Wednesday, December 19, 2012

The Rainforest

Recently, I was asked to assist in the design for an enclosed rainforest exhibit. The project has a lot of glass and is in a very southern climate zone (near the Gulf of Mexico). I have looked at a couple of other similar projects in the past and, of course, volunteered to assist. As a preliminary, we managed to get a tour of the Dallas Aquarium, which includes a large enclosed rainforest. 

We were met by the head of maintenance, who seemed to have worked there for 20 years and knew all the history of the HVAC system. Much of the air distribution system was custom built by the facilities folks themselves, in an effort to make the air outlets nearly invisible. I made several general observations concerning projects of this size.

Vents at the top of the structure are essential. At the Dallas facility, we were told that while the vents at the top of the tall glass wall were able to be opened and closed, they just leave them open year round. This seems to be a pretty general recommendation, as all tall structures exhibit stack effect, and releasing the hot air always seems to assist in controlling the space temperatures.

Clear glass seems unwise, especially on the roof. Having the east wall transparent seems to provide sufficient UV for the plants to thrive. It is unclear how much UV passes through the translucent panels used throughout most structures of this type. In general, it seems that displacement ventilation is a natural candidate for these types of spaces, especially as the use of relatively warm water (62°F) to the fan coils results in 65-68°F air at the outlets.

It was very interesting to see how the facilities folks managed the systems and how they often created their own air distribution devices.

Authored by: Dan Int-Hout, Chief Engineer Krueger

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

Thursday, December 6, 2012

ASHRAE Journal Article

In case you missed it, I have part one of a two-part article published in the November ASHRAE Journal. Part II will be in the December issue. I have been giving a talk, from which the articles are based, at ASHRAE chapters and engineer’s offices around the country for the past couple of years. The article’s premise, titled “Methods of Effective Room Air Distribution”, is that folks are responding to the request for more energy efficient systems by designing air distribution systems around alternates to overhead air delivery, which has been the design of choice for the past 30 years. Unlike overhead systems which have many years of data validation, the energy calculations for these newer systems have not been very well vetted. The result is that engineers must modify the inputs or calculations to come up with energy use values. The output, of course, is merely a guess. Hopefully it is an educated guess. This, however, is sent to the USGBC for a LEED rating.

Meanwhile, BOMA continues to report that occupants are often dissatisfied with the thermal environment we provide, even with tried and true overhead delivery systems. Now we are moving to underfloor and displacement systems. It remains to be seen if they are any better at satisfying occupants. The first part of the articles describes the “rules” for occupant comfort, including ventilation and acoustics. The second part outlines issues to be considered when designing overhead (including Chilled Beams), underfloor and displacement ventilation systems. 

Failure to comply with the “rules” (some being included in buildings and others just good practice) is likely to continue to result in unhappy occupants. And, of course, we will eventually find out if any of these designs are really more efficient than others.

Authored by: Dan Int-Hout, Chief Engineer Krueger