Tom Székely, P.E., LEED AP

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September 27, 2005

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Jurassic Park and Hurricane Katrina 

While I was channel surfing a few days ago, I came across that scene in Jurassic Park where the vehicles are leaving the exhibit center for the automated tour, and Jeff Goldblum’s voice is heard to say “God help us, we’re in the hands of engineers.”  We engineers have as much fun with the Dilbert comic strip as attorneys have telling lawyer jokes, but it’s not actors or attorneys who are drying out New Orleans, it’s engineers. 


As a matter of fact, everything which insulates humanity from the brutality of nature, and allows us to be so unconcerned with our day to day survival that things such as philosophy, poetry, and art are possible, only exist because of engineers.


We would do well to keep this fact in mind whenever anyone laments the effect of humanity upon the earth.


The “Eureka” moment


Speaking of nature, prehistory, and engineers, was the first engineer that person who first picked up a rock to crack open a bone for marrow? Or was it the one who saw the razor-sharp edges which resulted when the rock slipped out of his/her hand and struck the boulder below, splitting into two or more parts as it did so?  Actually, It was the latter.  The former was not a person, but was (and is) a Chimp.


Obsessive-Compulsive personality traits aside, lots of what we do used to be mind-numbingly boring repetitive calculations.  Now it’s mind-numbingly boring writing (as in specifications) and presenting in graphic form what we have pre-envisioned as necessary to meet the needs of a given project.


Every now and then, however, comes a “Eureka” moment, which reminds engineers why they got into the field to begin with. One of mine was the realization that linear diffuser plenums were a waste of sheetmetal and space.


And with that, enough of the philosophy; I closed the last issue warning you to wear sunglasses for this issue, so as to not be blinded by its technical brilliance.   Put ‘em on.


Of Diffusers and Airflow, or, What’s With All These Dampers and Access Doors?


Ducted Air heating and cooling systems have been around since at least the days of Ancient Rome, and maybe even longer than that.  Things changed substantially, however, with the advent of electric motors and fans.


Before such, ducts had to be enormous to move enough air to transfer the required amount of heat or “coolth” because the airflow velocities (and therefore, the amount of air delivered per unit of time) were limited by the forces generated by natural convection.


Even with fans, there are limits as to how much air can be pushed through a duct, relating to noise, power requirements, and temperature.


Taking them in reverse order, if you move air fast enough, intermolecular and duct friction will cause its temperature to increase, with the power required to move the air varying directly with the velocity, and with the cube of the amount of air moved in CFM (Cubic Feet per Minute).  If you move air slowly enough for its velocity not to cause noticeable temperature increase, it still can make a substantial amount of noise, both within the duct and at any outlets (diffusers).


Diffuser manufacturers list, in engineering catalog data sheets, along with how far into a space the air will be thrown, the noise generated at any outlets, with both dependent upon how much air is being forced through the diffuser, and at what velocity. By the way, contrary to popular belief in the construction and contracting industry, one should not design to throw air completely across a room.    A little over halfway is adequate for proper mixing, and actually more conducive to comfort vis-ŕ-vis drafts.


Anyway, since ductwork is custom fabricated stuff, there are no “catalog listings” for airflow noise generated within ductwork.  There are, rather, recommended velocity limits set via experience, listed in various engineering handbooks, with the noise at outlets usually characterized (to use the highly technical terms for which engineers are famous) as “whistling,” while that within ductwork is usually characterized as “rumble.”


Diffusers come in many categories among which are ceiling diffusers, sidewall grilles, and linear diffusers.


Ceiling diffusers usually are constructed with an integrated “pancake” plenum to allow a duct substantially smaller than the diffuser face to serve it, while “squeezing” the air so as to exit across the entire face of the diffuser.


Sidewall grilles are served by a duct or collar of the same size as the catalog dimensions of the grille.


Linear diffusers, on the other hand, are long narrow affairs which are suitable for use in ceilings or in walls, obtaining such dimensions (feet or yards in length rather than inches) as to conventionally require many “pancake” plenums with a duct connection and damper to each, with a step reduction in supply duct size after each plenum duct connection, to deliver air evenly across the entire length of a diffuser.


The dampers are necessary to throttle airflow at the duct connection nearest the source, gradually reducing such at each downstream duct connection so as to equalize the airflow across the face of the diffuser.  Each damper, however, requires an access door in the wall or ceiling to allow for the necessary adjustments, while each plenum duct connection effects a 90° direction change to have the air come out of the face of the diffuser.  While linear diffusers can be had with dampers just behind, or built into, the face of the diffuser, these are (as they are for all other kinds of diffusers) for fine rather than gross adjustments of airflow, and exacerbate outlet noise when used for gross adjustment, in place a volume damper in the duct branch just upstream of an outlet.


 A lot of money, a lot of complexity, and a lot of work.  Why not continuously taper the supply duct instead of step-reducing it, and subdivide it, with the cross-sectional area of each subdivision inversely proportional to how far down the length of the diffuser it goes, and put all the volume dampers up at the supply end of the trunk subdivisions?


I’ll show you a picture in the next issue.


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