Series and Parallel Fan Curve Calculator

Concepts behind this calculation

Combine identical fan curves in parallel or series, then find their intersections with the actual system curve instead of multiplying a single duty-point airflow.

Identical fans in parallel contribute flow at a shared pressure, stretching the combined curve horizontally. Identical fans in series contribute pressure at a shared flow, stretching it vertically. These are combinations of fan curves, not promises to multiply the installed system airflow.

The combined curve must still intersect the same quadratic-plus-constant system model. Increased flow raises friction resistance, so added parallel fans generally do not produce a proportional flow increase. The ideal combination also omits installation interaction, failed-fan backflow, unequal performance, and sequencing effects that can change real results.

How to use this calculator

1. Enter Identical fans and select Fan connection.
2. Provide the single-fan Supplied fan curve at the applicable speed and density.
3. Enter System reference airflow, Friction pressure at reference airflow, and Flow-independent pressure term, then inspect the combined operating candidates.

Calculation method and applicability

Parallel: add flows at equal pressure. Series: add pressures at equal flow. Intersect the combined fan curve with P_constant + kQ².

Q = combined airflow (CFM); pressure is in. w.g. The number of fans is a count. Series curves add pressure at common flow; parallel curves add flow at common pressure.

Input definitions and units

Identical fans (count)

Whole-number count from 1 to 100. All entered fans are assumed to have the same curve at compatible speed/density.

Fan connection

Parallel adds flow at equal pressure; series adds pressure at equal flow. Choices: Parallel — shared pressure; Series — shared airflow.

Supplied fan curve (CFM, in. w.g.)

One flow and matching fan pressure per line, sorted by strictly increasing CFM. Use actual manufacturer/test data at the selected speed/density and a consistent total/static basis. Example is synthetic, not a real fan.

System reference airflow (CFM)

A known airflow used to define the system friction curve, not an assumed operating point.

Friction pressure at reference airflow (in. w.g.)

Flow-dependent friction component only. Exclude the separate flow-independent term below.

Flow-independent pressure term (in. w.g.)

Enter zero for a purely frictional closed air circuit. This is a system-curve constant, not fan static pressure or TESP.

How to interpret the result

Read combined airflow and pressure at the system intersection, rather than multiplying a single-fan duty point. Multiple intersections still require separate stability and equipment review.

Worked examples

Assumptions and limits

• Fan data are interpolated linearly only between supplied points. No extrapolation, manufacturer data download or invented fan curve is performed.
• System resistance is modeled as a quadratic friction term plus a nonnegative flow-independent term. Real equipment, controls and flow regimes may differ.
• Multiple intersections are all shown. Dynamic stability, stall, surge, transient behavior, noise and safe fan operating range need separate review.
• All fan/system pressures must use compatible total/static definitions and air-density conditions. The synthetic example is not a real product selection.
• This ideal combination excludes fan-to-fan interaction, inlet/outlet system effect, unequal fans, failed-fan backflow, control sequencing and actual installation losses.
• More fans do not guarantee proportionally more system airflow. Series/parallel curve combination still requires the shared system intersection.

Sources

• ASHRAE 2024 Systems and Equipment, Chapter 21: FansPrimary fan/system operating-point and series/parallel context. Piecewise-linear interpolation is the disclosed numerical model, not a certified product selection.