Heat gain from Fans

[mildlyHVAC]

Hi all,
In Chapter 18 of ASHRAE Fundamentals 2021, we see this in the "Heat Gain From Fans" section (page 18.47):

"The power necessary at the input to the fan motor must account for fan motor inefficiencies and drive losses. Fan motor efficiencies generally vary from 80 to 95%, and drive losses for a belt drive are 3% of the fan power. This may be determined from

P[sub]M[/sub] = (1 + DL) P[sub]F[/sub]/(E[sub]M[/sub]E[sub]D[/sub])

where

P[sub]M[/sub]  = power required at input to motor, hp

E[sub]D[/sub] = belt drive efficiency, dimensionless

E[sub]M[/sub] = fan motor efficiency, dimensionless

P[sub]f[/sub] = power required at fan shaft, hp

DL = drive loss, dimensionless



Ok, so the question is, if as they say, "drive losses for a belt drive are 3% of the fan power"... does this mean for a belt-driven fan one sets DL=0.03? But if that is the case, isn't then using a value of E[sub]D[/sub] < 1 also accounting for losses associated with the belt drive? I'm confused by the presence of DL and E[sub]D[/sub] in this equation. Does anybody here know what's going on with this equation?


Appreciate any insights.

 

[IRstuff]

I would assume that they are referring to other drive losses like bearings, etc.

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[mildlyHVAC]

... but it says DL is drive loss for a belt drive.
So they are accounting for belt drive loss through DL AND E[sub]D[/sub]? This makes no sense.

 

[3DDave]

There is a possibility the document is wrong. If so, it is still guidance - look at your situation and proceed with the factors you know are contributors.