Heat rejected by fan gain

[MechEng92]

I am looking at two concept layout to blow fresh air into a room space. I am trying to determine how the fan motor is located in each instance and was hoping someone could give me their opinion.

Instance 1: The centrifugal fan is placed in a box attached the external wall of the room. The fan is blowing fresh air in the room through an aperture. The fan motor and impeller are in the same space/portion of the box that the fan is pressuring which blows the air through the aperture in to the room. Would this count as the motor and fan in the inlet air flow, or would this be the motor outside the air flow and the fan in the air flow?

Instance 2: The centrifugal fan is placed in the room positioned to draw air from outside and discharge sideways along the walls. The fan impeller and motor have a mounting plate between them, so both the fan and motor are in the room, but the motor isn't really in the air flow but the heat rejected would mix with the room.

I hope I've described this well enough, thanks for help in advance.

 

[RandomUserName]

All fans add heat. All of the fan shaft power eventually converts to heat in the system, either initially as fan losses or downstream as duct friction losses. Motor inefficiencies are an added load if that motor is in the airstream. Heat gain in medium-pressure systems is about 0.5°F per inch of water static pressure.

Specific information for calculating specific heat gain for your system can be found in the ASHRAE Handbooks.

 

[capliard]

If you do not want the air to be heated by the fan, set it up in a Pull configuration. Your building will then be in negative pressure, but *most of* the heat from the fan will be dissipated outside instead of inside your building. Otherwise, the fan will heat the builidng.

*Most of* because RamdomUser is right, Friction losses incurred during building entry will be disspated as heat. I'd say close to 0 unless air goes through filter.

Regardless of wether the motor is Inside or Outside of the building, a Push configuration will always add more heat to the building than a Pull configuration.

All based on underground mine ventilation. Proud miner!

Charles.

Ingenieur Minier. QuTbec, Canada.

 

[MTES93]

The fan will usually have more heat contribution to the airstream than the motor.

A 65% mechanical eff. fan will add 35% of shaft power as heat.
At 90% efficient motor would add 10% of the motor power output as heat if located w/in the airstream.