Diving air compressor

A diving air compressor is a gas compressor that can provide breathing air directly to a surface-supplied diver, or fill diving cylinders with high-pressure air pure enough to be used as a breathing gas.

Filling a cylinder from the panel

Machinery

Modern diving compressors are three- or four-stage-reciprocating air compressors that are lubricated with high-grade compressor oil (a few use ceramic-lined cylinders with O-rings, not piston rings, requiring no lubrication). Oil-lubricated compressor operators must only use lubricants specified by the compressor's manufacturer. Special filters are used to clean the air of any residual oil (see "Air purity").

The compression process helps remove water from the gas, making it dry, which is good for reducing rust in diving cylinders and freezing of diving regulators, but causes dehydration, a factor in decompression sickness, in divers who breathe the gas.

Air purity

The compressed air output by the compressor must be filtered to make it fit for use as a breathing gas. Filters remove:

• Water, using silica gel, activated alumina or a molecular sieve
• Oil, using activated carbon or a molecular sieve
• Carbon monoxide, using a catalyst (Hopcalite)

Oil, which must be used to lubricate the compressor's internal parts, can be particularly deadly if it enters the breathing gas and is inhaled as a mist. Petroleum-based oils cannot be absorbed and metabolized by the body and will coat the internal surfaces of the lungs, causing a condition known as lipoid pneumonia and leading to asphyxiation and death. For this reason, compressors must be carefully designed and maintained to ensure that oil contamination of the breathing gas is within safe limits. Petroleum-based oils must never be used within the compressor. Instead, vegetable-based or specifically formulated synthetic oils are used, which can be safely absorbed and metabolized by the body in small quantities, should a malfunction occur.

Carbon monoxide (CO) is a gas that is present in the exhaust gas of internal combustion engines, including those often used to drive compressors. It also comes from the breakdown of lubricating oil when compressors run too hot. CO is odorless, colorless, and tasteless. CO is deadly even in small quantities, because it readily binds with the hemoglobin in red blood cells and thus destroys the blood's ability to carry oxygen. Diving compressors must be carefully designed and placed so that the compressor's intake is located in fresh air well away and upstream from any engine exhaust.

Periodically, the air produced by a compressor must be tested to ensure it meets air purity standards. Frequency of testing, contaminants that must be analysed, and the allowable limits vary between applications and jurisdictions. The following impurities may be checked for:

• Carbon dioxide
• Carbon monoxide
• Lubricating-oil vapour
• Nitrogen dioxide
• Odor and taste
• Solid particles
• Water vapor

Air bank

Pressure

Diving compressors generally fall into one of two categories: those used for surface supplied diving and those used for filling scuba diving cylinders.

Surface supplied diving compressors are low-pressure and high-volume. They supply breathing gas directly to a diver, from a control panel sometimes called a "rack" via a hose which is usually part of a group of hoses and cables called an "umbilical". Their output is generally between 6 and 20 bar /100 and 300 psi. These compressors must be sufficiently powerful to deliver gas at a sufficient pressure and volume for multiple divers working at depths of up to about two hundred feet.

Compressors used in scuba are high-pressure and low-volume. They fill diving cylinders and storage flasks or banks of storage flasks. These compressors may be smaller and less powerful because the volume of gas they deliver is not so critical; a lower volume compressor can be used to fill large storage flasks during the long periods of the day when demand is low. This stored compressed air can be decanted into diving cylinders when needed. Common scuba diving cylinder pressures are 200 bar (2940 psi), 3000 psi (207 bar), 232 bar (3400 psi) and 300 bar (4500 psi).

Filling heat

If diving cylinders are filled too quickly, the gas inside them becomes hot as a result of adiabatic heating, increasing in pressure, which results in a drop in pressure when the cylinder cools later. Cylinders are often filled at a rate of less than 1 bar (100 kPa or 15 lbf/in²) per second to reduce this increase in temperature. In an attempt to cool the cylinder when filling, some people “wet fill”, immersing their cylinders in a cool water bath. There is an increased risk of internal cylinder corrosion caused by moisture from from the wet environment entering the cylinder due to contamination during connection of the filling hose during wet filling.

The bank

Often compressors are connected to a bank of large, high-pressure cylinders to store compressed gas, for use at peak times. This allows a cheaper low-powered compressor, which is relatively slow at pumping gas, to fill the bank automatically during idle periods, storing a large volume of pressurized air so that a batch of cylinders can be filled quickly one after the other at peak demand without being delayed by the slow-running compressor. In surface-supplied diving, high-pressure cylinder banks may be used as an emergency backup in case of primary compressor failure, or they may be used as the primary source of breathing gas, a system also known as "Scuba replacement".

Gas blending panel

Gas blending

Main article: Gas blending

Compressors are often linked to a gas blending panel so nitrox, trimix, heliair or heliox mixes can be created. The panel controls the decanting of oxygen and helium from cylinders bought from commercial gas suppliers.

As it is not possible to decant to a diving cylinder from a storage cylinder that holds gas at a lower pressure than the diving cylinder, the expensive gas in low pressure storage cylinders is not easily consumed and may go to waste when the storage cylinder is returned to the supplier. The cascade system is often used with a bank to economically consume these high cost gases so that the maximum possible gas is removed from the bank. This involves filling a diving cylinder by first decanting from the bank cylinder with the lowest pressure that is higher than the diving cylinder's pressure and then from the next higher-pressure bank cylinder in succession until the diving cylinder is full. The system maximizes the use of low-pressure bank gas and minimizes the use of high-pressure bank gas.

Another method for scavenging expensive low pressure gases is to pump it with a Haskel pump or other gas booster pump, or to mix it into the air at atmospheric pressure before compression in a mixer known as a blending stick.

External links

• The Case For Dry-Filling