Gas duster

A can of gas duster

Gas duster, also known as canned air, or compressed air, is a product used to clean electronic equipment and other sensitive devices that cannot be cleaned using water. The can comes with a straw to direct the forceful wind that it can produce. The gases inside the can have been compressed to the point that they have become liquid, and they evaporate before leaving the can, since the valve draws from the top (unlike most aerosol cans, which have a straw that draws from the bottom of the can). Despite the name "canned air," the cans actually contain gases that are much easier to compress into liquids, such as difluoroethane, trifluoroethane, or tetrafluoroethane (the main components of air, which are oxygen and nitrogen, are in fact very rarely used). Hydrocarbons, like butane, were often used in the past, but their flammability forced manufacturers to use fluorocarbons.

Applications

Cleaning

A gas duster is usually used to clean or dust delicate items or reach difficult areas. The dusters are particularly useful on ventilation fans and electronic heat sinks, which collect dust readily, and are otherwise very difficult to clean. The gases themselves do not leave residues on sensitive equipment, however the bitterant added to prevent abuse will leave a residue, making gas dusters an inappropriate choice for cleaning anything users will come into contact with such as keyboards. They can create static unless a specific ESD-safe compound is added.

The can must be held upright during use. Inverting, tilting or even shaking the can during use will result in the unevaporated liquid being forced through the nozzle instead of gas. The liquid will boil away almost instantly outside the can, producing extreme cold in the process. This sudden drastic change in temperature can mar or damage surfaces (as well as cause frostbite if it contacts skin).

Cooling

When the can is held upright and activated, gas flows out through the nozzle. The pressure inside the can therefore drops, and is no longer sufficient to keep the contents as a liquid; so some of the liquid boils, until the equilibrium pressure is re-established. The vaporization of a liquid is endothermic; thus, heat is absorbed, and the can becomes cold.

Continued use over a short period of time results in the can becoming increasingly cold. As the temperature drops, the vapor pressure of the liquid also drops, resulting in decreasing force of the gas at the nozzle. At some point, the force of the ejected gas at the nozzle is insufficient to accomplish anything useful in terms of dust removal. When the temperature of the can reaches the boiling point of the liquid (which is -25C for difluoroethane), the liquid no longer evaporates into gas in any useful quantity. At this point, one must stop and wait for the can to return to room temperature before it will again provide sufficient gas flow. Alternating between two cans (allowing one to warm while the other is being used) is one way to work around this problem during an extensive dusting job. Warming the can with a heat source can be dangerous as the can may overheat and explode.

A related category of product has an internal dip tube that reaches just about to the bottom of the can, so it sprays the liquid. It evaporates very quickly, seriously chilling anything it touches the way dry ice (solid CO₂) would. These "chill spray" cans were used by trained, responsible adults who troubleshot electronic equipment by chilling suspected components to make the equipment fail, if extreme cold would be a cause.

Alternative to 134a

Many gas dusters contain HFC-134a (tetrafluoroethane), which is widely used as a propellant and refrigerant. HFC-134a sold for those purposes is often sold at a much higher unit price, which has led to the practice of using gas dusters as a less expensive source of HFCs for those purposes. Adapters have been built for such purposes, though in most cases, use of such adapters will void the warranty on the equipment they are used with. One example of this practice is the case of airsoft gas guns, which use HFC-134a as a source of compressed gas. Several vendors sell "duster adapters" for use with airsoft guns, though it is necessary to add a lubricant when using gas dusters to power airsoft guns.

Safety

Since gas dusters are one of the many inhalants that can be easily abused, many manufacturers have added a bittering agent to deter people from inhaling the product. Because of the generic name "canned air", some people mistakenly believe that the can only contains normal air or contains a less harmful substance such as nitrous oxide. However, the gases actually used are denser than air, and inhaling can lead to paralysis, serious injury, or death. Recently, in the United States and Canada stores have begun to ask for ID to verify that the customer is 18 years or older (some include Fred Meyer, Office Max, Office Depot, Best Buy, Staples, RadioShack, Walgreens, Canadian Tire, Shopko, Target, Maplin Electronics, CVS, and Wal-Mart)^{[citation needed]}.

Though not extremely flammable in gaseous form, many dusters use a fluorocarbon which can burn under some conditions, and warn of this on the packaging. When inverted to spray liquid, the boiling fluorocarbon aerosol is easily ignitable, producing a very large blast of flame and extremely toxic byproducts such as hydrogen fluoride and carbonyl fluoride as a combustion product.

Fluorocarbons, although they replaced the older set of more flammable hydrocarbons, can still combust relatively easily, e.g., by holding a source of fire (such as a match or lighter) to the escaping fluid. They do, however, have a lower chance of exploding in a closed container by means of spontaneous combustion (which is what caused the general switch away from hydrocarbons).

The liquid, when released from the can, boils at a very low temperature, rapidly cooling any surface it touches. This can cause frostbite on contact with skin. As the can gets very cold during extended use, holding the can itself can result in frostbite.

Since gas dusters are often contained in pressure vessels, they are fairly explosively volatile.

Use of dusters can produce large clouds of dust, which may directly affect the lungs.

Environmental impacts

Difluoroethane (HFC-152a), trifluoroethane (HFC-143a), and completely non-flammable tetrafluoroethane (HFC-134a) are potent greenhouse gases. According to the Intergovernmental Panel on Climate Change (IPCC), the global warming potential (GWP) of HFC-152a, HFC-143a, and HFC-134a are 124, 4470, and 1430, respectively.^[1] GWP refers to global warming effect in comparison to CO₂ for unit mass. 1 kg of HFC-152a is equivalent to 124 kg of CO₂^[2]

Gas dusters sold in many countries are ozone safe as they use zero-ODP gases; however, this is a separate issue from the global warming concern.

Alternatives

An air compressor fitted with a small nozzle (as part of a compressor accessory kit) can be used in lieu for cleaning. This offers a fixed equipment cost, at the expense of portability. Care must be taken however, as water vapor may condense in the tank or at the nozzle.

Portable compressed air canisters that can be refilled with a bicycle pump are also an option. Electric dust blowers offer another alternative.

References

1. ^ http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf
2. ^ http://books.google.com/books?id=U9fMCaiNWkcC&pg=PA477&lpg=PA477&dq=12400+ozone&source=bl&ots=n7TYMwBz8r&sig=HbA9XstcNWLutnSIh7LD_mMJexY&hl=en&ei=LtkES6SzDoKutQOQk9jrDA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAgQ6AEwAA#v=onepage&q=12400%20ozone&f=false

External links

• Snopes: Adolescents huffing from cans of Dust-Off brand compressed air have died.