Fan efficiency too low?

[cokeguy]

We have calculated that ID fan efficiency for one of our scrubbers stands at around 50% or even lower, based on a 15 inH2O pressure difference between inlet and discharge, a 47,000 acfm flowrate, and the 250 HP fan motor running at full load. The scrubber´s fan is of the "wet" type, meaning that hot flue gases passing through the fan are already down to 60 degC and very humid after being washed down and scrubbed. Is this wet condition the cause of our low efficiency, or should we be able to achieve better efficiencies even under extremely wet gas conditions?

 

[KiwiMace]

Ask the manufacturer for the fan curve of the specific fan installed. This should give you a better idea of the achievable efficiency in this installation. I have never heard of the humidity of an air stream affecting the fan in such a manner. I'l leave it to others to educate here, but sounds far-fetched to me.

 

[cokeguy]

What efficiency should I expect in a fan this size?

 

[fredt]

High water content in vapour form reduces the gas density entering the fan. This in turn will reduce the pressure that the fan produces but does not directly affect efficiency. The efficiency is low but it depends where the fan is operating on its performance curve. I agree with CinciMace that you must obtain a performance curve and establish where the fan is operating on the curve as a starting point.

There is another possibility. If there is a large carry over of water droplets the fan will consume additional power "pumping" solid water. Check the operation of the scrubber to ensure that droplet carry over is minimal.