The design of the fan found on a vehicle depends on the air requirements of the engine's cooling system. Diameter, pitch, and the number of blades can be varied to attain the needed flow. A fan placed more than 3 inches from the radiator becomes ineffective. It merely recirculates the hot air around the fan blades. For this reason, some radiators are equipped with shrouds. A shroud is a large, circular piece of plastic, metal, or cardboardlike material that extends outward from the radiator to enclose the fan and increase its effectiveness. These shrouds should always be kept intact and should not be modified.