ID fan casing consists of three parts: an inlet guard, an interior casing, and an outlet guard. The inlet guard is typically made of steel or aluminum and is designed to separate the air entering the fan from the interior of the casing. The interior casing is typically made of sheet metal and is designed to contain the fan motor and related parts. It also serves to provide structural support to the fan. Finally, the outlet guard is typically made of sheet metal and is designed to separate the air exiting the fan from the interior of the casing. It also helps to keep dust and other particles from entering the interior of the fan.