The physical properties of gas mixtures can be calculated approximately from the physical properties of the individual components through interpolation and mixing rules. In the following denotes the molar mass, the mole fraction and the mass fraction of the -th species (component) of the gas mixture.
The following relationships apply to ideal mixtures:
Mean molar mass :
Specific enthalpy :
Specific entropy :
- is the diffusion coefficient of the species in the component
- the diffusion coefficient of the species in the mixture.
(Mixing formula according to Wassiljewa)
- According to Mason and Saxena, the correction factors result from the viscosity coefficients and the molar masses of the components:
The adiabatic exponent of an ideal gas mixture results from the adiabatic exponent of the individual components:
Gas mixers and gas mixing systems
Technical gas mixtures are generated with the help of gas mixers and gas mixing systems (other names are also e.g. gas mixers, blenders, gas mixing stations or gas mixing systems) from individual gases or from already mixed gases (gas mixtures), e.g. B. as
- Shielding gas for welding technology (e.g. Ar / CO 2 )
- Synthetic air for the medicine or chemical industry (mixtures of air with oxygen for enriched air or of air or oxygen with nitrogen for lean air )
- Protective gas for the food industry (e.g. N 2 / CO 2 )
- Reaction gas for thermal deformation technology (e.g. Ar / O 2 / H 2 )
- Forming gas for steel and rolling mills (e.g. N 2 / H 2 ) or protective gas moistened with water vapor ( N 2 / H 2 / H 2 O )
- Forming gas for glass production (e.g. N 2 / H 2 )
- Admixture of CO 2 in biotechnology for gassing fermenters z. B. to regulate the pH of the medium
- Test gas for high analysis accuracy due to high purity and low manufacturing tolerance
These and many other gas mixtures are used in many industries, e.g. B.
- Plant and mechanical engineering
- Automotive industry
- Ferrous / non-ferrous metals
- power supply
- Glass industry
- Semiconductor technology
- Industrial furnace construction
- Synthetic resins
- Copper processing
- food industry
- Medical technology
- Pharmaceutical industry
- Welding and laser technology
Gas mixers are mostly used for mass-produced devices with capacities from a few liters per minute to approx. 500 Nm³ / h. Gas mixing systems, on the other hand, usually refer to devices with higher performance levels, which can be up to 10,000 Nm³ / h and are individually planned and manufactured.
Gas mixers are usually used when one or more gas mixtures are required in larger quantities or mixtures with variable components. In such cases, the use of a local gas mixer or a gas mixing system is more economical than the external supply of gases in gas cylinders or gas bundles. For example, the economical use of gas mixers to generate a typical welding gas mixture (8% CO 2 in 92% argon ) begins with around five welding points, depending on the gas purchase price and duty cycle. With a normal material flow, these locations would have a monthly gas requirement of 750 liters (corresponds to about 40 bottles of 18L), so that savings can be made when the unmixed gases and mixtures are obtained on site with the help of a gas mixer. The same applies to variable gas points such as hospitals, industrial plants, measuring systems and similar applications with variable scattering.
Gas mixers can be used as static, i.e. H. with one or more control valves or as dynamic systems, i.e. with automatic control valves. The gas mixtures are often checked by a gas analyzer and stored in buffer containers and normalized for pressure before they are sent to the point of consumption.
Types of mixed media
Gas mixers can be designed in four different versions. The difference lies in the mixing technique used and is determined by the application.
- Gas mixer with mechanical mixing valve
- Gas mixer with electronic mixing valve
- Gas mixer with pneumatic mass flow controller
- Gas mixer with mass flow controller
Gas mixer with mechanical mixing valve
Gas mixers with mechanical mixing valves have been in use since the early Middle Ages . With this method, gas mixtures can be reliably generated from all gases . The mechanical mixing valve has gas inlets and a gas outlet for the mixed gas. By turning the valve , the flow rates of the individual gases are regulated in an interplay of orifices and pistons, thus generating the required gas mixture. Gas mixers with mechanical mixing valves are suitable for continuous withdrawal or, with a gas container, also for discontinuous withdrawal.
Gas mixer with electric mixing valve
As with the mechanical mixing process, proportional or individual mixing valves for the inlet gases are moved here. A piston in connection with different orifices regulates the flow rate of the gases and thus produces the desired mixture. In contrast to gas mixers with mechanical mixing valves, the electric mixing valves are not operated by hand using a rotary knob, but by means of electric motors. These electric motors are usually operated via an electronic control.
Gas mixer with pneumatic flow regulator
Gas mixer with flow regulator
In contrast to the mechanical gas mixer, the volume flows of each individual gas involved are regulated here. A mass flow controller (MFC) is used for each gas. The volume flow of the gases is recorded in the respective mass flow controller by means of thermal conductivity and then regulated.
The volume flows of the individual gases are only then combined into a mixture. This enables a control that compensates the mass flow of the gases against disturbances such as pressure fluctuations or temperature influences.
Classification in the scheme of chemical substances
|Schematic classification of the substances|
- VDI Society for Process Technology and Chemical Engineering (GVC): VDI-Wärmeatlas; 6th edition, VDI-Verlag, Düsseldorf 1991
- Mason, EA, et al. SC Saxena: Phys. Fluids 1 (1958), 361
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- Calculation of the thermal conductivity of gas mixtures with improved interaction coefficients, In: Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications , NASA Reference Publication 1311, 1994, p. 22