A
Diving Air Compressor is a
gas compressorA gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
that can provide breathing air directly to a surface-supplied diver, or fill
diving cylinderA diving cylinder, scuba tank or diving tank is used to store and transport high pressure breathing gas as a component of SCUBA . It provides gas to the SCUBA diver through the demand valve of a diving regulator.Diving cylinders typically have an internal volume of between and a maximum pressure...
s with high-
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
air pure enough to be used as a
breathing gasBreathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...
.
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Modern diving compressors are three- or four-stage-reciprocating air compressors that are lubricated with high-grade compressor oil (a few use
ceramicA ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
-lined cylinders with O-rings, not
pistonA piston is a component of reciprocating engines, pumps and gas compressors. It is located in a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod...
rings, requiring no lubrication).
A
Diving Air Compressor is a
gas compressorA gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
that can provide breathing air directly to a surface-supplied diver, or fill
diving cylinderA diving cylinder, scuba tank or diving tank is used to store and transport high pressure breathing gas as a component of SCUBA . It provides gas to the SCUBA diver through the demand valve of a diving regulator.Diving cylinders typically have an internal volume of between and a maximum pressure...
s with high-
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
air pure enough to be used as a
breathing gasBreathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...
.
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Machinery
Modern diving compressors are three- or four-stage-reciprocating air compressors that are lubricated with high-grade compressor oil (a few use
ceramicA ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
-lined cylinders with O-rings, not
pistonA piston is a component of reciprocating engines, pumps and gas compressors. It is located in a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod...
rings, requiring no lubrication). Oil-lubricated compressor operators must only use
lubricantA lubricant is a substance introduced between two moving surfaces to reduce the friction between them, improving efficiency and reducing wear often used while having sex...
s 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 cylinderA diving cylinder, scuba tank or diving tank is used to store and transport high pressure breathing gas as a component of SCUBA . It provides gas to the SCUBA diver through the demand valve of a diving regulator.Diving cylinders typically have an internal volume of between and a maximum pressure...
s and freezing of
diving regulatorA diving regulator is a pressure regulator used in a scuba set that supplies the diver with breathing gas at ambient pressure from one or more diving cylinders. The gas may be air or one of a variety of specially blended breathing gases...
s, but causes
dehydrationDehydration is defined as excessive loss of body water. It is literally the removal of water from an object. In physiological terms, it entails a relative deficiency of water molecules in relation to other dissolved solutes...
, a factor in
decompression sicknessDecompression sickness describes a condition arising from the precipitation of dissolved gasses into bubbles inside the body on depressurisation...
, in divers who breathe the gas.
Air purity
In addition, the compressed air output by the compressor must be
filterA particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. A chemical air filter consists of an sorbent or cataylst for the removal of airborne molecular contaminents such as volatile organic compounds...
ed to make it fit for use as a breathing gas. The following filters remove:
- Water
Water is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
, using silica gelSilica gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid....
, activated aluminaActivated alumina is manufactured from aluminium hydroxide by dehydroxylating it in a way that produces a highly porous material; this material can have a surface area significantly over 200 square metres/g. The compound is used as a desiccant and as a filter of fluoride, arsenic and selenium in...
or a molecular sieveA molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...
- Oil, using activated carbon
Activated carbon, also called activated charcoal or activated coal, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions....
or a molecular sieve
- Carbon monoxide
Carbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
, using catalyst
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 no oil enters the breathing gas. 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
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates....
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 cold air well away and upstream from any engine exhaust.
Periodically, the gas produced by a compressor must be tested to ensure it meets air purity standards. The following impurities are checked for:
- Carbon dioxide
Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
- Carbon monoxide
Carbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
- Lubricating-oil vapour
- Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula . It exists as a radical in nature. One of several nitrogen oxides, is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp,...
- Odor and taste
- Solid particles
- Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice...
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Pressure
Diving compressors generally fall into one of two categories: those used for
surface supplied divingSurface supplied diving refers to divers using equipment supplied with breathing gas using an umbilical cord from the surface, often from a diving support vessel but possibly, indirectly via a diving chamber...
and those used for filling
scubaScuba is an acronym for self contained underwater breathing apparatus. It may also refer to:* Scuba diving, the use of a self-contained breathing set to stay underwater for periods of time* Scuba set, the equipment used to do scuba diving...
diving cylinderA diving cylinder, scuba tank or diving tank is used to store and transport high pressure breathing gas as a component of SCUBA . It provides gas to the SCUBA diver through the demand valve of a diving regulator.Diving cylinders typically have an internal volume of between and a maximum pressure...
s.
Surface supplied diving compressors are low-pressure and high-volume. They supply breathing gas directly to a diver, from
a control station called a "rack" via a hose called an "umbilical". Their output is generally between 6 and 20
barThe bar is a unit of pressure equal to 100 kilopascals, and rougly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the decibar , centibar , and millibar...
/100 and 300
psi-As a letter or word:* Psi a letter of the Greek alphabet* Psi , a letter of the early Cyrillic alphabet, adopted from Greek-In science and technology:* Psi , a popular XMPP client program...
. These compressors are often very large and powerful, because they must be able to deliver gas at a sufficient pressure and volume to multiple divers working at depths of several hundred feet as the divers breathe.
Compressors used in scuba are high-pressure and low-volume. They fill diving cylinders and storage flasks or banks of storage flasks. These compressors are generally 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 232 bar / 3000 psi and 300 bar / 4500 psi.
Compare this to a small workshop or garage compressor which works to 3-15 bar and you can see why diving compressors are so large and expensive and why safety precautions are required.
Filling heat
If diving cylinders are filled too quickly, the gas inside them becomes hot as a result of
adiabatic heatingIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid. The term "adiabatic" literally means impassable, coming from the Greek roots ἀ- , διὰ- , and βαῖνειν ; this etymology corresponds here to an...
, 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
barThe bar is a unit of pressure equal to 100 kilopascals, and rougly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the decibar , centibar , and millibar...
(100
kPaKPA may refer to:* Kenya Ports Authority* Kilopascal , a unit of pressure* Known-plaintext attack, a method of cryptanalysis* Korean People's Army* The Kosovo Property Agency* Also refers to the Montagnard name meaning "straight"....
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. This increases the risk of internal cylinder corrosion by moisture from damp components entering the cylinder during 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 cheap and low-powered compressor, which is are 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.
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Gas blending
Compressors are often linked to a
gas blendingGas blending or gas mixing is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox.- Hazards :There are several hazards with gas mixing:* cylinders are being filled with high pressure gas...
panel so nitrox, trimix,
heliairHeliair is a breathing gas consisting of mixture of oxygen, nitrogen and helium and is often used during the deep phase of dives carried out using technical diving techniques. This term is mostly used by Technical Diving International....
or
helioxHeliox is a breathing gas composed of a mixture of helium and oxygen .Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of...
mixes can be created. The panel controls the decanting of
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
and
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
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 systemA cascade storage system is a cascade storage tank system which is used for the refilling of smaller compressed gas cylinders or air cylinders. Each of the large cylinders will be filled by a compressor, but the cascade system allows small cylinders to be filled without the presence of a compressor...
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.