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Is it essential to apply variable speed fan to DOAS?

asked 2014-11-06 19:57:35 -0500

yongqingzhao gravatar image

updated 2015-11-14 16:13:18 -0500

Recently, with building energy efficiency work,more and more energy efficiency measures have being applied, including variable frequency technology. But when I perform building energy analysis for DOAS-applied variable frequency fan, I find out that the energy saving effect is not obvious. And I think it is because variation range of outdoor airflow is small. Do you think so?

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What simulation program are you using?

__AmirRoth__ gravatar image __AmirRoth__  ( 2014-11-07 06:38:05 -0500 )edit

Also, how is your system actually controlled/designed? Are you implementing a demand controlled ventilation operation, or free cooling? Otherwise, putting a VFD on a fan set to run at constant speed is useless.

Julien Marrec gravatar image Julien Marrec  ( 2014-11-07 11:39:34 -0500 )edit

I am using eQUEST software.But I think it is not the problem of simulation software,and a DCV operation is not implemented.My opinion is minimum outdoor air will maintain in cooling and heating season,and increase in transition season if air economizer is used,however, the transition season is short in most region,so I think outdoor air variation is so small that the energy saving effect of VFD is not obvious.And even if the DCV(Demand control ventilation) have been implemented,it is only appropriate to the building whose occupancy density variation is big.@Amir Roth @Jullien

yongqingzhao gravatar image yongqingzhao  ( 2014-11-09 22:45:35 -0500 )edit

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answered 2014-11-10 16:12:26 -0500

To answer your question, it largely depends on several factors that would reduce your opportunity for energy use reduction:

1) You have a constant or nearly constant need for the same quantity OA.

2) Your need for OA quantity changes, but there aren't controls in place (in the design AND in your model) that allow the quantity of OA to be reduced at times.

3) The OA quantity is small and the resulting DOAS fan motor horsepower is small. Although the total magnitude of savings may be little, a VFD may still provide an ROI in this case.

4) You have many economizer hours where it is beneficial to take a larger quantity of outside air, even though the OA requirement is less.

@Julien Marrec - there still could be a small energy benefit if the VFD could be used for airflow balancing, or to adjust pressure to maintain constant airflow over time while filters loaded and then were replaced. Large motors that start and stop frequently may also benefit from use of a VFD at startup as an operational (non-energy) measure.

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Asked: 2014-11-06 19:57:35 -0500

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Last updated: Nov 10 '14