The air circulation through ventilation ducts
The air circulation inside a ventilation duct is affected by several factors: the pressure drop, the speed, and the obstacles it encounters, mainly bends and changes in section.
Elements that obstruct air circulation. Speed
These types of obstacles cause an increase in the pressure drop that occurs within the duct caused by friction with the walls. The circuit that air usually follows inside a duct typically starts from a grille from which it expands, ventilates a battery with a heat exchanger, passes through the fan's damper, and begins to circulate through the ventilation system. From that moment on, the air acquires a speed that can be of three types:- Capture speed (Va), which is the speed necessary to ventilate an area located at a certain distance
- Entry speed at the mouth through which the air is drawn (Ve)
- Speed in the plenum (Vp). The plenum is a box, cabin, or section of the duct of large section where the speed decreases considerably. This element is used to make the airflow more uniform.
- Speed in the duct (Vc), also known as pneumatic transport speed.
Once the air is in motion within a ventilation duct, it must overcome the obstacles it encounters, mainly the bends and branches. These can be resolved using "Y" or "T" shaped joints, both in circular ducts and rectangular ducts. To mitigate the loss that occurs in the bends, guide vanes can be used, evenly distributed and arranged in such a way that they can cover the entire curvature of the bend. These guide vanes can be of uniform thickness, made of sheet metal, or adopt aerodynamic profiles. The vertical outlet of a ventilation duct is also a singular point that must be properly resolved. They should be protected with a small finishing piece. At the end of the duct, a tubular covering is placed that will act as a vertical diffusion nozzle that drains any possible rainwater that may enter.
Sometimes, due to construction constraints, it is necessary to change the section of the ducts either by increasing or decreasing it. In both cases, it is necessary to avoid abrupt changes, instead seeking an installation that allows for a progressive change in section. This way excessively high pressure losses will be avoided.
Siber Ventilation
Related posts
How does air flow through ventilation ducts?
C.T.E. DB-HS 3. The air quality. Thermodynamics and aerothermal energy
