Familiarise with High Pressure Terminology

Compressor Dictionary - D

Drying Agent

Despite having passed through the separator tank and the » activated carbon filter (which is itself a poor absorber of water vapour), compressed breathing air still contains a substantial amount of water vapour.

Moisture inside a breathing air cylinder is always a problem. Firstly, water vapour may condense -especially during cold water dives- on the cylinder walls, making steel cylinders rust. Secondly, water vapour may cause the regulator to freeze up and free flow. This is because air cools down during pressure reduction in the regulator's first and second stage. Thirdly, the algae, fungi and bacteria that thrive in moist cylinder bottles may be extremely harmful.

Hence the DIN EN 12021 (former 3188) standard requires breathing air from compressors to contain less than 25 mg/m³ of residual moisture at a pressure of 200 bar. This is equivalent to 50 mg of water in a standard 10-litre diving cylinder. For breathing air at 300 bar, the limit is set at 35 mg/m³. Given these maximum humidity levels, the pressure dew point lies at 0°C. Condensation will not occur unless the air temperature drops below zero.

If the content of a standard diving cylinder (10 litres at 200 bar) were allowed to expand to atmospheric pressure, 2 000 litres of air would contain merely 50 mg of water. Hence, air from a diving cylinder appears bone dry! Relative humidity is well below 1%. As a diver, you may have experienced dry lips and a dry throat. At atmospheric pressure, the air's dew point now lies at -50°C.

The lowest dew points and the stipulated air quality can be attained by combining two methods, refrigeration drying and chemical absorption:

Refrigeration dryers

Air is cooled down to a temperature below the pressure dew point. Residual water vapour condenses and can be drained. However, air should not be cooled below 0°C because the condensate will freeze.

Desiccating agents

Hydrophilic substances, such as molecular sieves, trap water molecules within a crystalline structure. Molecular sieve is a synthesised aluminium silicate. Like zeolith rock, which occurs naturally, the substance give off water vapour when heated and absorbs water when cooled. Industrially produced, molecular sieve can be made to trap a specific compound inside its molecular lattice, such as carbon monoxide or water. Hydrophilic crystals are grown in a carefully controlled environment, then treated with a chemical hardener as to form granules. Then, residual water vapour is removed at high temperatures, leaving behind an anhydrous drying agent. The molecular sieve must be store in an absolutely dry place immediately. Otherwise, the substance will absorb moisture from the surrounding air, reducing the absorption capacity. Storing molecular sieve in plastic bags is ineffective; plastics are water-permeable. Even aluminium-coated plastic envelopes are not entirely airtight. Therefore, drying cartridges should not be stored for more than six months.

Molecular sieve is fully saturated once it has absorbed 20% of its dry weight in water. The level of saturation can, hence, be determined by weighing the cartridge. The crystals can be regenerated at high temperatures. The procedure does not, however, stand up to a cost-benefit analysis.

During absorption, molecular sieve radiates heat. Try the following experiment: place a few granules of molecular sieve into your open hand, add water and experience the crystals heating up.

Special molecular sieves have been designed for the absorption of carbon dioxide.

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