The rapid flow of air from a hole into an open space produces a relatively high velocity jet.  The flow is extremely turbulent with the air particles swirling about as they travel in the jet stream.

This turbulent jet flow expands at an included angle of about 22º due to its rapid entrainment and mixing action with the surrounding air.  At a distance of only 20 diameters from the hole in the tube, the jet will slow down to a centerline velocity of about 20% of its original outlet velocity and will be 90% mixed and tempered with the surrounding building air as shown in the two diagrams at right.

For example, a jet of air flowing from a  2½” hole in the tube at a velocity of 2000 fpm will slow down to approximately 400 fpm within a distance of only 4 ft.  The air continues to slow down in direct proportion to its distance from the tube and at 8 ft. will have a velocity of 200 fpm.

Effective tempering of cold make-up is quickly achieved.  Air at 0º jetting from the tube into the surrounding building air at 80ºF will warm up to 73ºF within a distance of only 4 ft. from the tubes.  Further tempering continues until the make-up air reaches the temperature of the building space.