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Duct fitting

Duct fitting


Duct fitting

As has been seen, in a distribution of ducts you can find some accessories that are part of it and condition its behavior. There are both for circular, oval or rectangular ducts.


The elbows change the direction of the current by a certain angle. The elbow can be; straight if the two directions intersect in a straight line or curved if they intersect through a continuous curve. The curve can also be discontinuous and be made up of three or more segments. Obviously, the smoother the change of direction, the less pressure drop and the less noise.


They are used to divide or branch the air stream. The branch current exits laterally to the main conduit and can do so; forming a right angle, which is not highly recommended, or forming an angle between 15 and 45º with the main current and in the same direction, although later it is necessary to change its direction with an elbow. The derived flow may or may not imply a reduction in section at the outlet of the main duct.

When the main current branches off into two symmetrical equals, forming a Y, the branch is known as a pant.

Gates and flow regulators

They are accessories that are used to totally or partially close the passage through the duct. The penstocks are usually made up of an aluminum frame, the interior of which is closed by flat or aerodynamic fins, each of which pivots jointly with a longitudinal axis that passes through the center of the fin and rests on both sides of the frame. The axes are unified outside the frame with a single command to move them manually or by coupling a servomotor.

Flow regulators or registers are small gates that are attached to the outlets to control their flow. They may be; shutter as in the gates, butterfly, when a single flap covers the entire passage section, or sliding, when a sliding screen is the one that closes the passage section.

duct fitting


In a duct layout, the section is normally reduced after each outlet, except if the flow rate decrease is very small. The piece that joins two sections of duct with different sections is called a reduction and must be executed in such a way that the transition from one section to the other is as smooth as possible. Slopes between 15% and 20% are recommended.

When the duct changes shape, for example from round to rectangular, the fitting is known as a transformation.

When several small ones have to be derived from a large duct, it can be done through a large duct or box to which both the large inlet duct and the small outlet ducts are attached. This box or conduit is known as a plenum and guarantees pressure stabilization and a homogeneous distribution of flows.


They are not accessories of the ducts, although it is very common to find them in the installations. These are elements of the building or other installations, such as pipes, cables, etc. whose passage through the conduit is not recommended, but on many occasions it is irremediable. When this happens, the obstacles must be lined with an aerodynamically shaped cover that minimizes load losses and prevents noise.


They are elements that are not currently used much, however they are very useful in balancing a duct system. They consist of sheet metal or even fiberglass plates, with the same shape as the section of the section in which they are going to be inserted. Holes are drilled into the plate. The number of perforations and their diameter, as well as the thickness of the plate, determine the pressure drop that will occur in the air flow.

Bottom view of galvanized sheet metal duct connected to a diffuser.


The inlets are the outlet and inlet terminals of a duct distribution. There are no more inlets than those for the return to the generator, or the outside air intakes, both generally made up of grilles.

The outlets can be basically; ceiling and wall. Both have many variants. Their selection is critical, since they constitute the fundamental element to guarantee a homogeneous distribution of the treated air in the premises to be heated and temperature and speed conditions that ensure thermal well-being13 in occupied areas. Its calculation and selection belong to an independent technique, which is known as air diffusion.

rectangular duct reducer

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