Centrifugal fan for hot air transportation 1

[kompiuteris]

What is special in construction of direct driven centrifugal fans for hot air transportation (air temperature over 200 Celsius degree)? My guess main requirement is motor selection with right thermal insulation. Any other suggestions?

Thank You

 

[zekeman]

Why does it have to be direct drive? Big problem with bearings and wiring would need motor special. You would be better off with belt drive if possible.

 

[MintJulep]

Just off the top of my head:

Post fan fabrication stress relief to prevent thermally induced distortion of the fan wheel, and associated imbalance.

Differential thermal expansion of the fan hub, shaft and whatever coupling may exist between the two.

Heat transfer to bearings and appropriate selection of lubricant.

 

[kompiuteris]

Its just because at the moment I working with direct driven centrifugal fans labeled for normal conditions (max. 130 C) and for high air (max. 250C). And I dont see any difference exept the price, so wanna know where is the trick

 

[marcoh]

The difference is in the bearings, lubrication, and motors if they are in the air stream. If all these are not rated for continuous operation at higher temperatures they you will get premature failure.

Also need to be conscious when the selecting the fan duty, do you select the fan at standard room temperature conditions, or at 250 degrees?

 

[kompiuteris]

marcoh, yes that thing about temperatures still bothers me.
I have found this page

http://www.engineeringtoolbox.com/air-density-temperature-fan-capacity-d_144.html

about this matter, but still don’t understand. Why in the "Fan with Hot Air" example they leave 10 000 m3/h without correction, and in "Fan with Combustion Air" they have corrected to 15,500 m3/h value.

 

[fredt]

1. The fan has to be correctly selected for the air density at temperature which is much less than the ambient density. Fans are volumetric devices which means that compared to handling cold air in simple terms the volume will be about the same, the pressure and power will be reduced by the density ratio.
2. The fan impeller and shaft need to be designed to account for the reduction in material properties at high temperature. 200C is usually easy to handle.
3. Mounting the impeller directly on the motor shaft is not recommended but might be possible if appropriate measures are taken. Usually the impeller is mounted on its own shaft and bearings and heat flingers used to reduce heat conduction to bearings.