The arc shape of the air-inlet is reasonable designed, which can make the gas pass into the fan smoothly and stably. The air-inlets come in 14 sizes:  9 inch, 10 inch, 12inch, 15inch, 18inch, 20inch, 22inch, 25inch, 28inch, 30inch, 33inch, 36inch and 39 inch. The product can also be processed according to the requirement of customer.  http://www.blower-wheel.com