AFT Arrow FAQ's

There are several aspects to AFT Arrow verification. First, AFT Arrow has been compared to published data where it exists. Available published data is usually for a single pipe, which is a trivial solution for a network solver. Models, comparisons and explanations for such published predictions are given the Verification folder within AFT Arrow. Second, it can be verified that the AFT Arrow results agree with mass and energy conservation along with the fundamental equations for compressible flow in single pipes and networks. Third, AFT Arrow offers two independent solution methods, which can be compared against each other. Fourth, incompressible gas flow predictions for networks show good agreement with incompressible calculation methods such as AFT Fathom. Finally, AFT Arrow predictions have been compared to test data and other analytical methods on numerous occasions and good agreement has been shown. For example, see Winters, B.A., and T.W. Walters, "X-34 High Pressure Nitrogen Reaction Control System Design and Analysis".

Yes, AFT Arrow can run off the network or local PC. When installed on a network, the number of concurrent users is limited to the number of licenses purchased. See the Installation page for information on installing the software with either a USB key or an eLicense.

AFT Arrow accurately models sonic choking at all three possible geometric conditions. See Sonic Choking for more information.

No, AFT Arrow cannot model liquid systems. AFT Arrow’s solution methodology is built on a gas equation of state (Equation 9.5), which is not compatible with liquid flow calculations. AFT Arrow can, however, model incompressible gas flow such as occurs in venting systems and many HVAC systems.

AFT Arrow’s solution methodology is based on the ideal gas equation modified with a compressibility factor. This compressibility factor can be obtained from an equation of state model or property database. The effect of the compressibility factor is carried through the fundamental equation derivations, and thus accounted for directly.

Heat transfer effects show up in several places in AFT Arrow. First, in pipes users may assign a convective coefficient and/or allow AFT Arrow to calculate convective heat transfer based on standard methods. These heat transfer calculations and the associated physical properties are determined over each solution section. Thus properties and convective coefficients can and do vary along the length of the pipe. Second, heat transfer can occur in Heat Exchanger junctions. Here the user assigns a heat load or chooses a heat transfer model for the heat exchanger. AFT Arrow performs the appropriate energy balance calculation across the heat exchanger. Third, heat transfer occurs in compressor/fans due to heat of compression. Finally, at each branching location in the network an energy balance is performed so that energy is balanced from one pipe to the next.

Yes, AFT Arrow accounts for heat of compression based on the polytropic constant specified by the user.

Yes, every junction except for heat exchanger and compressor/fans is assumed to be isenthalpic. To be more specific, to satisfy the energy equation the process involves constant stagnation enthalpy. The static (thermodynamic) enthalpy can and does change based on the energy equation.

There are no theoretical limits to model size, but there are a few practical limits. First, AFT Arrow accepts pipe and junction ID numbers up to 9999. This limits the model size to 10,000 pipes and 10,000 junctions. Before you reach that limit, however, you will likely encounter a limitation of your available RAM to hold all of the solver parameters. To determine how much RAM you need, add up the number of branches and tees in the model. Take the square of this number. Then multiply it by 32 to get the amount of RAM that must be available. For example, with 1,000 branches and tees, the square is 1 million, and after multiplying by 32 you need 32 million bytes of RAM (i.e., 32 MB).

Not directly, but one can effectively model a turbine using the heat exchanger junction. By specifying a heat exchanger pressure drop vs flow corresponding to the turbine pressure drop and specifying a heat rate out, you can accurately model the both the pressure drop and enthalpy reduction imposed by a turbine.

No, AFT Arrow models only non-reacting flows.

Whereas the Output Control window allows you specify units for all parameters, when in the Output window viewing results you can quickly change the units for parameters in the tabular displays by double-clicking the column header.

Open the Visual Report Control window, change to the Show Selected Pipes/Jcts folder tab, and use the provided features to specify which pipes and junctions should display data and which should not.

Open the Output Control window from the Analysis menu, change to the Show Selected Pipes/Jcts folder tab, and select the pipes and or junctions you want to display.

