Difference Between Pipe, Tube & Duct

Pipe:
A pipe is a tubular section or hollow cylinder, usually circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders, masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.
In common usage the words pipe and tube are sometimes interchangeable, but in industry and engineering, the terms are uniquely defined. Depending on the applicable standard to which it is manufactured, pipe is generally specified by a nominal diameter with a constant outside diameter (OD) and a schedule that defines the thickness.

Tube:
A tube, or tubing, is a long hollow cylinder used to convey fluids (liquids or gases).
The terms "pipe" and "tube" are almost interchangeable, although minor distinctions exist — generally, a tube has tighter engineering requirements than a pipe. Both pipe and tube imply a level of rigidity and permanence, whereas a hose is usually portable and flexible. A tube and pipe may be specified by standard pipe size designations, e.g., nominal pipe size, or by nominal OD, outside or inside diameter and/or wall thickness. The actual dimensions of pipe are usually not the nominal dimensions: A 1-inch pipe will not actually measure 1 inch in either outside or inside diameter, whereas many types of tubing are specified by actual inside diameter, outside diameter, or wall thickness.

Duct:
Industrial exhaust ducts are light weight (thin wall) pipe like systems that connect hoods to industrial chimneys through other components of exhaust systems like fan, collectors etc. Ducts are low-pressure pneumatic conveyors to convey dust, particles, shavings, fumes or chemical hazardous components from air in vicinity to a shop floor or any other specific locations like tanks, sanding machine, or laboratory hood. Ducts can be fabricated from a variety of materials including carbon steel and stainless steel. [1]

HVAC Duct systems do not include this category of industrial application namely exhaust systems. A distinction from HVAC system duct is that the fluid (air) conveyed through the duct system may not be homogeneous. An industrial exhaust duct system is primarily a pneumatic conveying system and is basically governed by laws of flow of fluids.[2]