution systems often have terminal units in the branch ducts. Usually there is one terminal unit per thermal zone. Some types of terminal units are VAV 'boxes' of either single or dual duct, fan-powered mixing boxes of either parallel or series arrangement, and induction terminal units. Terminal units may also include either, or both, a heating or cooling coil.
Air terminals
'Air terminals' are the supply air outlets and 'return' or 'exhaust air inlets'. For supply, diffusers are most common, but grilles, and for very small HVAC systems such as in residences, 'registers' are also used widely. Return or 'exhaust grilles' are used primarily for appearance reasons, but some also incorporate an air filter and are known as 'filter returns'.
Duct cleaning
The position of the U.S. Environmental Protection Agency is that "If no one in your household suffers from allergies or unexplained symptoms or illnesses and if, after a visual inspection of the inside of the ducts, you see no indication that your air ducts are contaminated with large deposits of dust or mold (no musty odor or visible mold growth), having your air ducts cleaned is probably unnecessary." Studies by the EPA and the Canadian Mortgage and Housing Corporation (CMHC) in the 1990s has lead CMHC to conclude that "duct cleaning will not usually change the quality of the air you breathe, nor will it significantly affect airflows or heating costs".
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