AFT Arrow employs two fundamental pipe system constructs: pipes and junctions.

Pipes are conduits for steady-state, compressible, one-dimensional gas flow. The flow rate through the entire length of the pipe is always constant. Pressure drop due to friction occurs in pipes. Heat transfer may or may not occur depending on the type of model you select.

Junctions are connector points for pipes and are elements at which flow balances are made. Some junction types can only connect to one pipe; others can connect with up to twenty-five.

In addition to balancing flow and energy, junctions also influence the flow or pressure behavior of the system. For example, a tank junction applies a constant pressure at a location, and the flow there is free to adjust in whatever manner is consistent with the governing equations. In contrast, an assigned flow junction applies a known flow rate at its location, allowing the pressure to adjust to that level dictated by the governing equations. The various junction types allow you to specify special kinds of irrecoverable pressure losses for gas behavior. In some cases you can specify special kinds of thermal behavior.

Junctions communicate with each other through the pipes connecting them. Each pipe must be connected to two junctions. There are no exceptions to this rule.

A pipe differs from a junction in that it has a reference positive flow direction. To say a pipe has a flow rate of 1 lbm/sec is meaningless unless the flow direction is specified.

In general, you do not need to specify the actual flow direction in a pipe, because AFT Arrow sorts out the true physical flow directions of the system you define. For example, if you specify the reference positive flow direction from left to right, and the solution is in fact the opposite direction, then that information will be presented in the output as a negative flow rate. Inlet and outlet conditions correspond to the reference positive flow direction, not the actual flow direction.

However, it is important to specify the proper flow direction for pipes which connect to junctions such as compressor/fans  or valves. AFT Arrow uses the flow direction of the pipes to determine in which direction to increase or reduce pressure.