AIR DIFFUSION IN HVAC SYSTEM BASIC INFORMATION AND TUTORIALS


The airstream leaves a supply outlet as a jet. This air jet creates motion in the surrounding air because of friction, which induces some of the surrounding air to become part of the jet.

As a result of this induction and friction, the jet becomes larger and more diffuse and slows down, until finally it loses all noticeable form and velocity.

In a free jet—one not influenced by cross drafts, adjacent surfaces, or obstructions—this effect is predictable with a fair degree of accuracy and is dependent on the initial velocity, size, and temperature. The throw is the distance that the stream travels before velocity is reduced to some defined speed, usually 50, 75, or 100 ft/min.

The threshold at which people sense air movement is about 50 ft/min. The drop is the distance that the lower edge of a horizontal jet falls below the outlet at the end of its throw. The spread is the divergence of the airstream after it leaves the outlet.

Most HVAC applications are not ideal. The throw, spread, and drop are affected by the outlet design. All grilles and diffusers have a fixed or adjustable pattern which controls the initial direction and spread of the airstream. Ceiling diffusers spread the air out along the ceiling or throw it downward in a tight jet, or do something in between.

Grilles may have two-way spread (one plane only) or four-way spread (both planes). Adjusting for wide spread decreases the throw. The aspect ratio (ratio of length to width) of an outlet also affects the jet properties. The smaller dimension tends to govern the throw.

When the outlet is located in the wall near a floor or ceiling (or in the ceiling or floor near a wall), the jet tends to hug the adjacent surface. Cold air supply streams tend to fall because of higher air density, while hot airstreams are buoyant and tend to rise.

Thus, ideally, warm air should be supplied horizontally from low sidewall outlets, and cold air from high sidewall outlets or ceiling diffusers adjusted to discharge the air horizontally. One of the most satisfactory compromises, where the air supply temperature varies seasonally, is a floor outlet near the outside wall. Interferences, such as beams, columns, and light fixtures, can degrade the air distribution pattern.

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