- Source: Air lock
- Source: Airlock
An air lock is a restriction of, or complete stoppage of liquid flow caused by vapour trapped in a high point of a liquid-filled pipe system. The gas, being less dense than the liquid, rises to any high points. This phenomenon is known as vapor lock, or air lock.
Flushing the system with high flow or pressures can help move the gas away from the highest point. Also, a tap (or automatic vent valve) can be installed to let the gas out.
Air lock problems often occur when one is trying to recommission a system after it has been deliberately (for servicing) or accidentally emptied. Take, for example, a central heating system using a circulating pump to pump water through radiators. When filling such a system, air is trapped in the radiators. This air has to be vented using screw valves built into the radiators. Depending on the pipe layout – if there are any upside down 'U's in the circuit – it will be necessary to vent the highest point(s). Otherwise, air lock may cause waterfall flow where the loss of hydraulic head is equal to the height of airlock; If the hydraulic grade line drops below the output of the pipe, the flow through that part of the circuit would stop completely. Note that circulating pumps usually do not generate enough pressure to overcome air locks.
Fig 1 shows a reservoir which feeds a gravity distribution system – for drinking water or irrigation. If the ground in which the pipe is laid has high points – such as Hi1, 2 etc. and low points between them such as Lo1, 2 etc., then if the pipe is filled from the top, and was empty, the pipe fills OK as far as Hi1. If the water flow velocity is below the rising velocity of air bubbles, then water trickles down to the low point Lo2 and traps the remaining air between Hi1 and Lo2. As more water flows down, the upward leg Lo2 to Hi2 fills up. This exerts a pressure on the trapped air of either H2 m of water (WG = water gauge) or H1, whichever is less. If H2 is greater than H1, then you have a full air lock, and the water level in the up leg Lo2 to Hi2 stops at H1 and no further water can flow. If H1 is greater than H2, then some water can flow, but the full pipe hydraulic head H3 will not be reached and so flow is much less than expected. If there are further undulations, then the back pressure effects add together. Long pipelines built across fairly level, but undulating land are bound to have many such high and low points. To avoid air or gas lock, automatic vents are fitted which let air or gas out when above a certain pressure. They may also be designed to let air in under vacuum. There are many other design considerations for design of water pipeline systems, e.g.
The air lock phenomenon can be used in a number of useful ways. The adjacent diagram shows an 'S' trap. This has the properties a) that liquid can flow from top (1) to bottom (4) unhindered and b) that gas cannot flow through the trap unless it has enough extra pressure to overcome the liquid head of the trap. This is usually about 75 to 100 mm of water and prevents foul smelling air coming back from water drainage systems via connections to toilets, sinks and so on. 'S' traps work well unless the drainage water has sand in it – which then collects in the 'U' part of the 'S'.
See also
Flush toilet – tank style with siphon-flush valve
Siphon
Vapor lock
References
An airlock is a room or compartment which permits passage between environments of differing atmospheric pressure or composition, while minimizing the changing of pressure or composition between the differing environments.
An airlock consists of a chamber with two airtight doors or openings, usually arranged in series, which do not open simultaneously. Airlocks can be small-scale mechanisms, such as those used in fermenting, or larger mechanisms, which often take the form of an antechamber.
An airlock may also be used underwater to allow passage between the air environment in a pressure vessel, such as a submarine, and the water environment outside. In such cases the airlock can contain air or water. This is called a floodable airlock or underwater airlock, and is used to prevent water from entering a submersible vessel or underwater habitat.
Operation
The procedure of entering an airlock from the external or ambient pressure environment, sealing it, equalizing the pressure, and passing through the inner door is known as locking in. Conversely, locking out involves equalizing pressure, unsealing the outer door, then exiting the lock compartment to enter the ambient environment. Locking on and off refer to transfer under pressure where the two chambers are physically connected or disconnected prior to equalizing the pressure and locking in or out.
Before opening either door, the air pressure of the airlock chamber is equalized with that of the environment beyond the next door. A gradual pressure transition minimizes air temperature fluctuations, which helps reduce fogging and condensation, decreases stresses on air seals, and allows safe verification of critical equipment.
When a person who is not in a pressure suit moves between environments of greatly different pressures, an airlock changes the pressure slowly to help with internal air cavity equalization and to prevent decompression sickness. This is critical in underwater diving, and a diver or compressed air worker may have to wait in an airlock for a number of hours in accordance with a decompression schedule. A similar arrangement may be used for access to airtight clean spaces, contaminated spaces, or unbreathable atmospheres, which may not necessarily involve any differences in pressure; in these cases, a decontamination procedure and flushing are used instead of pressure change procedures.
History
= 19th century
=The first airlock patent was granted in 1830 to Thomas Cochrane, who came up with the idea to help facilitate underground tunnel construction. It was put into use in 1879 during an attempt to dig a tunnel under the Hudson river.
= 20th century
=The Apollo program involved developments in airlock technology, as airlocks are critical to allow humans to enter and exit the spacecraft while on the Moon without losing too much air due to its scant atmosphere.
