Never quite understood why Londoners refer to their subway system as "The Tube" rather than "The Pipe"? Well, now you know. That is, until someone inquired as to the distinction between tube and pipe, which we didn't know. Despite the fact that tubes and pipes are all just long hollow cylinders, these terms are not interchangeable in the engineering world.

Definitions of a Hollow Cylinder
There is a significant difference in how hollow cylinders are measured or specified, and this in turn is determined by the application for which they are designed. Because it is used to transport fluid or other media, the internal diameter (ID) or bore of aluminum pipe is defined as follows:In this case, the ID is important because it determines the capacity of the pipe, which is typically the property that the buyer is most concerned with.

The outside diameter (OD) of an aluminum round tube defines its shape.  Because tube is used in structural applications, this is the case.  When it comes to the end use, the bore isn't particularly important, but the outside diameter (OD) most likely is. The tube does not have to be round, as a matter of factThe term "tube" refers to a closed internal passageway in most cases.  Square and rectangular tubing, as well as round drawn aluminium tubing, are available for purchase.

Manufacturing of pipe and tube
Instead of defining the ID and OD separately, why not define them both together? The answer comes down to the manufacturing processes and where you want the natural process variation to be located in the process.  The consistency of the bore or the consistency of the OD can both be achieved, but not both simultaneously.  A high degree of precision, at the very least.

Extrusion is used to manufacture the majority of small bore metal pipe and tube.  That is the process by which a billet of material is squeezed through a die to produce a long length of material with a uniform cross-sectional shape. The most effective materials for extrusion are ductile materials, which explains why drawn aluminium tube is used so extensively.

Extruding pipe or tube entails forcing metal around a mandrel, which creates the internal passageway in the pipe or tube.  The wall thickness varies as a result of the difficulty in maintaining this internal bore's concentricity with the OD in reality.  Either the bore or the outside diameter (OD) are under the control of the manufacturer, but neither is.

After extrusion (or, in some cases, instead of extrusion), some tube will be drawn in order to thin it out and improve dimensional consistency.  Drawn material passes through a die and over a mandrel in a manner similar to that of extruded material; however, the material is pulled rather than pushed through the die.

Extruded round pipe has a variety of applications
Heat exchangers are one of the most common applications.  This is due to the fact that aluminum has excellent thermal conductivity and is easy to weld together.  Being lightweight, it is also in high demand for solar water heating systems that do not place an undue burden on a roof's structural integrity.

Extruded tube has a variety of applications
A wide variety of architectural and structural applications make use of extruded aluminum tubes.  Handrails, support columns and gates are all examples of structural elements.  Also found in trailers and other transport applications where weight is a concern, and even in electrical bus conductors, it is a lightweight material with many applications.

Aluminum tube and pipe are made of a variety of materials
The majority of aluminum tube and pipe is extruded from 6061 or 6063 grade aluminum alloys.  The reason for this is that these materials have a lower tendency to work-harden, allowing the extrusion equipment to operate more quickly.  Although 6061 is stronger, 6063 has a more pleasing appearance due to its finer grain structure, and it anodizes well when a colored finish is required.

While they are similar, they are also distinctive.
However, while pipe and tube are both terms that can refer to hollow cylinders, they are not the same thing.  The inside of the pipe is utilized, whereas the outside is utilized by tube