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Heat Gain from Electric Motor High?

Heat Gain from Electric Motor High?

Heat Gain from Electric Motor High?

(OP)
The ASHRAE estimate for heat gains from electric motors seems very high.  For the motor and driven equipment in room equation, they're saying all the real electrical power put into the motor is converted to heat???  What about the energy transferred to the driven equipment?  Anyone else think this is way over estimated?  If you had a room full of motors, I think this would throw you way off.

Motor & driven in equation:
qem = 2545(P/eff)FumFlm
Fum-use factor
Flm-load factor
(both Fum and Flm are normally 1, unless you want to include known intermittent use and partial loading)

For motor in, driven equipment out, it's the same equ. multiplied by 1-eff, so maybe that would include scenarios where the driven equipment's energy is transferred out of the room, like by a belt for example?

RE: Heat Gain from Electric Motor High?

It would make sense that the driven equipment is doing work on something, or it wouldn't be driven.  If it was doing work on something then that would eventually be dissipated as heat.

If the driven equipment is in the room, then the heat eventually ends up in the room.  For a steady state assumption that is the same as the motor input power.

If the heat is dissipated elsewhere (condenser?) then you allow for that.

RE: Heat Gain from Electric Motor High?

(OP)
I can see that, probably a good enough approximation.  I guess what bothered me is that ASHRAE doesn't explain the condenser, coil, hydraulics, or pumps in general scenario where the driven equipment is the pump, but the fluid is carrying all the energy out of the room.  Closer look at the driven equipment out of room equation and it's clear they fall under it.

RE: Heat Gain from Electric Motor High?

(OP)
Saw that, thanks.  That's about the (1-eff), or ~10%, factor.

I'm still reluctant to think that a fan-in-room for example will transfer all the electrical energy into heat.  I wonder how much is lost to entropy, eddy currents, turbulence, etc., etc.  I guess it's close enough.

RE: Heat Gain from Electric Motor High?

Not just close enough...

If you take a spoon and stir a cup of water, you will heat it up a very small amount thus; When you stir, you transfer kinetic energy from the spoon to the water.  The water will move, then eventually stop moving due to friction - water/water, water/cup, water/air.  As the water slows and stops moving, all the kinetic energy is converted to [insert answer here].

So, if your fluid is contained within the room, then all of the work done on that fluid is converted to [same answer as above, I don't want to give it away] within the room.  Air, water or turnip juice, it don't matter.

 

RE: Heat Gain from Electric Motor High?

(OP)
Yes, but how much is lost to entropy, eddy currents/turbulence/or any other remaining kinetic energy that never truly settles out in the real world...but I'm still guessing it's pretty close.

RE: Heat Gain from Electric Motor High?

"kinetic energy that never truly settles out in the real world" = perpetual motion.

RE: Heat Gain from Electric Motor High?

(OP)
kinetic energy that never truly settles out in the real world = constantly circulating air in the room.  Am I that bad of a communicator, or are you guys bored?

RE: Heat Gain from Electric Motor High?

That's a baseline thermal energy that exists simply because everything is above absolute zero.  It's there before, and its there after.  You don't count that unless there's a temperature change, and if there is, you account for it in the delta temperature in your basic calculation.

TTFN

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RE: Heat Gain from Electric Motor High?

(OP)
You're right, the delta is all that's important.  So that just leaves entropy.  When I get unlazy I'll try and figure out how accurate the all electric energy to heat equation is.

Thanks for the input, guys.

 

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