Duct Classes

CLASS I. Includes non-abrasive applications; makeup air, general ventilation, gaseous emission control.

CLASS II. Includes applications with moderately abrasive particulate in light concentration; i.e. buffing and polishing, woodworking, grain dust, etc.

CLASS III. Includes applications with highly abrasive material in low concentration; i.e. abrasive cleaning operations, dryers and kilns, boiler breeching and sand handling, etc.

Minimum* U.S. Standard Gauge for Industrial Duct
Diameter of Straight Duct	Non- Abrasive Class 1	Medium Abrasive Class II	Highly Abrasive Class III
To 8" Over 8" to 18" Over 18" to 30" Over 30"	24 22 20 18	22 20 18 16	20 18 16 14

*NOTE: The chart above shows minimum recommended gauges. The spans (distance between hangers), negative working pressure, and the abrasiveness of particulate being conveyed should be considered when selecting duct gauges. Certain heavy industries such as high production grey iron foundries and steel mills have led to adoption of heavier gauges as listed below:

Class IV

Diameter of Pipe

Gauge

To 18"

18" To 30"

Over 30"