Reading a Blower Datasheet Line by Line: What Each Number Hides

By the NextAir Systems engineering team. Last reviewed October 2026.

A blower datasheet is a table of numbers, and almost every number has a second question attached to it. Cubic metres per hour of what, at what temperature? Pressure measured where? Efficiency defined how? Two datasheets can look identical and describe machines that perform very differently, or look different and describe the same machine.

This guide goes down the sheet line by line. For each one it explains what the number means, the question to ask, and where the reading goes wrong.

The short version

• Always ask whether airflow is actual or normal. 50,000 Nm³/h at 0 °C is about 77,000 m³/h at 150 °C.
• Pressure on a datasheet may be static or total. Total pressure is static plus the velocity pressure at the fan outlet, and efficiency is defined on whichever basis is stated.
• Tip speed, π × diameter × rpm, is a fast sanity check. 93 m/s for a 1.2 m wheel at 1,480 rpm.
• AMCA’s Publication 201 explains why a fan’s installed performance can fall short of its rating, which is why the test basis and the system effects are part of the datasheet.^[1]

Line by line

Line	Ask	Why it matters
Airflow	Actual (m³/h) or normal (Nm³/h)?	50,000 Nm³/h is 77,457 m³/h at 150 °C. A 55% difference.
Pressure	Static or total? At what density?	Total = static + dynamic. At 18 m/s outlet, dynamic pressure is about 190 Pa at 1.2 kg/m³.
Gas condition	Temperature, altitude, composition, moisture	Density falls from 1.2 kg/m³ at 20 °C to 0.83 at 150 °C, and pressure and power fall with it.
Efficiency	Total or static? At the duty point or the peak?	Total efficiency is higher than static efficiency for the same fan.
Shaft power	At the duty point, for this gas density	Motor rating must cover the worst case (cold start, high density), not the hot duty.
Speed and tip speed	rpm, wheel diameter, tip speed	A 1,200 mm wheel at 1,480 rpm has a tip speed of 93 m/s.
Sound	Sound power or sound pressure, and at what distance?	Sound pressure at 1 m and sound power are different quantities.
Test basis	ISO 5801 or AMCA 210?	See the guarantee conditions and the tolerance.

Take each in turn.

Airflow

Volume flow depends on temperature and pressure, so “50,000” means nothing without a unit and a condition. Normal cubic metres (Nm³/h) refer to 0 °C and 101.325 kPa. Actual cubic metres are at the gas condition at the fan. The ratio for dry air is the ratio of absolute temperatures: at 150 °C the actual flow is 1.55 times the normal flow. A buyer who thinks he is getting 50,000 m³/h and receives a fan for 50,000 Nm³/h has bought a bigger fan than he asked for, and the reverse mistake leaves the process short of air. The correction is the subject of gas temperature, altitude and density correction. Gas composition shifts density too, as the gas composition article shows.

Pressure

Static pressure is the pressure that overcomes the system resistance. Total pressure adds the velocity pressure of the air leaving the fan. Datasheets quote either, and some quote “pressure” with no qualifier. The difference is small for low-velocity fans and large for high-speed ones. For an 18 m/s outlet at 1.2 kg/m³, velocity pressure is 0.5 × 1.2 × 18² = 194 Pa. Whichever basis is used, ask for the pressure at the stated gas density, because pressure scales with density.

Efficiency

Efficiency is airflow times pressure divided by shaft power. Total efficiency uses total pressure, and static efficiency uses static pressure, so a fan’s static efficiency is always lower. A datasheet that states 84% without saying which is not complete. Ask for it at the duty point, because the peak efficiency of the fan is higher than the efficiency you will actually get if the duty point is off the peak.

Power and the motor

Shaft power at the duty point is airflow × pressure ÷ efficiency. The motor must cover more than that. Cold air is denser, so a fan sized for a hot gas will draw more power at start-up before the gas heats, and the motor and starting method must cope. Our guides on motors, drives and starting and motor efficiency cover the detail.

Speed and tip speed

Tip speed is π × diameter × speed ÷ 60. A 1,200 mm wheel at 1,480 rpm runs at 93 m/s. Tip speed drives stress in the wheel, noise, and wear in dusty gas. A datasheet that gives speed but not diameter hides it, so compute it yourself.

Sound and the test standard

Sound power is a property of the fan. Sound pressure depends on distance and environment, so “85 dB(A)” without a distance is not comparable. The last line, the test basis, decides whether the performance is a guaranteed value measured on a standard rig. AMCA 201 explains that fans in real systems can perform below their laboratory rating because of the connections around them, and the design engineer has to allow for it.^[1] Our guide to ISO 5801 and AMCA 210 explains the standards.

A short checklist

1. Convert every airflow to one basis before you compare.
2. Convert every pressure to total, or to static, and every efficiency to match.
3. Compute shaft power yourself from airflow, pressure and efficiency, and check that the sheet agrees.
4. Compute tip speed.
5. Ask for the performance curve and the operating point on it, as in the operating point guide.

Where this fits with NextAir

NextAir quotes list airflow on an actual basis with the gas condition, state static or total pressure and efficiency explicitly, and give the curve with the duty point marked. Send your datasheet requirements to our centrifugal blower manufacturer team. Our centrifugal blower guide covers selection, and steel plants are a typical high-temperature duty.

Sources

1. AMCA / ANSI Webstore. AMCA 201-23 Fans and Systems.
2. Plant Engineering (US DOE Motor System BestPractices material). Building energy efficiency into fan systems.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

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