Absolute pressure
In the construction of apparatus for thermal and mechanical process engineering, system pressures play an important role. For example, process engineering apparatus has a maximum permissible overpressure due to its design.
Furthermore, not all apparatus is suitable for withstanding a vacuum inside. A liquid container or a silo for powdery products can very quickly be damaged (dents) if a vacuum is applied inside. On the other hand, excessive pressure can also irreparably damage a device. It is therefore particularly important to clearly define the type of pressure.
The permissible excess pressure is based on atmospheric pressure. It is therefore a relative pressure indication. It can be positive or negative. For example, an indication may be as follows: Permissible excess pressure: -50 mbar to plus 50 mbar.
At sea level, atmospheric pressure fluctuates between 950 mbar and 1020 mbar depending on the weather. (At the top of Mount Everest, at an altitude of 8850 metres, atmospheric pressure is only 320 mbar).
In physics and research, fine vacuum and high vacuum play an important role. The following definitions are used
- Rough vacuum: 1 bar to 1 mbar
- Fine vacuum: 1 to 10-3 mbar
- High vacuum: 10-3 to 10-7 mbar
- Ultra-high vacuum: less than 10-7 mbar.
In synthesis reactors and vacuum mixing dryers, there are two different pressure chambers. These often have different values.
- Process chamber: This is the chamber in which the products are mixed. This also includes the attached filter.
- Jacket chamber (double jacket): This is the chamber in which the heat transfer medium (thermal oil, water or steam) circulates.
The permissible excess pressure has a great influence on the design of the apparatus and is extremely relevant to the price.
In the event of failure, pressure-bearing equipment can pose significant hazards. They must therefore be approved by the relevant authority for operation. The national standards/guidelines must be observed. These concern the type of strength calculation, the type of production, the certificates and the rules for approval.