Why the same fan makes less pressure in hot gas, at altitude or under suction — and how to refer a duty to standard air before you read a curve.
Reviewed by Jitamitra application engineering
Share on LinkedInWork out the gas density at the fan inlet from the site’s barometric pressure, the inlet suction and the gas temperature. Convert any normal or mass flow to actual m³/h, then scale the fan static pressure by 1.2 divided by that density to read the standard-air curve. The flow stays the same; pressure and power scale with density; the construction is still chosen for the real temperature.
Every fan curve in a catalogue, and every test result, is quoted at one air density. Almost no real duty runs at it. A fan in a hot-gas line, on a plant several hundred metres above sea level, pulling hard through a filter, sees gas noticeably thinner than the curve assumes — and the same fan then makes less pressure and draws less power than the curve says. This note shows how we refer every duty to standard air before it is compared with a curve, and the arithmetic behind it.
A duty is the actual volume at the fan inlet, the fan static pressure, and the density of the gas at the inlet. The density carries everything else: gas temperature, the site’s elevation (its barometric pressure), the suction at the fan inlet, humidity and gas composition. The elevation that matters is the site the fan is going to, so we ask for it on every enquiry rather than assume it from where the order comes from — one customer’s fans often go to several plants.
Standard air is 1.2 kg/m³ at a barometric pressure of 101.325 kPa, roughly 20 °C at sea level. It is the basis AMCA 99 defines and the one fan curves and test results are quoted on. To compare a real duty with a curve, refer the duty to that density first.
At a given speed a centrifugal fan is a constant-volume machine: it moves the same actual m³/h whatever the density. The pressure it develops and the power it draws both vary in direct proportion to density. So:
Two orders of work follow. A flow given in normal or mass terms (Nm³/h, kg/h) is converted to actual inlet m³/h before anything else. Then the pressure is scaled to standard air and the fan is read off the curve at that pressure.
Elevation alone takes a real share off the curve, before any heat or suction. Dry air on a 40 °C day:
| Site elevation | Barometric pressure, kPa | Air density at 40 °C, kg/m³ | Share of standard air |
|---|---|---|---|
| Sea level | 101.33 | 1.127 | 94% |
| 216 m (Delhi) | 98.76 | 1.099 | 92% |
| 555 m | 94.83 | 1.055 | 88% |
| 920 m (Bengaluru) | 90.75 | 1.010 | 84% |
A fan selected at standard air for a 40 °C plant at 920 m develops only about 84% of the pressure its curve shows. That gap is why the elevation question is on every enquiry.
A process exhaust stated as 50,000 Nm³/h of gas at 200 °C, with 400 mmWC of suction at the fan inlet, on a site 555 m above sea level, needing 300 mmWC fan static pressure at site:
| Step | Value |
|---|---|
| Stated duty | 50,000 Nm³/h at 0 °C and 101.325 kPa; gas at 200 °C; 400 mmWC suction at the fan inlet; site at 555 m; 300 mmWC fan static pressure at site |
| Barometric pressure at 555 m | 94.83 kPa |
| Absolute pressure at the fan inlet | 94.83 − 3.92 = 90.91 kPa |
| Actual inlet volume | 50,000 × (473.15 / 273.15) × (101.325 / 90.91) = 96,532 m³/h |
| Gas density at the fan inlet | 0.669 kg/m³ |
| Static pressure to read on a standard-air curve | 300 × 1.2 / 0.669 = 538 mmWC |
| Cold start at 30 °C, same site and suction | 1.045 kg/m³, so 1.56 × the hot running power |
Read on the stated normal flow, the fan would be undersized by nearly half. Read at standard-air pressure without the elevation and the suction, it would still fall short of its duty in service. The elevation and the suction are small corrections on their own; with the temperature, they decide whether the fan reaches its duty.
Inlet suction. AMCA 210 takes fan air density at the total pressure at the fan inlet. A fan pulling through a bag filter or a long suction duct sees gas below atmospheric pressure, so its density is lower than the room’s. Leave the suction out and the density is overstated.
Compressibility. Above about 2.5 kPa (around 255 mmWC) of fan pressure, the gas compresses measurably on its way through the fan, and a straight density ratio no longer holds exactly. AMCA 99 applies a compressibility coefficient, Kp, for this. We calculate it on every high-pressure duty rather than assume it.
Referring a duty to standard air is a way of reading the curve, not a description of the fan. The construction is chosen at the real maximum inlet temperature: materials, the heat slinger and its guard, the bearing arrangement and any cooling. And the motor is sized for the densest gas it will meet, which is usually the cold start. A hot-gas fan started cold draws far more power than it does in service, unless a damper is held closed while it comes up to speed.
The duty-point calculator does the density arithmetic for you, and how to specify a fan duty covers the other parameters an enquiry needs.
Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.
Sources & basis. Standard air (1.2 kg/m³ at 101.325 kPa) and the compressibility coefficient Kp: AMCA 99. Fan air density at the total pressure at the fan inlet: AMCA 210. The effect of temperature and altitude on fan pressure and power, and on motor sizing for a cold start, follows from the fan laws: at a given speed, pressure and power scale with gas density. Every figure on this page follows from the ideal-gas law for dry air (R = 287.055 J/kg·K) and the ISA barometric formula, worked as shown. Customer names and job numbers are withheld.
Air at 1.2 kg/m³ and a barometric pressure of 101.325 kPa, roughly 20 °C at sea level. It is the basis AMCA 99 defines, and fan curves and test results are quoted on it. Any real duty is compared with a curve only after it has been referred to that density.
The same volume, but less air by mass. At a given speed a centrifugal fan moves the same actual m³/h, so the volume stays the same while the kilograms of air per hour fall with the gas density, and so do the pressure it develops and the power it draws. At altitude, or in hot gas, the same fan makes less pressure than its standard-air curve shows.
The gas entering the fan is below atmospheric pressure by the inlet suction, and lower absolute pressure means lower density. AMCA 210 takes fan air density at the total pressure at the fan inlet, so a fan pulling hard on a filter or a long duct sees thinner gas than the atmosphere outside.
For the cold start, unless a damper is held closed while the fan comes up to speed. Cold gas is denser, so the fan draws more power until the system warms up. In the worked example on this page the cold start draws 1.56 times the hot running power.
No. Only the duty is standardised, so that it can be read off the curves. Materials, the heat slinger, the bearing arrangement and cooling are chosen at the real maximum inlet temperature.
Flow, static, gas temperature, application — or attach a spec, GA drawing or a multi-fan schedule. Engineer to engineer.
ISO 9001:2015 quality system · performance-tested to IS 4894 / ISO 5801 / AMCA 210 method · witnessed FAT on request.
*For our standard range. ATEX and special projects need 5 to 7 working days.
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