Resistance Curve Loss Model

The resistance curve loss model is available in many of the junctions in Arrow. It is useful when measured flow vs loss data is available for a component.

It is not uncommon to obtain from an equipment manufacturer a single data point. For example, the data might be 3 psi pressure drop at 1 lbm/sec. Along with this data point, another always comes along for free. The second data point is zero pressure drop at zero flow.

The pressure drop characteristics of most equipment follows the square law. As the flow rate increases, the pressure increases with the square of the flow rate.

Therefore, in our example, an increase in the flow rate to 2 lbm/sec would result in a pressure drop of 12 psi. Although this may not be exactly correct, it is much better than assuming a flat 3 psi drop for all flow rates, which we know for certain is not correct.

The Fill As Quadratic button found in the Curve Fit window can be used to create a quadratic curve out of a single data point as was shown in the below example. To do this, just enter the single data point in the table, click the Fill As Quadratic, then click Generate curve Fit to finish creating the curve.

Table 1: Example data points for pressure drop modeling using manufacturer data

Data Point Flow Rate
Pressure Drop
1 0 0 Always valid for passive equipment
2 1 3 Given by manufacturer
3 2 12 Extrapolated assuming square law