Reference Positive Flow Direction

Next to each pipe's ID number is an arrow that indicates the reference positive flow direction for the pipe. AFT Fathom assigns a flow direction corresponding to the direction in which the pipe is drawn. You can reverse the reference positive flow direction by choosing Reverse Direction from the Arrange menu or selecting the reverse direction button on the Toolbar. With exception to some junctions, the reference positive flow direction is used for reference purposes only and does not need to correspond with the real direction of flow calculated by the Solver (the part of AFT Arrow that applies the governing incompressible flow equations to obtain a solution to the pipe system). If the flow direction obtained by the Solver is opposite of the reference positive flow direction, then the output will show the flow rate as a negative number.

Note: The reference positive flow direction is important for compressors because the pipe directions will define which side is suction vs discharge. Furthermore, reverse flow through components may result in a Caution message which may or may not be valid depending on the intended direction of flow.

Convention for Flow Entering and Exiting

Many of the junction types permit gas flow to come into or pass out of the system in a prescribed manner. The most obvious of these is the Tank junction. Tanks maintain a constant pressure and temperature at a node, and the flow adjusts according to the governing equations. While in principle the gas is stored or removed from the tank, AFT Arrow considers the gas as coming into or going out of the system. The stored tank fluid is not part of the system.

The convention for defining inward or outward flow for the system or any AFT Arrow object is that flow in is positive (that is, the object/system gains the flow) and flow out is negative (the object/system loses the flow).