When plotting compressor/fan and system curves with two or more parallel compressors/fans, AFT Arrow offers two methods: Enhanced and Traditional.

It should be understood by the engineer that it is not possible to create an unambiguous composite compressor/fan and system curve for parallel compressor/fan systems. The reasons why are discussed here.

First, let's discuss the pressure curve. A composite compressor/fan curve shows a single pressure curve with values at particular flow rates. However, when there are parallel compressors/fans with unequal pressure curves or even equal pressure curves with unequal flow distribution, it is not clear what flow rate to use in the composite pressure curve. Further, with unequal flow distribution, different pressures for each parallel compressor/fan can result. But the composite pressure curve shows only a single pressure for any given flow. What pressure is this? The maximum, minimum, arithmetic average, flow rate weighted average, or some other value? A composite curve has uncertainty over what is meant by the composite flow rate and composite pressure.

Composite system curves have similar issues to composite pressure curves. The entire system performance is boiled down to a single pressure vs. flow rate curve. What flow rate is being referred to and what pressure is being referred to which goes with a given flow rate?

AFT Arrow attempts to create as meaningful and helpful composite compressor/fan and system curves as possible, but the fact remains that the curves AFT Arrow creates are ambiguous because of the nature of composite pressure vs.system curves.

Enhanced

The "enhanced" method was introduced in AFT Arrow 4.

For composite pressure curves, it assumes that the parallel compressor/fan pressure curves are added to the right with respect to flow rate. No attempt is made to apportion flow splits among the compressor/fan curves. With the pressure curves added to the right, there is a unique pressure value for every flow rate. In reality, the parallel compressors/fans may not work this way, depending on the flow split among the compressors/fans, whether or not the pressure curves are identical, and whether there are check valves at each compressor/fan discharge. This is, however, a reasonable approximation and represents the historical way that parallel compressor/fan pressure curves are added together.

No matter what method is used for creating a composite system curve, composite system curves are created by fixing some flow rate to each compressor/fan and then determining the required pressure from the compressor/fan to deliver the assumed flow rate.

The enhanced system curve takes the flow split at the operating point and assumes that the flow split remains the same at all flow rates. It collects the required pressure for each compressor at each flow rate and then averages these pressures together arithmetically.

If the compressor/fan pressure curves are identical and the flow split among compressors/fans is proportional, this method will give the same results as the Traditional method.

Traditional

The "traditional" method was used in AFT Arrow 2.0 and 3.0. The traditional method assumes that the flow splits equally among all compressors/fans at all flow rates, regardless of whether compressor/fan curves are identical or not.

The composite pressure curve is created by taking the composite flow rate, splitting it equally among each compressor/fan, obtaining the pressure for each compressor/fan at the split flow rate, and then taking the arithmetic average of the compressor/fan pressures.

The composite system curve is obtained by taking the composite flow rate, splitting it equally among each compressor/fan, running the system model with these flow rates, and then taking the arithmetic average of the required pressures for each compressor/fan.