Friday, October 21, 2011

ASHRAE 113 & Standard 55

It has come to my attention that a number of engineers are requiring the use of ASHRAE Standard 113 to verify compliance to ASHRAE Standard 55. That is not likely to be a successful venture, as the 113 Standard is really intended as a laboratory method of test.

I was the original author of Standard 113, back in 1979. It was originally an appendix to an ASHRAE Technical paper on how to get repeatable measurements of room air motion in an office space mock up, written to prove compliance, or in this case, non-compliance, to a GSA specification. The tests conducted under the protocol which became Standard 113 proved that in fact it was not possible to meet the GSA specification with any air distribution system. The GSA subsequently modified the specification, and shortly after dropped the requirement entirely. It also proved that getting repeatable results required very expensive instrumentation, modifications to the controls of the space being measured, and the addition of simulated loads to maintain the steady-state conditions required for the measurements, which often took an hour or more. For a number of reasons, it is unlikely that one can conduct a true ASHRAE 113 test in an office space.

I was Chair of ASHRAE 55 Standards committee when the 2004 standard was released. Compliance paths have always been a goal, but have never been fully or properly defined, due to the inherent difficulties in measuring the highly variable and often non-steady state conditions that exist outside of laboratories. We are still working towards that end and are developing compliance paths for different types of air delivery systems. That is still a work in progress. They will not, however, include a requirement to conduct air distribution tests per ASHRAE 113.

The relationship between measures of room air motion and Standard 55 are not clearly spelled out in the literature. ADPI (Air Diffusion Performance Index) can be predicted from manufacturer’s throw data and an analysis of diffuser spacing from tables in Chapter 20 of the ASHRAE Fundamental’s Handbook. ASHRAE 113 data can, and has been used to calculate an ADPI under steady-state conditions. An ADPI of 80% or greater will ensure that there is less than the ASHRAE 55 vertical temperature stratification limit of 5.4 degrees F in the lower 6 feet of a space. That is only a small part of the Standard 55 requirements, however.

The likely steps to prove compliance to Standard 55 will involve temperature difference measurements only, as these are much more stable than velocity measurements, and can easily be taken in occupied spaces. Air speed measurements are so highly affected by local loads that any measures are unlikely to be repeatable or meaningful, in practice. There may be some meaningful data obtained at the midpoint between diffusers to look for jet collisions at high air flow rates, or under diffusers at low air flows looking for excessive drop (dumping). These would involve some visualization and single point measurements, a type of measurement not included in ASHRAE 113.

In short, the only practical way to place ASHRAE 113 in a specification is to require its use in a full scale mockup to verify the predicted ADPI from the ASHRAE handbook table calculations. This will likely be a very expensive test and should not be employed except for very large projects where the cost might be justified.

Authored by: Dan Int-Hout, Chief Engineer Krueger