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Fan Power Inputs in OpenStudio

asked 2014-10-28 13:56:50 -0600

JO gravatar image

updated 2014-10-28 20:39:23 -0600

Hey all. Is there a way to input fan BHP, kW, or kW/CFM in OpenStudio/energyplus? When trying to update a Packaged Rooftop VAV supply fan through OpenStudio the only fields i'm seeing are Fan Efficiency, Pressure Rise, and Motor Efficiency.

With Appendix G requiring us to calculate the total fan system kW it seems strange that OS/E+ wouldn't have an input to accept this value. Am I missing something?

Thanks, JO

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answered 2014-10-28 15:05:23 -0600

updated 2014-10-28 18:16:33 -0600

No, EnergyPlus doesn't use these as inputs, instead it calculates W and W/cfm from the fan efficiency, design air flow rate, and pressure rise (see Fan section of eplustbl CSV or HTM file for fan power outputs). Because of this it requires some reverse engineering to predict what the fan power output will be from EnergyPlus.

For an Appendix G Baseline system I use USGBC's "Revised Section 1 4 Tables" XLS along with some fundamental power equations to solve for the required pressure rise to exactly match the allowed fan power. For an Appendix G Proposed system I use the data from the HVAC designer's schedule to calculate the fan power and then solve for the pressure rise. If fan BHP is given you can calculate the fan efficiency and if it's not I use the default from OpenStudio to calculate the pressure rise. This method for the proposed fan power will not predict the fan power exactly like the baseline method will, but it's close.

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answered 2014-10-28 15:02:02 -0600

updated 2014-10-28 15:07:27 -0600

IDL-Boise has created this spreadsheet tool that will calculate fan power given a few input parameters. It will aid you in determining Appendix G requirements and E+ inputs.

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Looks like a great resource, thanks @Lincoln.

MatthewSteen gravatar image MatthewSteen  ( 2014-10-28 15:18:52 -0600 )edit

Thanks for sharing this tool. Looks like just what I need. JO

JO gravatar image JO  ( 2014-10-28 15:20:58 -0600 )edit

answered 2014-10-28 15:00:00 -0600

You are not missing something. The fan BHP has to be done as an external calculation. PNL has documented their assumptions in the technical support documents which support the ASHRAE design guides. Refer to section 4.3.7 in the linked pdf.

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answered 2014-10-28 14:53:09 -0600

Entering the pressure rise through the fan, airflow and fan/motor efficiency will calculate the fan kW for you. I doubt that's the answer you are looking for, though.

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Asked: 2014-10-28 13:56:50 -0600

Seen: 1,406 times

Last updated: Oct 28 '14