These are made of 10 to 14 gage steel.
The principal they work on is that the relatively small opening (compared to a large hood) creates a high velocity and sucks up the smoke and flame. The body acts as an expansion chamber and funnels the hot air and smoke into the flue. Some are built WITH a smoke shelf, others are not. The general design replicates in steel the brick side draft flues used for centuries.
The fire pot sits immediately below and in front of the intake. This leaves the fire open on three sides. Sometimes the intake is a foot above the firepot. If this style is used be sure to close the gap between the forge and the hood/flue. A gap lets in cold air increasing the load on the flue while reducing the efficeincy at the same time.
Often these need a sheet of burning newsprint tossed in to start the draft just before starting the forge so that your shop doesn't get smoked up. A small flap, hinged or removable extension "hood" can also increase the efficeincy for when the fire is fresh or just smouldering.
References and Links