Ductwork is often treated as a separate discipline from fan selection. The duct designer calculates sizes and pressure drops, the fan selector picks a unit to match the total, and the two meet only at the system curve. This separation works on paper. In practice, ductwork design decisions made early in a project determine whether the selected fan can actually deliver the required performance-or whether it fights the duct system at every operating point.
Duct Sizing Sets the System Curve
The relationship between airflow and resistance in a duct system is not linear. Pressure drop increases approximately with the square of velocity. Double the airspeed, and the resistance quadruples. This means duct sizing decisions have an outsized influence on the system curve and, consequently, on the fan's operating point.
Undersized ducts create a steep system curve. The fan must operate at high pressure and low airflow relative to its capacity. This pushes the operating point toward the left side of the curve, where efficiency drops and power consumption may rise unexpectedly. Oversized ducts reduce resistance but increase material and space costs. The right balance depends on the specific project constraints, but the fan selector needs to know the resulting system curve accurately.
Fittings Generate Most of the Loss
Straight duct runs are rarely the dominant source of resistance. Elbows, transitions, branch takeoffs, and reductions account for the majority of pressure drop in most systems. A single square elbow can add as much resistance as 10 to 15 feet of straight duct. The cumulative effect of multiple fittings, especially when placed close together, can shift the system curve significantly.
Computational tools can model these losses, but the input data must be accurate. Duct designers sometimes use approximate fitting loss coefficients that don't reflect actual geometry. A rounded elbow with turning vanes has a fraction of the loss of a miter elbow. The difference shows up on the fan curve as a shifted operating point.
Velocity Considerations
Duct velocity affects more than pressure drop. High velocities increase noise, which may require silencers that add their own resistance. Low velocities reduce transport capability, allowing dust or moisture to settle in horizontal runs. The chosen velocity range influences both the system curve and the air distribution quality at the terminals.
For fan selection, the critical velocity is at the fan inlet and outlet connections. As discussed previously, poor inlet and outlet conditions caused by abrupt transitions or close-coupled fittings reduce fan performance. The ductwork within the first few diameters of the fan has a disproportionate impact on actual output.
System Effect from Duct Connections
The interaction between the fan and the connected ductwork creates system effects that shift the operating point. A discharge elbow immediately at the fan outlet creates asymmetric pressure at the wheel periphery. An inlet elbow introduces swirl that the impeller cannot fully recover. These effects are quantifiable using AMCA Standard 201 methods, but they are often overlooked in the selection process.
The consequence is a fan that meets the specification in the laboratory but underperforms in the field. The gap between catalog performance and installed performance is frequently the result of duct connection geometry that wasn't accounted for during selection.
Coordinating Design and Selection
Effective coordination requires the duct designer and fan selector to work from the same system curve. The curve should reflect actual fitting losses, not just straight-duct equivalents. It should include the effect of filters, coils, and other components discussed in our filtration and AHU articles. And it should account for system effect at the fan connections.
MIDI Industrial provides selection support that integrates ductwork considerations into the fan specification. We work with system designers to develop accurate system curves that reflect real installation geometry, not just theoretical calculations. The result is a fan selection that performs as expected from the first day of operation.