During the 1969 Apollo 11 mission, there was no room that was primarily designed to be an airlock; instead, they used the cabin as an airlock. It had to be evacuated and depressurized before the door was opened, and then once the door was closed it had to be re-pressurized again before anyone could safely reenter the cabin without a space suit.
= 21st century
=When the International Space Station (ISS) first began to house humans in November 2000, it did not include an airlock, and all extravehicular activity had to be facilitated by the airlock on the Space Shuttle until the Quest Joint Airlock module was installed in July 2001.
The first ever commercial space airlock was the Nanoracks Bishop Airlock, installed on the ISS in December 2020. It is "bell-shaped" and is designed to transfer payloads out from the ISS interior and into space. As of July 2023 it is the largest airlock of its kind on the station, capable of fitting "payloads as large as a refrigerator."
Air environments
Airlocks are used in air-to-air environments for a variety of reasons, most of which center around either preventing airborne contaminants from entering or exiting an area, or maintaining the air pressure of the interior chamber.
One common use of airlock technology can be found in some cleanrooms, where harmful or otherwise undesired particulates can be excluded by maintaining the room at a higher pressure than the surroundings, alongside other measures. Conversely, particulates are prevented from escaping hazardous environments, such as nuclear reactors, laboratories of biochemistry, and medical centers, by keeping negative room pressure - maintaining the room at a lower pressure than the surroundings, so that air (and any particulates that it carries) cannot escape easily.
A lesser-known application of an airlock is in architecture: inflatable buildings and air-supported structures such as pressurized domes require the internal air pressure to be maintained within a specific range so that the structure doesn't collapse. Airlocks are generally the most cost-efficient way to allow people to enter and exit these structures.
Airlocks are utilized to maintain electron microscope interiors at near-vacuum so that air does not affect the electron path. Fermentation locks, such as those used in alcohol brewing, are a type of airlock which allow gases to escape the fermentation vessel while keeping air out. Parachute airlocks are necessary because airfoil collapse due to depressurization can result in dangerous loss of altitude.
Since the 1980s, airlock technology has been used to explore newly detected chambers in the Egyptian pyramids, to prevent the contents from beginning to decompose due to air contamination.
Underground
Civil engineering projects that use air pressure to keep water and mud out of the workplace use an airlock to transfer personnel, equipment, and materials between the external normabaric environment and the pressurized workplace in a caisson or sealed tunnel. The airlock may need to be large enough to accommodate a whole working shift at the same time.
Locking in is usually a quick procedure, taking only a few minutes, while the decompression required for locking out may take hours.
Underwater
Underwater applications include:
Hyperbaric chambers, to allow entry and exit while maintaining pressure difference between chamber and environment;
Submarines, closed diving bells, and underwater habitats that are not at ambient pressure, to permit divers to enter and exit. In submarines and underwater habitats they may also be called diver lock-out compartments; and
Torpedo tubes and escape trunks in submarines.
= Saturation diving
=In saturation diving, airlocks are crucial safety elements; they serve as pressurized gateways to safely manage the transfer of divers and support personnel between the saturation system (living quarters) and the diving bell, which shuttles divers to their underwater worksite.
Airlocks in saturation diving are equipped with safety features such as pressure gauges, manual overrides, and interlocks.
Saturation systems typically feature a variety of airlocks, including a stores lock for the transfer of supplies and a medical lock for secure passage of medical necessities or emergency evacuations. Complex "split-level" systems, which house divers at different pressure levels for varied work depths, may necessitate additional airlocks.
Decompression post-dive is a gradual process, often taking a full week. During this time, the airlocks allow divers to shift to a decompression chamber where pressure is progressively reduced back to surface levels. In emergencies, airlocks can facilitate transfer to a hyperbaric escape chamber or lifeboat without significant pressure changes.
= Hyperbaric treatment chambers
=In any hyperbaric treatment chamber capable of accommodating more than one person, and where it may be necessary to get a person or equipment into or out of the chamber while it is pressurized, an airlock is used. There will usually be a large airlock at the chamber entry capable of holding one or more persons, and a smaller medical lock for locking in medical supplies and food, and locking out waste.
Outer space
Airlocks are used in outer space, especially during human spaceflight, to maintain the internal habitable environment on spacecraft and space stations when persons are exiting or entering the spacecraft. Without an airlock (or similar technology, such as a suitport) the air inside would be rapidly lost upon opening the door due to the expansive properties of the gases that comprise breathable air, as described by Boyle's law. An airlock room is needed to decompress astronauts after they suit up in specialized space suits in preparation for extravehicular activity, and then to recompress them upon return. Airlocks such as the Nanoracks Bishop Airlock also allow payloads to be released into space with minimal air loss.
Other examples of airlocks used in space include the Quest Joint Airlock and the airlock on Kibō (ISS module).
See also
Cabin pressurization
Dysbarism – Medical conditions resulting from changes in ambient pressure
Mechanisms with similar functions:
Lock (water navigation) – Uses water levels instead of air
Mantrap (access control)
Revolving door – Regulates building air pressure and temperature
Sally port – Focused on security rather than air pressure
Suitport – Alternative technology to enable extravehicular activity
Vestibule – Regulates building temperature
"Light-locks" in planetariums and darkrooms
Crew and Science Airlock Module
Notes
References
External links
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