Open the Output Control window from the Analysis menu, change to the Format & Action folder tab, and choose to select the Transfer Results to Initial When Done, Transfer Valve States When Done, and Save Model When Results are Transferred options.

Yes, the Scenario Manager lets you create dependent design cases where changes are inherited by children.

Multiple models can be run sequentially using the Batch Run feature.

No. Depending on your goal, you need only specify the costs for which you are interested. If you are looking for just a first-cost estimate then you need to only specify Material and possibly Installation. Remember, these are very broad categories and can be tailored to meet your specific application.

Yes. You can specify an Interest Rate and an Inflation Rate which are used over the System Life.

Costs are entered in a one or more user-defined Cost Databases using the Cost Database editor. You can keep separate databases for each type of cost or put all the costs for a project into one database. These databases can also be used in multiple models and can be shared among all the users over a network.

Yes. The Cost Report is fully customizable through the Output Control. You can set the column types and order, units used, number of significant digits and order of magnitude displayed.

See ‘Libraries’ in the AFT Arrow help file.

See ‘Libraries’ in the AFT Arrow help file.

AFT Arrow solves the governing equations of compressible flow without simplifying assumptions, and is thus more accurate than traditional handbook methods. See the Solution Method Overview article in the AFT Arrow Help system for more information.

A junction has several advantages. First, solutions are given at all junctions, so the user can check the results at the junction. In contrast, Additional Losses are lumped into the pipe and it is not possible to give results at the loss. Second, many junctions (such as valves) have the ability to specify a CdA for sonic choking calculations. No such ability exists for Additional Losses; thus, sonic choking is always ignored for Additional Losses. Third, when using a junction the location in the pipe system of the pressure loss is specified. In other words, the upstream and downstream pipe lengths are specified. Since the location of the junction loss along a pipe can affect how much pressure is lost, the junction loss calculation is more accurate than the Additional Loss. A short explanation of this is that the pressure loss due to a K factor depends on the velocity squared. Typically, the velocity of the gas will change along the pipe, so the pressure drop of the K factor loss will depend on the local velocity. In the case of Additional Loss, it assumed to act like a friction factor and be evenly distributed along the pipe. The Additional Loss approach has the advantage of being able to specify multiple losses quickly and easily, and not cluttering up the model Workspace with numerous junctions.

There is an overlap in capability between the Tank junction and an Assigned Pressure junction, for example both allow up to 25 pipes connecting. Here are the differences. The Tank junction input pressure and temperature always corresponds to stagnation properties. In the Assigned Pressure junction, they can correspond to either static or stagnation properties. The ability to represent static conditions is the reason the Assigned Pressure can connect only to one pipe. Static conditions have a flow velocity associated with them and multiple pipes would have multiple velocities. If the stagnation option is used in the Assigned Pressure junction, it will behave identically to a Tank junction. Finally, the Tank junction can act as the reference pressure for a closed system, while the Assigned Pressure cannot.

AFT Arrow uses the most sophisticated models available to calculate losses at tees and wyes. The methods, which come from Idelchik, take into account losses that depend on the flow split, area change and angle of the branch pipe.

See Size a Compressor or Fan.

Enter fan data in the Compressor/Fan Properties window, and set the fan setpoint directly. No entry is assumed to be 100% speed. For speeds other than 100%, AFT Arrow uses the affinity laws to adjust the curve.

Enter the fan data in the Compressor/Fan Properties window, select the option to give a reference density, and then set the reference density for that fan curve. AFT Arrow will then modify the curve based on operating densities.

Enter the fan data in the Compressor/Fan Properties window and set the desired flow or discharge pressure in the “Variable Speed” tab. AFT Arrow will calculate the fan speed required to meet the setpoint and display the speed in the Compressor/Fan Summary report in the output.

Select the pipe or junction you want to close and choose Special Condition from the Edit menu. By default, AFT Arrow will display a red “X” next to the pipe or junction label on the Workspace. It will also redraw your Workspace and show the closed sections of the model with dashed lines for the pipes and dashed outlines for each junction. Some special condition settings do not close the junction but have different purposes. For example, the normal condition for a relief valve is to be closed, so applying special conditions force it to be open.

