Fan power and fan heat gain/generation

[momomomoses]

fellow engineers,

hi I'm totally new here so hello everyone.

I'm a mechanical HVAC design engineer and I have a question about fan power and fan heat gain/generation. My main concern is the temperature rise in the air passing through the fan.

I've been researching in books and on the internet but haven't got any concrete result.

I'm trying to think in terms of energy. It all started with electric energy to the fan, then it is converted to the following
1) kinetic energy of the fan
2) kinetic energy of air, which causes an increase in air temperature
3) friction loss of the fan system, which is released as heat i.e. the kinetic energy or air which causes an increase in air temperature.

however at this website "www.engineeringtoolbox.com/electrical-motors-heat-gain-d_707.html"[/URL] it states that ALL fan power is converted to heat to its surrounding. I've also confirmed something like this with a Carrier engineer.

Is is possible? I always think of "air heat gain" as an increase in air motion. Am I missing something important here?

Thanks

 

[SteamKing]

Fans have been known to stir up air. Some of the energy from the fan motor will cause the fan blades to turn, causing the air to move.

 

[momomomoses]

thanks for you reply.

that's my point exactly. only some of the fan motor energy out work to the air, so only part of the fan energy is "heating up" the air, right? so how would you determine the amount of heat gain from the fan by the air?

 

[russ_watters]

When you turn off the fan, does the air continue to move? Where did the kinetic energy go?

 

[momomomoses]

thanks for your reply.

after the fan is off the air keeps moving slower and slower because of friction, releasing the "heat" to the surrounding.

So are you suggesting that the electric energy is converted to
(1.1) kinetic energy of the fan
(1.2) friction loss of the fan, which is released as heat and increases the air temperature

and then the kinetic energy of the fan (1.1) is converted to
(2.1) kinetic energy of air, which causes an increase in air temperature

therefore the entire heat given out from the fan is (1.2) and (2.1), which is the electric energy input?