[Equest-users] [Bldg-sim] Proposed Case fan power modelling

Tue Jul 18 10:02:46 PDT 2017

Terminology can be a beast with fan/pump motor power calcs.

“Fan absorbed power” isn’t a phrase I’ve encountered frequently but suggests to me work done by the fan on the air (or vise versa), presumably at design conditions for your case.  I would intuitively use this in combination with your knowledge of the nameplate HP rating to determine appropriate fan power inputs.  Leverage the nameplate HP rating of 7.5 to determine a reasonable motor efficiency first.

So – quick aside for Recommended further reading:

·         The “I don’t have time!” suggestion:  Chapter 10 tables from 90.1

·         “Hit me with the good stuff!”  https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwitnt6Y_ZLVAhUCeSYKHcRdCZ0QFggiMAA&url=https%3A%2F%2Fenergy.gov%2Fsites%2Fprod%2Ffiles%2F2014%2F04%2Ff15%2Famo_motors_handbook_web.pdf&usg=AFQjCNE2OJ-TJR5_EFu34EoAiPO1PMP9BA

·         Highlights include:

o   Table 3-3 detailing solid documentation for full load efficiencies for older large motors (excludes winding degradation losses for motors that haven’t been serviced).

o   Table 2-1 should look familiar to those perusing 90.1 over the past decade.

o   I recall on skimming today that Chapter 3 was super helpful for me learning to both digest and intelligently request direct measurements of operating motor performance in the field.

So back to the problem.  Descriptively:  [Power In] = [Work accomplished by the fan: 4.73 bhp] / [Motor efficiency].  For a new motor that should lead you to something in-between 4.73 and 7.5 hp.

Since you CC’ed eQuest-users specifically:  For developing eQuest inputs, additionally consider your flow rate associated with that design operating point to determine a kW/CFM input:  [kW/CFM] = 4.73* 0.7457 / motor-eff / CFM.  Best paired with a corresponding system design airflow rate input that isn’t getting overridden due to site elevation (air density) or zonal flow demands.

A related nugget perhaps helpful to share, as this confused me for some years:  motor efficiencies in broad strokes do vary, but not much between 50-100% of nominal horsepower load with all else being equal (distinct from motor speed!).  It’s pretty normal to see motors “oversized/underloaded” motors with high nameplate horsepower relative to their actual full-load operating power requirement, and that isn’t necessarily a bad thing.  The above document includes some examples and further explanatory text illustrating where “right-sizing” a fan motor can result in identical or worse power input, where the points of efficiency gained for operating closer to full capacity are countered by the points of efficiency lost inherent to smaller motor sizes.

~Nick

[cid:image001.png at 01D2FFA2.BC049F10]
Nick Caton, P.E., BEMP
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  Energy and Sustainability Services
  Schneider Electric

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From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jim Dirkes via Bldg-sim
Sent: Tuesday, July 18, 2017 6:27 AM
To: Mayank Bhatnagar <mayank.23aug at gmail.com>
Cc: equest-users at lists.onebuilding.org; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Proposed Case fan power modelling

If the fan is using 4.73BHP, that's the value to consider for energy cost. It is very common for a motor to have greater capacity than is actually used.

On Tue, Jul 18, 2017 at 7:24 AM, Mayank Bhatnagar via Bldg-sim <bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>> wrote:
Hello Everyone,

Ours is a Residential Project with PTAC units and DOAS in the Proposed design. The DOAS nameplate motor HP is 7.5 where as the fan absorbed power is 4.73 BHP. If we model 7.5 HP; we are falling short of the minimum LEED energy performance requirements. Modelling
4.73 BHP just gives us enough savings.

Which one is the correct fan power to model?

I have been through the archives and didn't find appropriate answer. Please help!

Regards,

Mayank.

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