It was a fan that computer guy had given to me to stick on top of a video card that wa running a little hot way back when I was into computer. Actually any fan that will fit inside a 2" PVC fitting would work, its funny I was just thinking the other day about trying to cast one see if I could get it to come out of the sand in one piece.
I did and was able to get a good but rough casting of the cooling fan off on the 2 HP motor on my milling machine. So I don't know why a guy couldn't cast up a couple of em. The blade that I have has a magnet in it but I'm not into all that crap about magnetic s like some people are, its there because it was part of the fan, and to get it out would tear up the blade.
What it does is to give the air and gas stream a spin, sort of like the burners we use at work, but closer to the blower. The burners at work that I work on have spin vanes in them the angle of the vanes sets the size of the flame, but they are also close to the nozzle end of the burner. In my burner the fan blade is used to cause turbulence so that the air and gas will mix better, this allows the gas upon hitting the end of the burner pipe/tube to ignite from the heat in the pipe end ( had to put a stainless steel tip on the burner pipe plus it slides into a 2" stainless steel sleave in furnace) (burned the end off of the tube the fires time I fired the thing with the blade in it) Its very easy to achieve a proper air/fuel ratio. If the ratio is off just like you lawn mower the burner will not work worth a damn. The numbers say that roughtly for every 50 cubic feet of propane gas your going to need roughtly 10.5 CF of O2, so your blower is to use that 50CF of fuel is going to need to put out Air is only 21% Oxygen, so your going to need a even for a burner like mine a pretty hefty blower. The blower that I have is capable of I think around 250 CF of air which equals to about 62 cubic feet of O2 so I can burn with complete combustion about 50 cubic feet of propane vapor.
That burner with a furnace build with the right stuff will melt your 30 pounds of brass in and hour, less of you use all high temp white alumina oxide brick to build it out of or a good insulating refactory. My choice if I wasn't going to be moving the furnace around a lot would be an insulating castable for the wall and lid, and a high strength 3000 castable for the floor or bottom. Ceramic fiber is fragile after its been fired doesn't like to be packed to tightly, if I was making a furnace with it, I would use 1 inch thick fiber, installed in 2 layers with seams off set. in a 12 LB density fiber blanket, or if you could find an outfit that sold preformed shapes in the size you were needed for you furnace I'd go that route, use some thin flashing to keep water from the fiber and to keep from crushing it with pouring the sleave. PVC pipe greased up makes a good form, I use sonotube but always end up with an out of round furnace. Lid I would use 2 inch thick fiber board, with one inch of castable on the furnace side a stainlesss steel cover on the outside with the castable extending into the Exhaust Port to protect the fiber.
The picture below is of my first furnace running on a 1 N/A burner on 17 PSI of propane gas, the furnace body is 3000 degree castable, the lid is 2600 degree brick, when at Iron melting temp the lid is red complete through, I have a choke that I put on top of the lid to close the exhaust down, you take it off and the lid will be red, should of turned the brick up on there side.
Oh yea you asked about the Fan, go to a computer store one that sells parts look for a 2 inch fan or one of the size that will work for you. that came from a fan that I just happened to have laying around. Hobby shop that sells boat parts get a prop it will do the same thing.