Thursday, January 26, 2012

40 FPM, Round 2

As I mentioned in the previous blog, there is an issue with a limit of 40 fpm in the occupied zone, a limit which is likely to always be exceeded somewhere. I proposed the following to the TC 5.3 (Space Air Distribution) handbook committee last week at ASHRAE. It was determined that it belongs in the Applications Handbook, which is a couple years away before needing revision.

We presented it to the Standard 55 committee, which determined it to be inappropriate to include in the Standard, but would be useful in the upcoming Design Guide (as yet unfunded). So, I’m posting it here for comments. I have sent it to the LEED IE TAC for inclusion in the LEED 2012 reference guide, now being put together. (Send comments to dint-hout@Krueger-hvac.com.)

Proposed prescriptive compliance path for overhead, well-mixed air distribution, at the design stage.

6.2.1 At a minimum, for compliance to ASHRAE Standard 55 requirements in Section 6.2, when using an overhead (well-mixed) air distribution system in an office environment, the following shall be considered a set of minimum design considerations:

6.2.1.1 An interior space with multiple diffusers shall be selected so that in cooling mode, the calculated Air Diffusion Performance Index, based on table 6, ASHRAE Fundamentals, Chapter 20, is at least 80% at all expected loads and flow rates. Achieving this requirement allows the designer to assume average room air speeds are less than 40 fpm, the room environment is thermally uniform, and the graphical or PMV methods of determining room setpoints may be allowed. It also assures compliance to the maximum room air stratification limit of 5.4F in the occupied zone.

6.2.1.2 In addition to the requirements of 6.2.1.1, an interior space in heating mode shall be supplied by a room to discharge temperature differential of no greater than 15F. This assures compliance to the maximum room air stratification limit of 5.4F in the occupied zone.

6.2.1.3 A perimeter space in heating mode shall be supplied with diffusers that result in the 150FPM throw making it to within 4.5ft of the floor along the perimeter wall, and having a room to discharge temperature differential of no greater than 15F. This assures compliance to the maximum room air stratification limit of 5.4F in the occupied zone.

6.2.1.4 A perimeter space in cooling mode with both perimeter and interior diffusers, such as an open office space, shall have a calculated ADPI for the collision zone between perimeter and interior diffusers of at least 80%, assuring no objectional drafts will be experienced by the occupants.

6.2.1.5 All diffusers shall be located so as to avoid obstructions that would cause the discharge jet from the diffuser being directed into the occupied space. This assures that no excessive air speeds will be encountered by the occupants, which would void the use of the PMV or Graphical methods to determine a proper room setpoint.

Authored by: Dan Int-Hout, Chief Engineer Krueger