Monday, November 19, 2012

Ignorance is Bliss

I spent three days in upstate New York last week. I was presenting “The Basics of Air Distribution” to a Young Engineers at ASHRAE (YEA) in Rochester. I also made a few calls on design engineers. As I have said before, it is a bit surprising how few practicing engineers have a full understanding of the requirements of ASHRAE Standard 62.1 (2010) Ventilation Rate Procedure and recommendations regarding the ASHRAE Comfort Standard (55-2010). Standard 62.1 is referenced in the 2009 International Building Code, which has been adopted fully or in part by most local codes. While I expected some of the young engineers to be ignorant of the requirements, it is always a bit of a surprise when no one in the room knows the overhead heating rule: Maximum delta-t (difference between room and discharge), when heating from the ceiling is 15°F. Exceeding this has two consequences:

1.) 25% more outside air must be supplied to that zone (per 62.1 VRP). I would assume that when one is in heating mode, the outside air is cold.

2.) One can no longer claim compliance to ASHRAE 55-2012, Thermal Comfort, as there will certainly be more than 5.4F thermal stratification in the occupied zone. While Standard 55 is not a part of the Mechanical code like 62.1, it is often referenced. When it is more fully restated in code language (in progress) I expect it will become code in many locations.

Some engineering firms, to their credit, have internal standards limiting discharge temperatures. The engineers I speak to in these firms are glad to finally know why this seemingly arbitrary rule has been put in place.

Surprisingly, I still get push-back from engineers who claim they have been exceeding that for years and no one has complained. This is notwithstanding the fact that the Building Operators and Managers Assn. survey had shown dissatisfaction with the thermal environment continues to be the #1 reason for not renewing the lease in a high rise office.

I guess what they say is true: Ignorance is bliss.

Authored by: Dan Int-Hout, Chief Engineer Krueger

Tuesday, November 6, 2012

Room Design

The K-Select program for grilles and diffusers contains a feature called “Room Design”. This feature was designed 20 years ago to help optimize the selection of overhead diffusers in large spaces, to get the best arrangement of diffusers in the space. In practice, it a great tool for large open plan spaces, for which it was designed. It does not, however, work very well in small spaces with just a few diffusers, or along a perimeter. The problem lies in applications where the space being supplied by the diffuser results in asymmetrical throws. The “characteristic room length” is defined as half the distance to the adjacent diffuser or the distance to the wall. Unfortunately, there just isn’t any way to use the Room Design feature if the distance to the wall is not the same as half the distance to the adjacent outlet. In most cases, to get the 150fpm throw to comply with ASHRAE Standard 62.1 requirement that it make it half way down the window, the diffuser must be much closer to the window than half the space to the next diffuser.

Small rooms (such as a classroom) also have the same problem with a wall about 30 feet from the window. One diffuser has to be close to the window, but will likely overthrow the one towards the opposite wall. Overthrow results in drafts at the midpoint between diffusers where the jets collide, and primary air then enters the occupied zone. (This is a bad thing). To meet the stringent sound limitations in classrooms, four diffusers are probably required, but the throws will often collide.

One proven classroom solution is to use a 3-way diffuser near the windows (with the non-open portion facing away from the window) and a 4-way behind it. The throw from the rear 4-way diffuser will simply combine with the 3-way, towards the window. The 3-way, and 4-way diffusers should be spaced so that they don’t collide in the parallel-to-the- window direction. Remember, it is usually no issue if one overthrows a wall, as the comfort zone starts a couple feet from any wall. The result will be that the diffusers won’t be located evenly, but likely somewhat more towards the side walls of the room.

The K-Select “room design” program is an interesting tool, and works well selecting diffusers for large spaces with no walls. In smaller spaces, ensure that the sum of half the diffuser spacing plus the ceiling height less 6ft is always less than the 50fpm throw values at maximum airflow.

Authored by: Dan Int-Hout, Chief Engineer Krueger

Thursday, November 1, 2012

Acoustical Corrections

Once again, I have been asked to explain how to correct sound data for size, in this case, the length of a linear slot diffuser. The basic equation is that sound output is proportional to 10*Log (A), where A is the area of the sound generating source. In practice, this means that for doubling the area, and at the same time doubling the flow rate, the measured (and reported) sound level would be expected to increase by 10* Log(2*A), which is 3 dB. That is the rule; add 3dB for doubling the area of a sound generating source.

This comes to play when looking at the sound traveling down a duct. If the duct is split and feeds two separate spaces, any sound in the duct would be expected to be 3dB lower at it enters the space. If, of course, the two ducts exit into the same space, the sounds would recombine, thus adding the 3dB back. This effect is the only sound parameter that is independent of frequency.

For continuous slot diffusers, we provide a correction table for length, for lengths up to 10 feet. Someone wanted to know the correction for longer lengths. Applying the above formula to the base data, which is based on a 4 foot length, going to 8 feet would increase the sound by 3dB. It would go up by another 3dB at 16 feet (all assuming the flow rate per foot is kept constant). The problem is that by the time we get to 16 feet, the observer is so far away from the added length that one can no longer hear the sound being generated there. So we stop the published correction at 10 feet.

The posted sound data for continuous slots assumes that the supply plenum isn’t adding any sound. In practice, however, it is likely that there is some noise added by the supply plenum. As a rule, the larger the plenum, the less noise it will create. A “step sided” plenum (wider than the opening at the top of the diffuser) will generate less noise than one that is only as wide as the opening. A taller plenum is quieter than a shorter one, and allows for a round, rather than an oval, inlet. An oval inlet has less area than a round one with the same perimeter. As a “rule of thumb”, add 1dB for every 100 fpm velocity above 800 inlet velocity into the plenum.

Finally, insulating the inside of a plenum decreases the interior volume, raising velocities in the plenum, and negating the sound absorption of the insulation. This is one reason that plenums are insulated with thin insulation, because increasing the thickness of the insulation will likely result in more sound generation. In practice, spaces with a return air plenum are seldom faced with condensation on the plenum, as the space (and therefore plenum) dew point is almost always higher than the supply air temperature in the plenum. If the space has ducted returns, I recommend field installed external insulation on all exposed surfaces, especially in humid climates.

Authored by: Dan Int-Hout, Chief Engineer Krueger