Tubes are used in applications where the outside diameter must be
precise, like in cooler tubes, heat exchanger tubes, boiler tubes etc.
Pipes are generally used to carry fluids and must contain them. They
also have pressure rating and hence are scheduled.
In tubes the thickness increases in standard steps like 1 mm thick, 2
mm thick etc. In pipes however the thickness depends on the schedule
of the pipe and there is no fixed step.
Pipe joining is more time consuming . For e.g. welding, threading,
flanges with bolts etc. The tubes joining are faster using flaring,
brazing, couplings etc.
Tube dimensions are actual dimensions. Whereas the pipe
dimensions are only nominal. That means that a 1” tube will have
actually OD as 1”. The pipes on the other hand are named nominally,
which means only for name. The 1” schedule 40 Nominal size pipe has
an ID of 1.049”, OD of 1.32” and a wall thickness of 0.133”.
Tube fittings are compression fittings such as ferrule and union nut,
flared fittings, biting fittings, mechanical grip type fittings. The pipe
fittings on the other hand are pipe to pipe butt welding, threaded pipe
fitting connectors, flange to flange bolted fittings etc.