Tuesday, July 12, 2016

ADPI and Standard 55

I have been campaigning for 40 years to get a correlation between ASHRAE Standard 55 and the concept of ADPI. I chaired Standard 70, (Method of Test, Air Diffusers), Standard 113 (Method of Test, Room Air Distribution, including ADPI) and Standard 55 itself. I also chaired TC 2.1, Thermal Comfort and TC 5.3, Space Air Distribution. I Chaired TC 4.10, Calculation of room air motion (CFD), when it moved from a TG to a TC.

In prior blog posts, I have described the issues with the 40 fpm limit on using the graphical method of Standard 55 to show compliance and wrote an ASHRAE Journal article on how to manage the process. After several false starts, ASHRAE finally approved the creation of a user’s manual for Standard 55. While I was listed as a reviewer during its creation, I was either on the Technical Activities Committee or Standards Activities Committee and felt some conflict of either jurisdiction or time. I have been on ASHRAE’s board for the past two years and have attempted to remain neutral. The process of creating a user’s manual was not without conflicts of opinions.

The document, nonetheless, has been approved for publication. I couldn’t be more pleased with the result. I have attached a tiny piece of the user's manual:

Example 1: Using ADPI for overhead air distribution systems, the diffuser manufacturers often rate their products based on the Air Diffuser Performance Index (ADPI). ADPI is defined by ASHRAE Standard 113 as the percentage of points measured in a room that are within both the ASHRAE temperature and velocity ranges for comfort. ADPI is based on the acceptance and recognition that it is not possible to achieve a comfort level of 100 percent, but 80 percent acceptance is achievable and measurable. Using the example space identified in Figure 3-B, in cooling mode the designer needs to provide 79.6 W/m2 (25.2 Btu/h-sf) of sensible cooling.

As seen in Figure 3-E, the diffuser type is ceiling-mounted with 4-way throw and the return is at the ceiling. Characteristic length of the diffuser is 1.07m (3’-6”) to the wall, 2.23m (7’-4”) to midpoint between diffusers. From Table 4 Chapter 57, ASHRAE Handbook – HVAC Applications, 2011, the closest room load is 63 W/ m2 (20 Btu/h-ft22), with a maximum achievable ADPI of 93, a T50/L of 0.8 for maximum ADPI, and a range of T50/L of 0.7 to 1.3 to achieve an ADPI greater than 90. The designer has selected a diffuser with an isothermal throw of 1.2m (4-ft) to 0.25 m/s (50 fpm). X50/L for this selection is 1.12, which is within the range needed to have ADPI > 80, therefore ADPI requirements are met. Based on ADPI > 80, the designer can conclude that the average airspeed is less than 0.35 m/s (70 fpm) for greater than 80% of the space. The designer can also conclude from the ADPI calculations that the temperature stratification is less than 2.8 °C (5 °F) for locations between ankle and head level (1.8m (6-ft) above floor level) and at least 0.6m (2-ft) away from the exterior wall. In many cases, achieving ADPI of this value will also meet the requirement to not exceed 0.2 m/s (40 fpm) and even 0.15 m/s (30 fpm) if the operative temperature (to) is below 22.5°C (72.5°F). ADPI does not predict air speeds in heating mode, but if the overhead air system is the only potential source of elevated air speed (window drafts have been accounted for with perimeter baseboard heating systems), then it is also likely that the air speed criteria will not be exceeded in heating as the terminal box has been tuned to minimum vent rate in heating mode. See Section 5.5 for more discussion of this point.

The above is what I have been campaigning for as long as I have been in ASHRAE (since 1974). I was asked if referencing the user’s manual was the same as referencing the Standard itself. It would appear that it could. So at last, we have a path to use performance data in our catalog and design documents to be able to claim compliance to major portions of Standard 55.

Authored by: Dan Int-Hout, Chief Engineer Krueger