Overpressure

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Overpressure is the designation for pressure that is measured relative to atmospheric pressure or air pressure . The designation is used again in current standards and, according to their definition, can also be negative. This then corresponds to the term negative pressure, which is no longer used .

Pressure used to be measured in at (technical atmosphere) or atm (physical atmosphere), overpressure in atü (atmospheric overpressure), whereby the numerical value of the overpressure was exactly 1 lower than the absolute pressure. This unit is no longer used today, the unit Pascal or bar has taken its place .

Applications

Clean room

In a clean room , the ingress of dust is prevented by the fact that there is permanent overpressure in the room.

Tunneling

In the early days of tunnel construction , especially when crossing rivers, the ingress of water was prevented by the overpressure prevailing in the tunnel. The physical properties of an artificially built up overpressure are used, among other things, in modern tunnel construction as a safety measure by building a secondary gallery in which overpressure prevails when a tunnel system is being designed. If a fire breaks out, rescue units can get to the scene of the accident without having to use the smoke-filled tunnel.

Building security

Modern buildings, especially high-rise buildings, often have one or more stairwells that are equipped with a positive pressure system. Comparable to the application in tunnel construction, these also serve to keep toxic smoke away from the escape routes. If there is a fire on a floor, the pressure of the air increases due to the expansion (warm air expands). Since the volume is usually limited, there is an increase in pressure. If a person escapes from this floor, smoke from the floor could get into the stairwell. This is prevented by a higher pressure in the stairwell (15 Pa min to 100 Pa max compared to ambient pressure). This means that the stairwell remains smoke-free and can be used for escape without danger.

Bridge building

Establishing of bridge piers under the water surface are often open to the bottom caissons ( caissons used) in the external compressed air is introduced to displace the water and to allow a dry working.

military

Important military systems, such as bunkers, but also vehicles and tanks are equipped with an overpressure system to prevent the penetration of warfare agents ( NBC protection ).

Storage and transport of gases

Gases , especially compressed gases , are transported or stored under often high overpressure up to liquefaction. This happens both in pipelines (e.g. transport of natural gas) and in pressure vessels (e.g. natural gas and hydrogen tanks in vehicles) and gas bottles (mainly chemical and medical applications).

Other uses

generation

Overpressure in a room arises from the fact that at the same time a larger volume flow is introduced into this room than is discharged, or the existing room is reduced without material being able to leave the room (displacement in a piston).

See also

literature

  • W. Schüle: Technical Thermodynamics . First volume, fourth edition, Springer Verlag Berlin Heidelberg, Berlin / Heidelberg 1923.
  • Hugo Krause: Machine elements . Third edition, Springer Verlag Berlin Heidelberg, Berlin / Heidelberg 1920.

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