We are working on a Project with a central 100%OA AHU. We calculated the pressure drop adjustment for the baseline Systems 7 following ASHRAE 90.1-2007 table G3.1.2.9. Now we received the following review comment:
"The Baseline fan power calculations indicate that a pressure drop adjustment factor has been applied to the total supply air volume of each system in the Baseline model to reflect the high efficiency filters included in the actual design; however, the high efficiency filters only filter the outdoor air volume provided by the dedicated outdoor air systems in the actual design. Revise the Baseline model, as needed, so the pressure drop adjustment factor associated with the high efficiency filters is applied to the total outdoor air volume only, not the total supply air volume, in the Baseline model. Further, update Table 1.4.2 and provide revised fan power calculations reflecting the changes."
In our understanding, the review team wants a differentiation between OA and Return Air of System 7 for the pressure drop adjustment calculation.
ASHRAE 90.1-2007 G3.1.2.9 definition for the calculation is "the baseline system maximum design supply fan airflow rate".
Now it is unclear if the review team is wrong with their comment or if we have to make a differentiation of OA and return air. Any ideas?
Thank you in advance!
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
December 6, 2013 - 12:04 pm
The pressure drop adjustments only apply to the applicable air flow in the system. For example, if you have ducted returns the adjustment only applies to the the return air flow not the supply airflow. In your case if the baseline unit is 100% OA then the supply air and OA flows are the same and the PDA would be applied to the supply airflow. However, just because your proposed system is 100% OA does not necessarily make your baseline system 100% OA. The baseline supply air is auto-calculated by the software and the OA is identical to the proposed OA. So if the baseline supply air is greater than the proposed OA the baseline unit is not 100% OA. In this scenario if the filters are on the OA then only the OA flow would be used to apply the PDA. There is a spreadsheet which we created under the Documentation Toolkit above which is set up so that you can apply a portion of the air flow to an individual PDA.