Duct (HVAC)
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Image:Central ventilation tube2.jpg
Ducts are used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. These needed airflows include, for example, supply air, return air, and exhaust air.<ref>The Fundamentals volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, USA, 2005</ref> Ducts also deliver, most commonly as part of the supply air, ventilation air. As such, air ducts are vital parts of ensuring acceptable indoor air quality as well as thermal comfort.
A duct system is often called ductwork. Planning ('laying out'), sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct design.<ref>HVAC Systems -- Duct Design, 3rd Ed., SMACNA, 1990</ref>
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[edit] Duct materials
Ducts are most often made of galvanized steel in rectangular, round, and oval cross-sectional shapes. 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 noise reduce 'breakout' noise through the ducts' sidewalls.
While galvanized steel is still very common, many rectangular and even round ducts are manufactured from duct board, a rigid form of fiberglass that can easily be shop- or field-fabricated to make custom shapes and sizes. The fiberglass provides 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.
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's use in ducts as well as food cans. However, the term tin-knocker is still used in practice to describe sheet metal duct installers.
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.
[edit] Duct system components
Besides the ducts themselves, complete ducting systems contain many other components.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] Dampers
Ducting systems must often provide a method of adjusting the volume of air flow to various parts of the system. Balancing, flow-control, and fire/smoke 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.
[edit] 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.
[edit] 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.<ref>Designer's Guide to Ceiling-Based Room Air Diffusion, Rock and Zhu, ASHRAE, Inc., Atlanta, GA, USA, 2002</ref>
[edit] See also
[edit] References
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