Familiarise with High Pressure Terminology

Compressor Dictionary – O

Operation of a Three Stage Compressor

1. Intake filter and hose
   Air enters the compressor through an intake filter that traps solid contaminants (such as pollen or dust).
2. The filtered air then passes through a 2-3 metre, flexible intake hose. Its cross-section must be sufficiently large to ensure unobstructed air flow. Alternatively a longer intake hose can be employed. To prevent a drop in air delivery, the hose's cross-section has to enlarged accordingly. For compressors with electric drive, the intake filter and the intake hose may become redundant in an absolutely clean and dust-free environment.
3. The lifetime of the cellulose intake filter can be extended fourfold by turning it by 90 degrees once it becomes clogged. The precise filter capacity depends on the purity of the intake air. In a desert environment, consider yourself lucky if the filter lasts for four weeks.
4. First stage
   Alongside compression of the clean intake air to around 6 bar, the unit's first stage recycles the "blow by" (i.e. the volume of air that has escaped from the individual compressor stages, past the piston rings and into the crank case). Blow by amounts to 5-10% of the processed air. The suspended oil mist lubricates the cylinders' compression chambers.
5. Inter-cooler
   The inter-cooler between the first and second stage cools the hot, compressed air. This aluminium cooler lowers air temperature from 120 °C to 10–15 °C above ambient temperature.
6. The first stage's safety valve, incorporated in the second stage cylinder head, blows if the pressure in the inter-cooler between first and second stage rises beyond a certain level (for example, if the second stage's plate valves malfunction).
7. Second Stage
   The cooled air from the first stage is compressed from 6 to 45 bar in the second stage. Again, the inter-cooler lowers air temperature from 120 °C to 10–15 °C above ambient temperature. The safety valve blows if a critical pressure is reached.
8. Intermediate separator
   » Condensate, an emulsion of water contained in the moist intake air and oil from the compressor's lubrication system, is discharged during compression. This liquid is separated from the compressed air using sinter metal filter and a cyclonic separator, which can be drained manually or automatically.
9. Third stage
   It is only in the third stage that high pressure air is generated. The piston compresses air from 45 bar to a final pressure of 200 or 330 bar (depending on the compressor design). The after-cooler dissipates the heat emanated during compression. At the high pressure outlet, the air temperature is 10–15 °C above ambient.
10. The » safety valve for the third stage limits final pressure to 225 bar, even though the filling pressure for diving cylinders is 200 bar. The latter value is quoted for an ambient temperature of 15 °C. Since compressed air tends to be hotter, final pressure is set at a slightly higher level. For a filling pressure of 200 bar, for example the safety valve blows at 225 bar.
11. All safety valves have been sealed for your protection. They may not be adjusted. Tampering with safety devices renders the compressor unsafe.
12. Final separator
    The final separator mechanically removes condensate from compressed air (see the section about "intermediate separators" for details).
13. Purification
    The activated carbon filter absorbs gaseous contaminants (e.g. oil fumes). Next, air is dried by a molecular sieve, an extremely porous granulate of hydrophilic silicate crystals. Drying prevents corrosion inside the cylinders and safeguards against frozen, free flowing regulators, especially during cold water dives (<10 °C).
14. Pressure maintaining and non-return valve
    The » pressure maintaining valve keeps filter and final separator at a pressure of 150 bar, even if an empty cylinder is connected. The sluggish air flow in the filter cartridge improves filtration; the reaction time between the medium and the filter material is increased. The pressure maintaining valve also functions as a non-return valve. It prevents a reverse flow of air while the condensate drain is open.
15. Clamp and hose
    The filling assembly is equipped with a » pressure gauge and a three-way valve (used to adjust the flow of air into the cylinder and to depressurise the clamp assembly).

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