The Same Airflow and Pressure Numbers Can Mean a Different Fan Duty
A fan requirement such as 20,000 m³/h at 2,000 Pa may look complete. It is not. If the gas temperature is 20°C, 120°C or 200°C, the gas density changes. That change affects the pressure generated by the fan, absorbed power, mass flow and the way airflow data should be interpreted.
For high-temperature ventilation, boiler induced draft, kiln exhaust and process-gas systems, temperature is part of the fan performance condition - not only a material-selection condition.
The First Question Is Not Only "How Hot Is the Gas?"
For the same gas at approximately the same absolute pressure, increasing temperature reduces density. In simplified form:
Approximate dry-air values near atmospheric pressure for explanation only. Actual project density should be calculated from gas composition, temperature and absolute pressure.
What Does "20,000 m³/h" Actually Mean?
One of the most common problems in industrial fan RFQs is an airflow value without a reference condition. We still need to know whether the value is actual airflow at the fan inlet or airflow converted to a normal or standard condition.
Actual m³/h at the Fan Inlet
Example: 20,000 m³/h at 200°C means the fan actually handles that physical volume under the real operating condition.
Nm³/h or Sm³/h Requires a Defined Reference
"Normal" and "standard" are not defined identically in every industry or country. The reference temperature and pressure should be stated explicitly.
Hot Gas Requires More Actual Volume for the Same Mass Flow
20,000 m³/h
≈ 32,000 m³/h for similar mass flow
This simplified example assumes approximately the same absolute pressure and dry air. It shows why confusing actual m³/h with referenced m³/h can lead to a significantly incorrect fan selection.
Fan Pressure Changes with Gas Density at the Same Fan Speed
For the same fan geometry and approximately the same rotational speed, fan pressure at corresponding operating conditions is approximately proportional to gas density.
If a curve is based on ρ = 1.20 kg/m³ but the actual hot-gas density is 0.75 kg/m³, the density ratio is about 0.625. A 2,000 Pa reference-density point would therefore correspond to roughly 1,250 Pa at the lower density under equivalent fan conditions.
What Must Be Confirmed
Actual gas temperature
Absolute inlet pressure
Gas composition
Fan speed
Pressure definition
A pressure curve is meaningful only when its reference gas density is understood.
The Hottest Condition Is Not Always the Highest Motor Load
Aerodynamic absorbed power at corresponding fan conditions is also approximately proportional to gas density. Hot gas can therefore require less shaft power than colder, denser gas - but that does not mean the motor should automatically be reduced.
Cold Start
Colder gas is denser and can increase fan pressure and shaft load during start-up.
System Resistance
Damper position and actual system resistance can shift the operating point and motor load.
Speed / VFD
Speed variation can have a much larger effect on pressure and power than density correction alone.
Motor Margin
Motor sizing should cover the realistic operating envelope rather than one hot-gas point.
The hottest condition may control material design, while a colder condition may influence maximum fan power.
"2,000 Pa" Is Still Incomplete Without a Pressure Definition
Airflow Basis
Actual m³/h, Nm³/h or Sm³/h?
Pressure Basis
Static pressure, total pressure or measured differential?
Temperature Basis
Normal operating temperature or short peak?
Gas / Site Basis
Gas composition, absolute pressure and altitude?
Why a 20°C Curve Cannot Simply Be Read as a 200°C Curve
Total Pressure: 2,400 Pa
Shaft Power: 20 kW
Corrected Pressure: ≈ 1,500 Pa
Corrected Shaft Power: ≈ 12.5 kW
This is a simplified density-correction illustration, not a complete fan selection. The final engineering review still needs the actual system curve, required flow basis, gas composition, inlet pressure, fan speed, motor margin and mechanical temperature requirements.
Temperature Is Not the Only Factor That Changes Density
Altitude
Higher altitude usually means lower atmospheric pressure and therefore lower gas density.
Absolute Pressure
A fan inlet under suction or pressurization should not automatically be treated as atmospheric.
Gas Composition
Flue gas and process gas mixtures do not necessarily have the same density as dry air at the same temperature.
Send the Operating Condition, Not Only the Fan Model
Flow Basis
Actual m³/h or referenced flow
Reference temperature
Reference pressure
Pressure Basis
Total pressure
System resistance
Inlet / outlet pressure
Temperature & Medium
Normal temperature
Peak temperature
Dust / moisture / corrosion
Where the Fan Operates
Voltage / frequency
Available space
Existing GA for replacement
From Process Data to a Verified Fan Duty Point
Flow Basis
Actual or referenced airflow?
Gas Condition
Temperature, pressure, composition, altitude.
Gas Density
Determine density at the fan operating condition.
Performance
Correct pressure, power and operating point.
Mechanical Review
Material, shaft, bearings, drive and motor.
Verification
Curve, motor, GA and installation interface.
Performance Data Must Be Read Together with the Operating Condition
Density correction is part of fan selection - not a substitute for fan selection.
Five Errors That Can Distort High-Temperature Fan Selection
Reading Standard-Air Pressure Directly
A standard-density curve should not automatically be treated as the same pressure curve for hot gas.
Mixing Nm³/h and Actual m³/h
The same numerical airflow can represent a different mass flow depending on the reference condition.
Looking Only at Maximum Temperature
Maximum temperature does not define normal density, start-up load or peak duration.
Ignoring Altitude
Lower atmospheric pressure at altitude can further reduce gas density.
Reducing Motor Power Automatically
Cold start, system resistance, speed range and motor margin must still be reviewed.
Practical Questions for High-Temperature Fan Selection
Does a centrifugal fan deliver less airflow at high temperature?
Not necessarily in simple volumetric terms. For the same fan speed, pressure and absorbed power change with density, while the actual operating point depends on the complete fan and system curves.
Why does fan pressure decrease when gas temperature increases?
If absolute pressure is similar, increasing temperature lowers gas density. Pressure at corresponding fan conditions is approximately proportional to density.
Does high temperature reduce fan motor power?
Aerodynamic absorbed power generally decreases with density under corresponding conditions, but motor selection must still consider cold start, system resistance, speed control and margin.
What is more important: m³/h or Nm³/h?
Neither is universally better. The important point is to state clearly whether airflow is actual or referenced and define the corresponding temperature and absolute pressure.
Continue from Gas Condition to Verified Fan Selection
Static vs Total Pressure
Confirm what the pressure value in an RFQ actually represents.
Read pressure guide →GA Drawing Review
Verify dimensions and installation interfaces after aerodynamic selection.
Read GA guide →Nameplate Data Guide
Use old fan and motor data correctly in replacement projects.
Read nameplate guide →High-Temperature Industrial Fans
Review material, bearing, cooling and drive requirements for hot-gas applications.
Browse high-temperature fans →Send the Gas Condition - Not Only the Airflow and Pressure
For high-temperature, boiler, kiln, furnace or process-exhaust fan selection, QIYUE FAN reviews the gas condition together with aerodynamic performance and mechanical configuration before proposing a fan.