Air Compressors: High-Pressure/Low-Pressure

Breathing Gas Preparation and Contamination:  Contamination of a diver’s breathing gas can cause a variety of effects. These range from relatively mild discomfort resulting from an oily taste or odour to a mild headache, respiratory changes or respiratory distress, impaired cognition and consciousness, cardiac arrhythmias and death through toxicity or drowning. In earlier editions of this text, this chapter began by stating that the death of a diver as a result of breathing a contaminated gas mixture is an uncommon event. Unfortunately, this statement has become less true with the increased use of various forms of mixture diving and rebreathers. Readers need be aware of the causes of contamination and the methods of prevention and treatment.

Suitable technologies to compress breathing gas are essential to the conduct of underwater diving. Early ‘standard dress’ divers used simple hand pumps to provide a constant flow of compressed air to their helmets. Modern demand regulators used in both scuba diving and commercial surface-supply breathing apparatus (SSBA) require a supply of compressed gas at around 5 to 10 bar pressure that can be delivered either from the first-stage regulator on a dive cylinder or from the surface via a diving umbilical hose. Compressors are therefore necessary either to supply breathing gas directly to the diver or to fill the dive cylinders.

High-pressure air compressors

Compressors used to fill scuba cylinders take in air at atmospheric pressure and compress it to 200 to 300 bar. The high-pressure air is then filtered, dried and often stored in storage cylinders until it is decanted into scuba cylinders. Alternatively, small compressor units may be used to fill the diver’s cylinder directly.

If the air was compressed to high pressure in one step, there would be two major problems. First, the gas would become very hot, and subsequently a large volume of condensation would form when the gas cooled. Second, it would be difficult to design seals to prevent air leakage around the large pistons required. Because of these problems, most systems compress the air in three or four stages.

The basic design of a three-stage system is shown in Figure 19.1. At each stage the gas is compressed by a piston that moves up and down in a cylinder with inlet and outlet valves controlling the airflow. These compressors are oil lubricated, and small amounts of oil mist mix with the air as it is compressed. This must be removed, along with the water vapour in the air.

Diagramme of a three-stage compressor.
Figure 19.1 Diagramme of a three-stage compressor.

In the first stage, the air is compressed to about 8 bar. It then passes through cooling coils, and the resultant drop in temperatures causes some reduction in the pressure. Some compressors have an interstage filter between the first and second stages, although this is not standard in the diving industry. The second stage of compression raises the pressure further before the air is again cooled. In conjunction with an interstage filter, water and oil are condensed and filtered out. The final stage compresses the air to the pressure required for the storage tanks or scuba cylinders. A final filter system should be incorporated, which may include a fine filter (to remove further moisture), activated carbon (for odour) and molecular sieve material (to remove moisture and oil particles). In an environment where there is likely to be high levels of carbon dioxide, it may advisable to add an extra filter with carbon dioxide absorbent (e.g. soda lime). If carbon monoxide is of particular concern a catalytic converter element such as Hopcalite medium may also be added. A four-stage compressor differs from the three-stage device shown only in that there are smaller pressure increase steps over each stage.

The cylinder, piston and valves in each stage of the compressor are smaller than the preceding because the volume of gas decreases in accordance with Boyle’s Law. In the previous example, if the volume of the first stage cylinder was 1000 ml, to match the pressures given, the second stage would need to be less than 200 ml and the third stage about 25 ml.

The moisture that condenses is the water vapour in the air that was compressed. If the relative humidity of the air entering the compressor was 50 per cent, condensation would occur when the air pressure is increased to more than 2 bar (if the temperature remains constant). It is difficult and expensive to remove water from compressed air by chemical means only, so this is achieved throughout the process primarily by means of condensation as the air passes through pipes cooled by either fans (air) or water. Interstage separators, fitted with drains, then remove this excess (condensed) water. The process is optimized at the final filter stage by the fitting of a pressure maintaining valve (PMV). The PMV maintains pressure within the final filter (to around 160 to 190 bar, depending on the final pressure) and so increases the efficiency of (chemical) filtration.

A refrigeration system is sometimes used to increase condensation (Figure 19.2). This is fitted after the final (after) cooler and is usually sold separately from the compressor. Both a high outlet valve pressure and refrigeration will also increase the removal of any contaminants that condense with, or in, the water.

A portable four-cylinder Bauer compressor (Bauer AG, Schrobenhausen, Germany) in which some of the components have been cut away to show the internal design.
Figure 19.2 A portable four-cylinder Bauer compressor (Bauer AG, Schrobenhausen, Germany) in which some of the components have been cut away to show the internal design. To reduce the size of the compressor, the cylinders are arranged in a radial pattern. The spiral coils are to cool the air between stages of compression; cooling is aided by a fan attached to the far side of the unit. Contaminants are removed from the compressed air by chemicals in prepared packs that fit into two cut-away vertical cylinders in front of the compressor.

There is a divergence of opinion on how best to lubricate compressors. Some advocate synthetic oils, and others recommend mineral oils. The manufacturers’ instructions are the best guide to selecting the lubricant. Standard motor oils should not be used because they may break down under high pressures and heat.

Attempts have been made to design compressors that do not need lubrication. Another option is to use water as coolant and lubricant. Neither approach has won general acceptance. The reality is that all high-pressure compressors require oil, at least in the crank case.

Compressors range in size from small units that can be carried by one person to large units that weigh several tonnes. In output terms, these range from as little as 28 to 1500 litres per minute and even larger.

Low-pressure air compressors

SSBA involves much more complex equipment than scuba to provide safety for deeper occupational diving operations. Compressed air may be supplied from large cylinders filled before the dive or from a low-pressure compressor that runs throughout the dive. The air passes through a diver’s control panel and then via an umbilical hose to the diver’s helmet or demand mask. When a low-pressure compressor is used, a large-volume storage vessel and/or high-pressure cylinders may be incorporated into the system as a back-up. ‘Hookah’ is a colloquial term sometimes used to refer to minimalist versions of SSBA in which a simple low-pressure compressor supplies a mouth-held regulator via a floating hose. Hookah is commonly used in the seafood harvesting sector and by some recreational divers.

There is a larger variety of compressor types in use for SSBA and hookah diving, with currently used options including reciprocating piston and rotary screw-type compressors. Low pressure air compressors are either lubricated and sealed with oil, or of an oil-free “dry” design and there are advantages and disadvantages to each of the multiple available configurations. Because the compressor is running at the dive site and is usually powered by a diesel or petrol internal combustion engine, there is probably a greater risk of unexpected intake of contaminants compared with a properly installed high-pressure compressor at a fixed location. The quality of compressors used in practice varies dramatically, from well-filtered and well-monitored systems to inexpensive compressors designed for workshop compressed air and sometimes equipped with only makeshift filtration.