Engineering guide
The LUFT module calculates the thermophysical properties of dry air as a function of temperature and pressure according to the property data section of the VDI Wärmeatlas (VDI Heat Atlas, 12th edition 2019). Calculated quantities include density, specific heat capacity, enthalpy and entropy, thermal conductivity, dynamic and kinematic viscosity, thermal diffusivity, Prandtl number, real gas (compressibility) factor, speed of sound, as well as critical data and the composition of the mixture of nitrogen, oxygen and argon.
Anyone who needs to calculate air properties almost always needs them as inputs for further design work: air-cooled heat exchangers and dry cooling towers, compressed air systems, compressors and fans, drying and ventilation processes, or pressure drop calculations in ducts and piping. Since density and viscosity vary strongly with temperature and pressure, a single tabulated value is rarely sufficient; the module delivers consistent values for the actual operating point.
At moderate pressures, air behaves almost like an ideal gas, so many quantities can be estimated with simple equations of state; in addition, the module accounts for the real gas corrections and temperature dependencies according to the approaches of the VDI Wärmeatlas.
Scope
- Properties of air
Typical workflow
Define the operating state
The temperature and pressure of the state point for which the properties of dry air are required are entered, e.g. the mean air temperature of an air-cooled heat exchanger.
Composition and basic quantities
Dry air is treated as a mixture of nitrogen, oxygen and argon. From this, the molar mass and the specific gas constant are obtained as the basis for the caloric and thermal state variables.
Calculate thermal state variables
Density and real gas factor are determined from the equation of state; at moderate pressures the real gas factor is close to unity and the density practically follows the ideal gas law.
Evaluate caloric quantities
Specific heat capacity, enthalpy, entropy and the isentropic exponent are determined as functions of temperature from the correlations stored in the VDI Wärmeatlas; the speed of sound also follows from these.
Determine transport properties
Thermal conductivity and dynamic viscosity are calculated as functions of temperature. Derived from these are the kinematic viscosity, thermal diffusivity and Prandtl number, which enter directly into Nusselt correlations for heat transfer.
Frequently asked questions
Up to what pressure may air be treated as an ideal gas?
At ambient temperature, the real gas factor of air deviates from unity by only a few parts per thousand up to about 10 bar; for approximate density calculations, the ideal gas law is entirely sufficient there. At high pressures (compressed air storage, final compressor stages) or low temperatures near liquefaction, the deviations become significant and the module's real gas correction is required.
Does the module also apply to humid air?
No, LUFT provides the properties of dry air. The water vapor fraction noticeably changes density, heat capacity and enthalpy in particular. For state changes of humid air (humidification, dehumidification, mixing), a module based on the Mollier h-x diagram should be used, which explicitly balances the water content.
Which properties depend practically only on temperature?
The dynamic viscosity, thermal conductivity and specific heat capacity of ideal gases are nearly pressure-independent over wide ranges and increase with temperature. Pressure-dependent, on the other hand, are density, kinematic viscosity and thermal diffusivity, because density enters them directly. Taking the kinematic viscosity from tables at 1 bar and using it at higher pressure is a common and large error.
To which reference temperature should properties for heat transfer correlations be referred?
Most Nusselt correlations of the VDI Wärmeatlas use the mean fluid temperature (arithmetic mean of inlet and outlet) as the reference temperature; some correlations additionally require properties at the wall temperature for the correction term. The reference convention of the respective correlation must be followed, otherwise systematic errors in the heat transfer coefficient result.
Worked example
For the design of an air-cooled heat exchanger, the density and dynamic viscosity of dry air at 80 °C and 2 bar (abs.) are required — a worked example of how to calculate air properties for heat exchanger design.
Given
| Temperature t | 80 °C (T = 353.15 K) |
|---|---|
| Pressure p (abs.) | 2 bar = 200,000 Pa |
| Specific gas constant Rs | 287.06 J/(kg·K) |
Solution
Density from the ideal gas law
At 2 bar and 80 °C the real gas factor of air is practically 1, therefore:
ρ = p / (Rs · T) = 200,000 / (287.06 · 353.15) = 1.973 kg/m³
Dynamic viscosity according to Sutherland
With the Sutherland equation (η0 = 17.16 µPa·s at 273.15 K, S = 110.4 K):
η = η0 · (T/273.15)1.5 · (273.15 + S)/(T + S) = 17.16 · 1.470 · 0.827 ≈ 20.9 µPa·s
The viscosity is practically pressure-independent at this pressure.
Kinematic viscosity
ν = η / ρ = 20.9 · 10−6 / 1.973 = 1.06 · 10−5 m²/s
Note: compared with air at 1 bar, doubling the pressure halves the kinematic viscosity, because density increases in proportion to pressure.
Result
| Density ρ | 1.973 kg/m³ |
|---|---|
| Dynamic viscosity η | ≈ 20.9 µPa·s |
| Kinematic viscosity ν | ≈ 1.06 · 10⁻⁵ m²/s |
Technical content source: LV-Solved-Problems-Content 628d313247e7