This type of bus duct is the most common type of bus duct. It is normally made of PVC plastic, which is inexpensive and easy to maintenance.
One type is the large volume duct, which is better used as an air conditioner duct. If the air conditioner is inside of the vehicle, it is possible to create a large volume duct, with lots of air inside. Similarly, the locked bus duct is one of the most popular types, and it is important to have an air conditioner inside the duct itself and be in the form of a large volume duct.