See the Using Manufacturer Pressure Drop Data article in the AFT Arrow Help system.

You can model a relief valve using either a Relief Valve junction or a regular Valve junction. The Relief Valve junction is always closed when you run the model (unless you set its Special Condition), and AFT Arrow will run the Solver to determine if sufficient pressure exists to crack it. If so, it will run the Solver again with the relief valve open. If you know the condition you are modeling will crack the relief valve, it is more efficient to just use a regular valve junction. In this case, AFT Arrow assumes the valve is open from the start, and does not have to go through the extra step of solving the network to check for sufficient system pressure to crack it.

The definition of Cv for a gas valve differs among industries and vendors. AFT Arrow uses the ANSI/ISA-75.01.01-2012 standards to define Cv. If the valve Cv does not match that standard, the next best option would be to use a spreadsheet to generate a pressure drop profile over the anticipated flow rate range and enter that data into AFT Arrow as a Resistance Curve for the valve.

You must specify the CdA on the Optional tab for the junction. AFT Arrow uses this as the basis area for sonic calculations.

You can use a Branch junction to model a desuperheater. Enter an inflow (source flow) at the branch and specify the enthalpy of the source. AFT Arrow will perform an energy balance using this enthalpy, and assume the discharge is still superheated, but at the new lower energy level.

The Global Pipe Edit and Global Junction Edit windows offer tremendous power and flexibility in changing all or parts of your model input all at once. See Global Pipe Edit and Global Junction Edit topics for more information.

You can create gas mixtures using either the included NIST REFPROP database or the optional Chempak add-on database. The AFT Standard gas database does not support mixtures. To create a mixture, choose either the NIST REFPROP or Chempak databases, click the “Create New Mixture and Add…” button, and specify the desired components and composition of the mixture.

You can model dynamic gas mixtures using either the included NIST REFPROP database or the optional Chempak add-on database. The AFT Standard gas database does not support mixing. To model dynamic mixing, open the System Properties window from the Analysis menu, choose either the NIST REFPROP or Chempak databases, and then choose the gases in the list and add them to the model. You can add any number of gases, and also create gas mixtures and add them. Once the desired gases are added to the model, they can be assigned to different source junctions such as Tanks, Assigned Pressures and Assigned Flows. AFT Arrow applies the gas or gas mixture to the junction’s connecting pipes. It then solves the network, carrying the gas composition to each section of the model in accordance with species mass conservation. As the solution progresses, the mass flows will adjust as they approach convergence and AFT Arrow updates the concentration balance for each iteration based on the current global mass balance.

Set the elevations in the elevation fields for each junction. Pipes are assumed to be straight between junctions. If you need to model a system other than a stationary earth-based system, the gravitational acceleration can be changed in the System Properties window.

Specify the rotational speed in the System Properties window (opened from the Analysis menu). The zero datum is the rotational centerline. Elevation input in Junction windows take on the alternate meaning of distance to rotational centerline. See Rotating Systems for more information.

Yes, either Panhandle or Weymouth can be specified by selecting them in the Pipe Properties window.

Open the Junction Properties window, click the Optional folder tab, and then click the Change Icon button.

No, icons are in a resource file that cannot be edited by the user. 

There are three ways to enable or disable the Highlight feature. The first is toggling the option on the Options menu. The second is pressing the F2 function key while in a Pipe and Junction Specifications window. The third is double-clicking the anywhere in the Pipe and Junction Specifications window.

Select the pipe or pipes and choose the Reverse Direction feature on the Arrange menu.

 Use the Segment Pipe tool found on the Arrange Menu.

Use the Merge feature on the File menu to merge models together.

The Global Pipe Edit and Global Junction Edit windows offer tremendous power and flexibility in changing all or parts of your model input all at once. See Global Pipe Edit and Global Junction Edit topics for more information.

Open the pipe or junction specifications window, click the Optional folder tab, and check or clear the check boxes for showing the number or name. This will affect the current pipe or junction. You can set the default behavior in the Workspace Preferences window available on the Options menu.

You can use the Global Edit windows to change the current model settings for all pipes or junctions or only selected ones. See the Global Pipe Edit or Global Junction Edit window for more information.

