As the output increases with the square of the screw’s diameter D, so is the surface of the barrel expressed in D lengths; therefore, the same number of diameter if required to finish the required heat. For example, for a 3” D screw, if a length of 15” or 5 times 3” is required, for a 4” D screw, a 5 times 4” or 20” of barrel length it required.