Monday, December 10, 2012

Free Jets

I’ve had a couple of inquiries about the throw of air jets, where engineers have asked why some manufacturer’s data is different than others. In truth, a jet of air is pretty much independent of the shape of the hole from which it is discharged. Long narrow jets behave differently than round ones, of course, but there is little actual difference between round jets from different manufacturer’s at a given flow rate. The old Air Diffusion Council (ADC) published a “typical” grille throw chart; most manufacturers use this chart, as it has proven to be close to their measured data. However, there is a huge difference with the performance of the jet with regard to “free” or “entrained” jet performance.

A basic fact of air jet dynamics is that a jet of air has negative static pressure. This results from the laws of conservation of energy and the translation of air from being restrained in a duct and entering a non-constrained space. As the sum of velocity and static pressure must be essentially constant, the total pressure of air in a duct, when discharged into an unrestrained space, results in most of the potential fan energy being translated into velocity pressure. The only way for the sum be constant is for the static pressure to be negative. This is why a jet of air induces air from the surrounding environment. The result is that the mass of moving air increases while the velocity decreases to maintain a conservation of energy. 

A jet that is discharged parallel to a surface experiences two phenomenon. The higher pressure of the air opposite the surface “pushes” the jet towards the surface. The rate of induction is reduced by the reduction in the exposed jet surface, where it is not touching the surface. In practice, an “entrained” jet has about 30% less exposed surface than a “free” jet. If one looks closely at the math, the ratio of most free jets to entrained jets is about the square root of 2, or 0.707.

Krueger, along with most other manufacturers, displays throw for most products as “entrained” jet throws. The ADC “standard” grille throw graph was, in fact, determined along the floor of a large warehouse many years ago. There are a few exceptions however. Drum louvers are seldom installed in a manner where the throw would be along a surface, so we present “free” jet throw data. The same is true of our round jet outlets and some vertical throw data for adjustable outlets. For duct mounted grilles, we provide both free and entrained jet throw data. Be sure to look at the data notes at the bottom of any manufacturer’s throw data to be sure what is being reported.

Authored by: Dan Int-Hout, Chief Engineer Krueger