Under Floor Air Distribution

[chims]

Has anyone found a software program (or other method) that will calculate cooling loads in the occupied zone using an underfloor air distribution system?

 

[tombmech]

This is genuinely a sincere answer: an underfloor distribution system does not change the cooling load calculation. There are peculiarities with the design, but it doesn't affect load.

Many are having good luck with the round, high-velocity swirling nozzles. They seem better suited to underfloor distribution in high-occupancy environment. (As opposed to the classic perforated floor tiles in a computer room.)

Here's Titus's version:

http://www.titus-hvac.com/ecatalog/framecontent.aspx?CatID=23#

This is also a great general article about the subject:

http://www.cbe.berkeley.edu/underfloorair/Default.htm

 

[chims]

I've heard this response before. I'm wondering if the air flows calculated for S/A at 55F will be the same for 65F for underfloor. This may be so, as cooling loads in the occupied zone will be less, i.e. some of the lighting and wall loads don't reach the occupied zone and will only serve to increase the return air temperature and affect the cooling coil load.

This seems to be the consensus on designing UFAD systems and I will proceed until a software program is developed to accurately take into account the temperature stratification of the UFAD.

 

[GMcD]

While there is currently no specific software to calculate room air quantities required for UFAD systems, there are some good calculation methods at the Lawrence Berkeley Labs website that Fred Bauman and his staff run at the CBE. If you haven't gone through it, here is the link:

http://www.cbe.berkeley.edu/underfloorair/

The basic logic is that the air supply to the space is calculated using an "adjusted" amount of people and equipment loads (and other cooling loads) to allow for part of the room heat gains to be stratified and drawn off into the return air side. The heat from lights is not a "space load" since it is removed by the return air path. The actual AHU cooling coil load, however, has to be enough for ALL of the total heat gains being extracted form the room. The actual supply air to the room is usually around 65% to 85% of what you'd normally use for a conventional overhead system. CFD modeling will provide a better analysis of the room loads and air temperature stratification.

 

[chims]

Thanks. I've been to the website and it has a lot of good information. I'm also going to attend Fred Bauman's daylong course at the ASHRAE show in January and hopefully learn a bit more.

I'm a little surprised that the air flow is that much less than a conventional system. I realize that the loads are less due to stratification, but I thought the higher supply temperature balanced that out and made the airflows close to the same.

 

[HVAC68]

Dehumidified air quantity = Room Sensible Heat
________________________
1.08 x (Room Temp - ADP)

As the room sensible heat reduces, the air quantity also will obviously reduce. The room sensible heat would reduce, eventhough the total heat is not reduced, if you are able to get the heat to be dissipated through the return air - In other words, it becomes part of the coil load, but is NOT part of the room sensible load.

Example - Heat dissipated by lights above a false ceiling which is used as a return air plenum.

I have also not come across any special software for underfloor cooling load calculations.



HVAC68

 

[tombmech]

"I've heard this response before. I'm wondering if the air flows calculated for S/A at 55F will be the same for 65F for underfloor."

With respect, you asked for "calculating cooling loads", not airflows. Neither did you ask to do a coil selection, determine ventilation rates, or calculate the sensible heat ratio. Space/zone loads differ from total load, as GMcD points out, but you did not ask that question, either.

Please note that my reply was also the first to include Mr. Bauman's link, as well. There's a lot of selective reading vs. selective writing going on, apparently.

 

[chims]

Apparently there are some very sensitive readers on this site. With respect, tombmech, I did acknowledge your website suggestion when I was writing my original response, but deleted it in my rewrite and neglected to put it back in. I had in fact already visited this website before your suggestion.

I thought the intent of this website was to generate a dialogue between professionals and share ideas, not to see who is the smartest. Unfortunately dialogue through the written word doesn't carry the same personal depth that the spoken does. I aplogize for inadvertantly hurting your feelings.

 

[tombmech]

chims:
my mistake entirely ... please forgive my obtuse sensitivity