Exhaust, Air Velocity near an Exhaust Inlet

Question: What is the velocity of air at a point near an exhaust inlet?


(1) A common misconception is that air can be "directed" into an exhaust inlet. Reality is that atmospheric pressure pushes air from all directions equally toward a location of reduced pressure (the exhaust inlet). Air from the side of the inlet rushes in almost as fast as air right in front of the inlet. Air in front of the inlet does not rush in as fast as you would like, because there is much more air rushing in from the sides, top, bottom, and maybe even behind.

(2) Imagine the shape of all the air that will tend to move toward the exhaust inlet. An inlet in mid-air pulls in a sphere of airflow. An inlet on a surface (such as a ceiling) pulls in a hemisphere of airflow. An inlet on the edge of two surfaces (such as a slot hood at the edge of a table) pulls in a quarter cylinder of airflow near the center of the slot, quarter spheres of airflow near the sides of the slot, and larger parts of spheres of airflow beyond the edges of the table.

(3) Imagine surfaces of equal pressure caused by the exhaust inlet. These are like shells inside shells. There is a shell of lowest pressure right at the inlet, next there is a shell of slightly higher pressure, etc. These shells are also imaginary surfaces of equal air velocity - a surface of highest air velocity right at the inlet, next a surface of slightly slower air velocity, etc. (These surfaces are called isovels.)

(4) For the point in question, define the surface or shell of equal air velocity that it is on. What is the shape of all the points experiencing equal velocity into the exhaust inlet?

(5) What is the calculated surface area of this isovel? A useful formula is the surface area of a full sphere is 4*pi*(r^2).

(6) (Flow, cubic feet per minute, cfm, of the exhaust inlet) divided by (area of the isovel, square feet) equals (velocity at the point in question, feet per minute, fpm).

(7) It will be found that without blocking airflow on sides, top and bottom, velocity into an exhaust inlet is very unimpressive. For example, an exhaust through a wall or ceiling surface, creating hemispheric isovels, pulling 1000 cfm airflow, creates only 10 fpm velocity at a distance 4 feet from the inlet.

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