Field calculator

Fan and pump affinity laws calculator

The affinity laws describe how a fan or pump responds when you change its speed with the impeller diameter fixed. Flow varies directly with speed, pressure or head varies with the square of the speed, and power varies with the cube. Enter the old and new speed (RPM or percent speed) and any value you know now, the flow in CFM or GPM, the pressure or head, or the brake power in horsepower, and the calculator scales each one to the new speed. The cube law on power is the reason a variable frequency drive saves so much energy: slowing a fan to 80 percent speed drops the power draw to roughly half. Treat these as ideal relationships and confirm the operating point against the actual fan or pump curve and the system curve, since the system curve and static head shift the real result.

anvilfield.com/calculators/fan-pump-affinity-laws-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.

More HVAC calculators

ACH to CFM calculatorFind the airflow for a target air change rate: CFM = room volume x ACH / 60, from floor area and ceiling height.Ventilation rate calculatorFind the breathing-zone outdoor air a space needs: Vbz = Rp x Pz + Ra x Az.Tons to BTU calculatorConvert a cooling load to tons of refrigeration: tons = BTU per hour divided by 12,000.Dew point calculatorFind the dew point from air temperature and relative humidity, and the surface temperature condensation forms at.Duct equivalent diameterConvert a rectangular duct to its equal-friction round diameter: De = 1.30 x (a x b)^0.625 / (a + b)^0.250.Duct airflow calculatorSolve duct airflow and velocity from the duct size using continuity (Q = V x A): enter a round or rectangular duct and either the CFM or the velocity, and get the other.

Affinity laws FAQ

How do I measure total external static pressure?

Zero a digital manometer, then drill 3/8 in test ports outside the cabinet and read the return side after the filter and the supply side after the coil. The return reads negative and the supply positive. Add the two magnitudes for TESP, with the system running in high-stage cooling.

What external static pressure is too high?

A reading is too high when it exceeds the equipment's rated maximum, often about 0.5 in. wg for residential PSC systems, but read the blower table. PSC systems normally run 0.3 to 0.5 in. wg and ECM systems 0.5 to 0.8. Above the rating, airflow collapses on a PSC blower.

PSC vs ECM: how does high static pressure affect each?

A PSC blower spins at a fixed speed, so high static drops its airflow sharply, which is why high static usually means low CFM on PSC equipment. An ECM constant-airflow motor speeds up to hold its target CFM, so high static shows up as wasted watts and noise instead of lost airflow, until it hits its limit.

Is 0.5 in. wg static pressure too high?

On most residential PSC equipment, 0.5 in. wg is at or near the rated maximum, so it is the ceiling, not a comfortable target, and you want headroom under it. On ECM equipment 0.5 in. wg is normal. Either way, read the blower table for the actual unit instead of the rule of thumb.

What do I do if my external static pressure is too high?

Reduce restriction, do not turn up the blower. Change the filter and re-measure first, since it is often half the problem. If the return side is high, add return area. Clean a dirty coil, open closed dampers, and replace the worst fittings. Re-measure after every change to prove it worked.

More in the External static pressure field guide field guide.