ABSTRACT
After the required rates of ventilation for various locations in an area have been established, as well as the types of hoods it is then necessary to design the ductwork, estimate system losses and static pressure needs, and select the fan. After selection of the path of the duct work, the next step is to fix the diameter of the duct. This can be done by reference to the duct air velocity. The interior of ducts exhausting mists from electroplating tanks or the like accumulate a crust of dry salts which can markedly modify the surface smoothness of the ducts. In addition to the energy loss due to friction at the wall of the duct there are losses in various fittings like elbows, enlargements, and contractions in the duct area and points of entrance from the atmosphere into the duct system. The duct diameter after the bell mouth entrance corresponds to the diameter of the vena contracta of the orifice.
