Heat Gain from Electric Motor High?

[fiberstress]

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?

 

[KiwiMace]

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.

 

[fiberstress]

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.

 

[imok2]

Try This

http://www.engineeringtoolbox.com/electrical-motor-heat-loss-d_898.html

 

[fiberstress]

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.

 

[KiwiMace]

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.

 

[fiberstress]

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.

 

[IRstuff]

Your last question is puzzling. What does "kinetic energy that never truly settles out in the real world" mean? I think you seem to have some misconception about this subject.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[ChrisConley]

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

 

[fiberstress]

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?

 

[IRstuff]

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

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[fiberstress]

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.