air lock

 

compartment connecting two different environments, usually at different pressures, that enables personnel to transfer from one environment to the other. Space capsules have air locks to enable astronauts to move between the pressurized cabin and the near vacuum of space. A more common example is the air lock between the outer atmosphere and the working chamber of a caisson. By its means access can be gained to the working chamber without loss of pressure. It is also used at the head of tunnel excavations underwater. There is a door at each end. When the outer door of the air lock is opened, men or material may be admitted into the compartment. After the outer door is closed, compressed air is admitted to raise pressure in the air lock to the level of the pressure in the working chamber, and the inner door can be opened. The reverse of this procedure takes place on leaving the working chamber. Great care must be exercised in passing workmen through an air lock, so that the change of atmospheric pressure takes place gradually. Too sudden a change of pressure may cause decompression sickness.

An airlock is a device which permits the passage of people and objects between a pressure vessel and its surroundings while minimizing the change of pressure in the vessel and loss of air from it. The lock consists of a small chamber with two airtight doors in series which do not open simultaneously.

An airlock may also be used for passage between environments of different gases rather than different pressures, to minimise or prevent the gases from mixing.

An airlock may also be used underwater to allow passage between an air environment in a pressure vessel and a water environment outside, in which case the airlock can contain air or water. This is called a floodable airlock or an underwater airlock, and is used to prevent water under high pressure flooding a submersible or an underwater habitat.

Use

Before opening either door, the air pressure of the airlock (in the space between the doors) is equalized with that of the environment behind the next door to open. This is analogous to a waterway lock: a section of waterway with two watertight gates, in which the water level can be varied to match the water level on either side.

A gradual pressure transition minimizes air temperature fluctuations (see Boyle's law), which helps reduce fogging and condensation, decreases stresses on air seals and allows safe verification of pressure suit and space suit operation.

Where a person who is not in a pressure suit is moving between environments of greatly different pressures, an airlock can change the pressure slowly to help with internal air cavity equalization and to prevent decompression sickness. This is critical in Scuba diving, and a diver may have to sit in an underwater airlock for some hours in line with decompression rules.

Applications

Airlocks are used in

• spacecraft, to prevent pressure loss when exiting or entering the craft.
• hyperbaric chambers, to allow entry and exit while maintaining the pressure difference with the surroundings.
• submarines, diving chambers and underwater habitats to permit divers to exit and enter underwater. Torpedo tubes and escape trunks in submarines also act as a form of airlock.
• 'clean rooms', protected environments in which dust and other contaminants are prevented from coming in by maintaining the room at a higher pressure than the surroundings.
• hazardous environments, such as nuclear reactors and some biochemical laboratories, in which dust and particles are prevented from leaking out by maintaining the room at a lower pressure then the surroundings.
• pressurized domes such as the USF Sun Dome and BC Place.

Similar mechanisms

• In cold climates, two doors arranged in an airlock configuration are common in building entrances. While not airtight, the double doors minimize the loss of heated air from the building. Revolving doors may be used for the same purpose.
• Some jewelry stores have an airlock-like portal to slow the egress of criminals.
• Butterfly farms usually have an airlock-like entrance to prevent the exit of butterflies—and entrance of predatory species.

Fictional airlocks

A four-door airlock (with, therefore, three interior chambers) was proposed by science fiction writer H. Beam Piper in his novel Uller Uprising. In the proposal, the atmosphere inside the structure was human-breathable, while the outside atmosphere was highly toxic. Only one door of the airlock was to be opened at a time, and the middle chamber of the three would always contain a vacuum. Piper implied that this design would minimize traces of the exterior atmosphere traveling inside.

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