Motor In Airstream fraction is acting weird

openstudio
air-loop
fan
motor
idf-file
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Found a weird thing with the Mortor in Airstream fraction that is input for Fans in OpenStudio. From what i understand, when the value is 0, it should not provide any heat to the airstream. But that is not the case, which made me wonder if something is wrong when it translated into the idf file.

I also noticed that there is a small difference between setting it to 1 or 0. But it still gain some heat from the fan, so is that cause by something else? The two examples are shown in the figures below:

Motor In Airstream Fraction = 0.0 image description

Motor In Airstream Fraction = 1.0 image description

The HVAC system used: image description

Hope someone can enlighten me :)

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Kingo

asked 2017-04-21 04:06:17 -0500

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__AmirRoth__

updated 2017-08-06 16:22:42 -0500

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This is interesting. As Adam mentioned, I checked the equations in 17.4 Air System Fans. From the equations (17.131 to 17.136), even if f_motortoair is 0, it does not imply the Q_toAir variable is 0, therefore, the enthalpy of air at the outlet is not equal to enthalpy at the inlet. (check the derived equations in image)

h_out > h_in

The psychrometric function (eq. 17.136) calculates fan outlet drybulb temperature higher than the inlet fan air temperature. Therefore even if f_motortoair = 0, there is addition of heat and T_out > T_in.

image description

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kpp

answered 2017-04-21 15:51:05 -0500, updated 2017-04-21 16:13:18 -0500

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From the engineering reference...

$$Q_{toair} = H_{fan} + (H_{motor} - H_{fan}) * f_{motortoair}$$

While not discussed in the fan section, I assume it's the same logic as pumps. Any shaft power is eventually realized as heat to the fluid due to friction losses. I'm not actually a fan of that because in the real world, you're not controlling to those losses because that heat would be added after any kind of discharge air controller.

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Adam Hilton

answered 2017-04-21 07:58:01 -0500, updated 2017-04-21 08:08:27 -0500

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Are you sure you didn't mean to have $Q_{toair} = H_{fan} + (H_{tot} - H_{fan}) * f_{motortoair}$? I could be misinterpreting your notation but the way you have it written if $f_{motortoair}=1$ you would have $Q_{toair} = H_{motor}$ but if the motor is totally in the air stream I believe you should have $Q_{toair} = H_{tot}$.