To deliver air from place to another space, air carriers are required, which are called ducts. Ducts work based on the principle of air pressure changes. If a pressure difference exists, air will flow from high pressure area to low pressure area.

There are three common classifications of ducts – conditioned air ducts, exhaust air ducts, and fresh-air ducts. Conditioned air ducts carry conditioned air from the air conditioner and distribute in to the specific area. Exhaust air ducts take air from the specific space and exhaust to atmosphere by treating it . Fresh air ducts bring fresh air into the air-conditioning system from outside to the conditioned space

Most common shape of ducts are

1. Round duct system

2.Square duct system

3.Rectangular duct system

4.Flat oval (spiral) duct system

The most efficient duct shape is round, less material is needed for the same capacity as a square or rectangular duct. Square or rectangular duct fits better to building construction. Ducts may be made up of metal, wood, ceramic, and plastic. Most commonly used is sheet steel coated with zinc. Now days sheet metal brakes and forming machines are using fabricating ducts..