What is "air lock"

Air \Air\ ([^a]r), n. [OE. air, eir, F. air, L. a["e]r, fr. Gr. 'ah`r, air, mist, for 'a[digamma]hr, fr. root 'a[digamma] to blow, breathe, probably akin to E. wind. In sense 10 the French has taking a meaning fr. It. aria atmosphere, air, fr. the same Latin word; and in senses 11, 12, 13 the French meaning is either fr. L. aria, or due to confusion with F. aire, in an older sense of origin, descent. Cf. A["e]ry, Debonair, Malaria, Wind.]

1. The fluid which we breathe, and which surrounds the earth; the atmosphere. It is invisible, inodorous, insipid, transparent, compressible, elastic, and ponderable.

   Note: By the ancient philosophers, air was regarded as an element; but modern science has shown that it is essentially a mixture of oxygen and nitrogen, with a small amount of carbon dioxide, the average proportions being, by volume: oxygen, 20.96 per cent.; nitrogen, 79.00 per cent.; carbon dioxide, 0.04 per cent. These proportions are subject to a very slight variability. Air also always contains some vapor of water.

2. Symbolically: Something unsubstantial, light, or volatile. ``Charm ache with air.''
   --Shak.

   He was still all air and fire. [Air and fire being the finer and quicker elements as opposed to earth and water.]
   --Macaulay .

3. A particular state of the atmosphere, as respects heat, cold, moisture, etc., or as affecting the sensations; as, a smoky air, a damp air, the morning air, etc.

4. Any a["e]riform body; a gas; as, oxygen was formerly called vital air. [Obs.]

5. Air in motion; a light breeze; a gentle wind.

   Let vernal airs through trembling osiers play.
   --Pope.

6. Odoriferous or contaminated air.

7. That which surrounds and influences.

   The keen, the wholesome air of poverty.
   --Wordsworth.

8. Utterance abroad; publicity; vent.

   You gave it air before me.
   --Dryden.

9. Intelligence; information. [Obs.]
   --Bacon.

10. (Mus.)

    1. A musical idea, or motive, rhythmically developed in consecutive single tones, so as to form a symmetrical and balanced whole, which may be sung by a single voice to the stanzas of a hymn or song, or even to plain prose, or played upon an instrument; a melody; a tune; an aria.

    2. In harmonized chorals, psalmody, part songs, etc., the part which bears the tune or melody -- in modern harmony usually the upper part -- is sometimes called the air.

11. The peculiar look, appearance, and bearing of a person; mien; demeanor; as, the air of a youth; a heavy air; a lofty air. ``His very air.''
    --Shak.

12. Peculiar appearance; apparent character; semblance; manner; style. It was communicated with the air of a secret. --Pope. 12. pl. An artificial or affected manner; show of pride or vanity; haughtiness; as, it is said of a person, he puts on airs. --Thackeray. 14. (Paint.)

    1. The representation or reproduction of the effect of the atmospheric medium through which every object in nature is viewed.
       --New Am. Cyc.

    2. Carriage; attitude; action; movement; as, the head of that portrait has a good air. --Fairholt. 15. (Man.) The artificial motion or carriage of a horse. Note: Air is much used adjectively or as the first part of a compound term. In most cases it might be written indifferently, as a separate limiting word, or as the first element of the compound term, with or without the hyphen; as, air bladder, air-bladder, or airbladder; air cell, air-cell, or aircell; air-pump, or airpump. Air balloon. See Balloon. Air bath.

       1. An apparatus for the application of air to the body.

       2. An arrangement for drying substances in air of any desired temperature. Air castle. See Castle in the air, under Castle. Air compressor, a machine for compressing air to be used as a motive power. Air crossing, a passage for air in a mine. Air cushion, an air-tight cushion which can be inflated; also, a device for arresting motion without shock by confined air. Air fountain, a contrivance for producing a jet of water by the force of compressed air. Air furnace, a furnace which depends on a natural draft and not on blast. Air line, a straight line; a bee line. Hence Air-line, adj.; as, air-line road. Air lock (Hydr. Engin.), an intermediate chamber between the outer air and the compressed-air chamber of a pneumatic caisson. --Knight. Air port (Nav.), a scuttle or porthole in a ship to admit air. Air spring, a spring in which the elasticity of air is utilized. Air thermometer, a form of thermometer in which the contraction and expansion of air is made to measure changes of temperature. Air threads, gossamer. Air trap, a contrivance for shutting off foul air or gas from drains, sewers, etc.; a stench trap. Air trunk, a pipe or shaft for conducting foul or heated air from a room. Air valve, a valve to regulate the admission or egress of air; esp. a valve which opens inwardly in a steam boiler and allows air to enter. Air way, a passage for a current of air; as the air way of an air pump; an air way in a mine. In the air.

          1. Prevalent without traceable origin or authority, as rumors.

          2. Not in a fixed or stable position; unsettled.

    3. (Mil.) Unsupported and liable to be turned or taken in flank; as, the army had its wing in the air.

       on the air, currently transmitting; live; -- used of radio and television broadcasts, to indicate that the images and sounds being picked up by cameras and microphones are being broadcast at the present moment.

       Note: In call-in programs where individuals outside a radio or television studio have telephoned into the station, when their voice is being directly broadcast, the host of the program commonly states ``You're on the air.'' as a warning that the conversation is not private.

       To take air, to be divulged; to be made public.

       To take the air, to go abroad; to walk or ride out.

n. a chamber that provides access to space where air is under pressure [syn: airlock]

An air lock is a restriction of, or complete stoppage of liquid flow caused by gas trapped in a high point of a liquid-filled pipe system. The gas, being less dense than the liquid, rises to any high points. When pressure is applied, instead of the liquid flowing as it would if the system was full of liquid, the system behaves like a series of manometers.

Flushing the system with high flow or pressures can help move the gas away from the highest point, or 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. 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). If not, air lock may stop flow through that part of the circuit 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 [ref], 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. Obviously long pipelines over 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 diagram to the right 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 foul 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'.