Gas Temperature, Altitude and Density Correction for Blowers

A blower moves volume, but the work it does depends on mass. Because pressure, power, and the flow the process actually needs all depend on gas density, a blower selected from a catalogue at standard conditions can fall short at a hot or high site, or overload its motor on a cold morning. Density correction is the single most common step that is skipped in blower selection, and it is easy to do once you see the pattern. This guide gives the formulas, computed tables for temperature and altitude, and the two effects that catch people out.

The Basic Relationship

Gas density depends on pressure and temperature. For air treated as an ideal gas:

Density = absolute pressure ÷ (287.05 × absolute temperature in K)

Standard air is about 1.204 kg/m³, at 20 °C and sea level. At constant speed, a blower moves the same volume of gas whatever its density, but its pressure and power are proportional to density.

Density by Temperature at Sea Level

Gas temperature	Density (kg/m³)	Ratio to standard	A 4,000 Pa blower develops
20 °C	1.204	1.00	4,000 Pa
100 °C	0.946	0.79	3,143 Pa
200 °C	0.746	0.62	2,479 Pa
300 °C	0.616	0.51	2,046 Pa
400 °C	0.524	0.44	1,742 Pa
500 °C (oven limit)	0.457	0.38	1,517 Pa
650 °C (kiln limit)	0.382	0.32	1,270 Pa
800 °C (furnace limit)	0.329	0.27	1,093 Pa

Density by Altitude at 20 °C

Altitude	Density (kg/m³)	Ratio to standard
Sea level	1.204	1.00
600 m	1.121	0.93
1,000 m	1.068	0.89
1,800 m	0.968	0.80
2,400 m	0.899	0.75

A blower rated at 400 mm WC at standard air develops about 300 mm WC at 2,400 m, at the same speed.

Effect 1: Converting Normal and Actual Airflow

Process specifications often state airflow in normal cubic metres per hour (Nm³/h), normalised to 0 °C and standard pressure. The blower moves actual cubic metres. The conversion is:

Actual flow = normal flow × (101.325 ÷ absolute pressure in kPa) × (temperature in K ÷ 273.15)

Example. A burner needs 6,785 Nm³/h of combustion air. At 20 °C and sea level, the actual flow is 6,785 × 293.15 ÷ 273.15 = 7,282 m³/h. At a site 2,355 m above sea level, with air at 25 °C, the absolute pressure is about 76.1 kPa, and the actual flow is 6,785 × (101.325 ÷ 76.1) × (298.15 ÷ 273.15) = about 9,870 m³/h. The blower has to move about 35% more volume at the high site to supply the same mass of air.

Effect 2: Converting Required Pressure to a Standard-Air Rating

A required pressure at site density becomes a higher standard-air rating:

Standard-air rating = required pressure × (1.204 ÷ site density)

Example. A burner needs 350 mm WC at a site where the density is 0.889 kg/m³. The standard-air rating is 350 × 1.204 ÷ 0.889 = 474 mm WC, close to our 500 mm WC standard maximum. At sea level with 20 °C air, it would be 350 mm WC. So a duty that is comfortable at sea level can approach the ceiling of the range at high altitude.

Effect 3: The Hot-Gas Trap

When gas is hot, the same mass of gas occupies more volume, so it moves faster through the ducts, and pressure loss rises. At constant mass flow, system resistance is proportional to one over density. So as temperature rises, the system asks for more pressure while the blower can supply less.

Gas temperature	Volume vs 20 °C	System resistance (from 1,000 Pa at 20 °C)	Standard-air rating needed
20 °C	1.00	1,000 Pa	102 mm WC
300 °C	1.96	1,955 Pa	390 mm WC
500 °C	2.64	2,637 Pa	709 mm WC
800 °C	3.66	3,661 Pa	1,366 mm WC

This is why hot-gas systems are designed with low resistance. See high-temperature blowers for furnaces, kilns and ovens.

Moist Gas

Water vapour is lighter than air, so moist gas is less dense than dry gas at the same temperature. Saturated air at 60 °C has a density of about 0.98 kg/m³, compared with about 1.06 kg/m³ for dry air, roughly 7% lower. In scrubber and dryer duties, use the actual gas composition and moisture content when calculating density.

Effect 4: Motor Power at Cold Start

Power is proportional to density, so it is highest when the gas is densest. A blower on a site at 1,800 m with design air at 35 °C (0.921 kg/m³) draws about 7% more power on a cold morning at 15 °C (about 0.985 kg/m³). At start-up in a hot-gas system, the gas may be cold and dense for a while, before it heats up, so the motor sees the highest power at the start. Size the motor for the coldest, densest condition the blower will meet, including start-up.

A Simple Correction Procedure

1. Establish the gas composition, temperature, and site altitude.
2. Calculate the actual density.
3. Convert the process airflow to actual volume, if it is given as normal flow.
4. Calculate the required pressure at actual conditions, remembering the hot-gas effect.
5. Convert to a standard-air rating using the density ratio.
6. Check the rating against the blower’s range, and check motor power at the coldest, densest condition.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure, rated at standard air, with high-temperature designs for ovens up to 500 °C, kilns up to 650 °C, and furnaces up to 800 °C. Tell us the gas conditions and site altitude, and we will do the correction and tell you whether the duty is within range.

Have Us Do the Correction

Send your gas conditions and site altitude, and we’ll correct the duty and confirm it is within range. Contact us for a free quote.

Frequently Asked Questions