Subject:- Refrigeration and Air Conditioning

Topic:- Duct Design, Controls & Applications

Difficulty:- Medium

Duct designing is about determining the dimensions of all ducts in the given system.

The ducts are required to carry the necessary volume of conditioned air from the fan outlet to the desired space with minimum frictional and dynamic losses.

The area changes must be gradual and limited to not more than 20° for diverging area and 60° for converging area.

An aspect ratio of upto 4 is desirable for rectangular ducts but it should never be greater than 8.

Minimum sheet metal should be required by the design. Hence square cross-sections are preferred for a given cross-section.

The duct layout must be so as to reach the outlet with the least number of bends, obstructions and area changes.

Velocities in ducts must be high enough to reduce size of ducts but also low enough to reduce noise and pressure losses. Some of the recommended velocities are:

recommended velocities are:

The important methods of duct designing are:

a) Velocity reduction method

b) Equal pressure drop method

c) Static regain method.