Use the Find feature to quickly find a pipe or junction.

If you hold down the CTRL key when completing the pipe drawing (just before releasing the mouse button), the Pipe Drawing tool remains active, and you can draw a series of pipes without returning to the Toolbox each time. If you double-click the Pipe Drawing tool it remains active until you click it again a single time. This allows you to draw a series of pipes without returning to the Toolbox each time.

In the Model Data tabular displays double-click on a row to launch that pipe/junction’s properties window.

The preferred units can be chosen when the software is first opened by selecting unit sets in the Startup window. Alternatively, this can be done by going to the Tools menu and opening User Options.  On the left pane select Preferred Units under the Unit System header. You can then select a commonly used set of units, or change individual unit types.

Arrow XTS can model a wide range of transients, including tank pressures, valves open/close, compressor changes, relief valve opening and more. Just as important, Arrow XTS can model time transients, event transients and inherent transients. Time transients are changes that are specified to occur based on the model time. Event transients, on the other hand, are triggered by an event that occurs in the system. Many kinds of events can be used, such as pressure, flow, velocity and liquid level. Inherent transients are changes that occur by the very nature of a system component. One example would be depressurization of a tank, while another would be a relief valve opening and closing as a result of pressure changes.

While both applications do time transient modeling, they use entirely different methods and are intended for different applications. AFT xStream models what are sometimes referred to as 'fast transients', transients dominated by wave speed, communication time, and momentum and pipe stiffness. Arrow XTS, for extended time simulation, models 'slow transients', slow in the sense that factors such as wave speed, momentum, etc. can be ignored. Where AFT xStream uses the method of characteristics to calculate transients, Arrow XTS runs a series of steady-state solutions with changing parameters, e.g. tank pressure, transferred from one-time step solution to the next.

Arrow XTS considers the thermophysical effects of pipe flow (such as the Joule-Thomson effect and heat of compression) and specified heat transfer (such as through a heat exchanger) but does not consider the heat transfer interactions between the fluid and pipe or to the surroundings.

This is because Arrow XTS does not capture the transitionary period between its assumed steady-state operating points. Instead, it is implied the entire system reacts instantly to changes in boundary conditions. This also implies that advective waves which travel with the fluid at the bulk velocity are also not considered, such as the introduction of a hotter stream to the system.

Without considering these transitions, Arrow XTS would assume heat transfer is always fully developed at these new intermediate operating points, which has more potential to produce unreasonable results.

No, there does not need to be a one-to-one relationship in the number of Arrow module licenses and AFT Arrow licenses. This provides users the flexibility to choose the level of capability they need. For example, you may have several AFT Arrow licenses deployed on your network but only need to use Arrow XTS half the time and, therefore, have a correspondingly lower number of XTS licenses. If you open a model previously saved using a Arrow module, AFT Arrow will first try to activate the module. If all of the module licenses are being used, a message will be displayed telling you module license is not available and provide you a choice of continuing to open the model or canceling the load. This same message will remind you that if you do open and work with the model, module specifications contained in the model may be lost.

Arrow GSC, Goal Seek & Control, extends Arrow's capabilities in two ways. One is goal seeking, automatically finding the value of one or more inputs that will produce a specify value for one or more outputs. The second is to extend AFT Arrow's control modeling capabilities to remote sensing. AFT Arrow can model flow and pressure control valves and controlled, variable speed pumps. With basic Arrow, the variable controlled is the one at the control component. That is, a pressure reducing valve controls the outlet pressure at the valve. With GSC, control can be based on a variable located anywhere in the system Instead of controlling the pressure at the valve outlet, it could be a pressure downstream at some other location. Additionally, control can be based on the difference between two variables, such as a differential pressure between two points, and on group sums, maximums or minimums, simulating some of the more sophisticated control logic that is sometimes implemented in real systems through computer based control systems.

While there is no limit, the time required to find a solution will increase with the number of variables and goals that must be solved since the number of possible variations increases exponentially with the number of variables and goals. Arrow GSC employs a robust and efficient numerical optimization engine to be able to handle models with large numbers of variables and goals.