electric motor heat gains

[Roach]

ASHRAE Fundamentals has a chart that says that an electric motor driving a fan generates as much heat as the the total power rating of the motor plus the inefficiency of the motor (assuming the fan and the motor are both in the conditioned space). This seems to negate the energy balance that says if i'm running a fan, most of the energy is moving air, not generating heat? The table states that i can generate 83 HP (212,000 btu/h) from a 75 HP motor?
Am i not understanding the table, or do i not understand the physics?
i.e .. i would think that if i am running at motor full load, my heat genrated would be:
Q = hp(1-motor eff.)

 

[MintJulep]

Motor does work to move the air.

Motor efficiency is less than 100%, so electricity in must be greater than work out. This difference is converted into heat by the motor.

As soon as the air leaves the fan, it starts to decellerate due to interaction with the air in the room. This kinetic energy is converted to heat.

Thus, all of the work done to the air, plus the motor inefficiency eventually gets turned to heat.

 

[25362]

I don't have access to the ASHRAE standards. Anyway, my assumption would be:

The energy delivered to a fan is totally converted into heat for air conditioning estimation. Thus we should add to a fan with an energy input of 75 hp, the efficiency loss of motor and transmission, which, when taken at 90%, result in

(75/0.9) 2544 Btu/h = 212,000 Btu/h​

The conventional U.S. conversion being 2544 Btu/h per hp.

Am I on the right track ?

 

[Roach]

THANKS FOR THE ANSWERS. IT MAKES SENSE THAT EVENTUALLY ALL THE POWER INPUT WILL TURN INTO HEAT FROM FRICTION LOSS. PLUS THE LOSS OF HEAT FROM MOTOR INEFFICIENCY. THAT BETTER EXPLAINS THE CHART THAT SPECIFIES WHETHER THE MOTOR, EQUIPMENT, OR BOTH ARE IN THE CONDITIONED SPACE, AND IF A FAN OR PUMP IS THE DRIVEN EQUIPMENT, AND IT'S MEDIA IS LEAVING THE CONDITIONED SPACE, THEN ONLY THE MOTOR INEFFICIENCY IS TAKEN INTO CONSIDERATION.

THANKS AGAIN

 

[imok2]

Since energy cannot be destroyed, and can only be changed to a different form, any electrical energy transmitted to motors inside a refrigerated space must undergo a transformation. Any motor losses due to friction and inefficiency are immediately changed to heat energy. That portion of the electrical energy converted into useful work, for example in driving a fan or pump, exists only briefly as mechanical energy, is transferred to the fluid medium in the form of increased velocity, and as the fluid loses its velocity due to friction, eventually becomes entirely converted into heat energy.

Btu per (hp)(hr) motor loses

Motor hp (1) (2) (3)
1/8 to 1/2 4,250 2,545 1,700
1/2 to 3 3,700 2,545 1,500
3 to 20 2,950 2,545 400

Load in refr space(1) Outside refr space (2)Load outside(3)

1. For use for when both useful output and motor losses are dissipated within refrigerated space; motors driving fans for forced circulation unit coolers.

2. For use when motor losses are dissipated outside refrigerated space and useful work of motor is expended within refrigerated space; pump on a circulating brine or chilled water system, fan motor outside refrigerated space driving fan circulating air within refrigerated space.

3. For use when motor heat losses are dissipated within refrigerated space and useful work expended outside of refrigerated space; motor in refrigerated space driving pump or fan located

 

[friartuck]

As a general rule of thumb, the air temp increases by 1 deg C per 1000Pa pressure rise across the fan.

Many air handlers incorporate filters, cooling coils and silencers and therefore require typically a 1000Pa to simply move the air through them plus an additional 300Pa or so to cater for the external ductwork. So a rise of 1.3 degrees is not uncommon.



Friar Tuck of Sherwood

 

[turbA]

hi all!

i have a question related to this one... i'm trying to figure out the heat emitted by an engine and a motor in a room that must be air conditioned. tell me if i am on the right track:

the heat energy per unit time emitted by the motor is equal to the electric consumption minus the output hp (since all friction losses etc. generate heat).

the heat energy per unit time emitted by the engine is equal to the input energy from the fuel (taking into account the combustion efficiency) minus the output power (which is the rated output for the engine, right?)

i'm not sure how to account for the energy lost as sound.

i'm also pretty sure that ASHRAE has some standard for industrial equipment loads. can anyone point me towards the relevant standards?

thanks!

 

[McCormick93]

Don't forget that the engine will expel significant heat outdoors through the exhaust.

Regarding energy as noise, this also becomes heat load as the building muffles the sound. The sound power level will not be significant anyway; consider a 500W car stereo making more noise than a sizeable diesel engine.