Lighter than air
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- The phrase lighter than air is used commonly, and in this article, to mean less dense than air.
Some gases are buoyant in air because they have a density that is less than the density of air. Lighter than air gases are used to fill craft called aerostats which include free balloons, moored balloons, and airship to make the whole aircraft, on average, lighter than air. (Heavier than air aircraft include aeroplanes, gliders and helicopters.)
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[edit] Hot air
The density of a gas can be reduced by raising its temperature while leaving the pressure unchanged (Boyle's law).
Heated air is widely used as a lifting gas in hot-air balloons. (The gas in a hot-air balloon is not only heated air, but also includes the products of combustion from the balloon's burner.)
The altitude of a hot air balloon is controlled by regulating lift. To increase lift, more heat is applied. To decrease lift slowly, the hot air is allowed to cool. To decrease lift quickly, hot air is vented. Unlike balloons using low molecular mass gases (see below), hot air balloons require continual burning of fuel in order to remain aloft.
[edit] Low molecular mass gases
Because any given volume of any gas at a given temperature and pressure contains the same number of molecules (Avogadro's law), any gas with a lower molecular mass than that of air will be lighter than air (at the same temperature and pressure).
A sealed balloon expands as it rises because air pressure decreases with increasing altitude. As buoyancy depends on the mass of the displaced gas (Archimedes' principle) and because air is less dense at higher altitudes, balloons which rise to high altitudes (such as weather balloons) have to be allowed to expand as they climb so as to support the same weight. Weather balloons are made with strong elastic "envelopes" so that they do not burst as they expand.
Determining which gases are lighter than air is relatively straightforward. These gases must have a molecular mass less than 28.97 (the average molecular mass of air) and exist as a gas at atmospheric temperatures and atmospheric pressures.
Assuming one atom per molecule of gas, the heaviest possible atom that could meet these criteria is silicon, which has an atomic mass of 28.1. However, silicon does not become a gas until it reaches a very high temperature. The same applies to the metals aluminum, magnesium, sodium, beryllium and lithium and the hydrides of these. Carbon and boron have high boiling points, but methane and borane (the hydrides of carbon and boron) are lighter than air.
The following is a list of all stable materials with a molecular mass under 28.8 and a boiling point under 100°C. Isotopes are not considered here.
| Compound | Formula | Mass | Comments |
|---|---|---|---|
| Nitrogen | N2 | 28 | Majority component of air (~78%) |
| Carbon monoxide | CO | 28 | Toxic, flammable |
| Ethylene | C2H4 | 28 | Flammable, reactive |
| Diborane | B2H6 | 27.6 | Spontaneously flammable in air |
| Hydrogen cyanide | HCN | 27 | Very toxic, flammable, and water soluble |
| Acetylene | C2H2 | 26 | Extremely flammable, reactive |
| Neon | Ne | 20.2 | Noble gas |
| Hydrogen fluoride | HF | 20 | Very toxic, very corrosive, and water soluble |
| Water | H2O | 18 | Liquid at room temperature |
| Ammonia | NH3 | 17 | Somewhat toxic, slightly flammable, and water soluble |
| Methane | CH4 | 16 | Flammable |
| Helium | He | 4 | Noble gas, expensive, very small size makes it prone to leakage, must be replenished often |
| Hydrogen | H2 | 2 | Very flammable, relatively inexpensive petroleum by-product, prone to leakage |
[edit] HAHAMICE
The acronym HAHAMICE was used to help emergency responders remember the gasses which are lighter than air. It stood for:
- H - Hydrogen
- A - Ammonia
- H - Helium
- A - Acetylene
- M - Methane
- I - Illuminating Gases (old term for natural gas)
- C - Carbon Monoxide
- E - Ethylene
This acronym left out several gases, and was later changed to 4H MEDIC ANNA:
- H - Hydrogen
- H - Helium
- H - Hydrogen Cyanide
- H - Hydrogen Fluoride
- M - Methane
- E - Ethylene
- D - Diborane
- I - Illuminating Gases
- C - Carbon Monoxide
- A - Acetylene
- N - Neon
- N - Nitrogen
- A - Ammonia
As is noticeable, the acronyms do not include all lighter than air gases.
Many of these gases are not practical for use in balloons. The following combine poor lift with objectionable properties: carbon monoxide, hydrogen cyanide, hydrogen fluoride, diborane, ethylene and acetylene. Nitrogen has negligible lift. Neon is harmless and offers a modest degree of lift; however it costs roughly the same as helium, another noble gas with far superior lift. The four remaining gases (ammonia, methane, helium, and hydrogen) have been used as balloon gases.
Ammonia has sometimes been used to fill weather balloons. Due to its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).
Methane (the chief component of natural gas) is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the small-moleculed hydrogen and helium. (Most lighter than air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons.)
[edit] Hydrogen and helium
Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. (Both provide about 1 kilogram of lift per cubic meter of gas at room temperature and sea level pressure.) Helium is preferred because it is not combustible.
Many countries have banned the use of hydrogen as a lift gas for manned vehicles. The Hindenburg disaster is frequently cited as an example of the risks posed by hydrogen. The high cost of helium (compared to hydrogen) has led researchers to reinvestigate the safety issues of using hydrogen as a lift gas: with good engineering and good handling practices, the risks can be significantly reduced. It has been suggested that policy might allow hydrogen for cargo airships (both those unmanned and those manned only by pilots) and require helium for passenger airships.
[edit] Water vapour
Although pure water is not a gas at room temperature and sea level pressure, water in the vapor phase mixes readily with dry air, as do any two gases, until the partial pressure of the water vapor reaches the saturation water vapor pressure at the current temperature. Such moist air is lighter than dry air at the same temperature, because the molecular mass of water is lower than the average molecular mass of dry air. Most hot air balloons burn propane (or some other hydrocarbon) to provide heat; the combustion products have an average molecular mass of 29.1; the "light" water vapor more or less compensates for the "heavy" carbon dioxide. Pure water vapor (steam) can be used to lift balloons; however, the question of condensation must be addressed somehow. One route would be simply to tolerate the condensation - this supposes a rather large balloon. Alternatively, the balloon could provide insulation (for example have a double-walled structure) or the water vapour (if pure) could be maintained at least at the boiling point of water at the altitude of use (100 degrees C at sea level; less higher) by a heating device. [There are intriguing possibilities in using a mixture of air and water vapor, at a temperature high enough for the water component to remain as vapor.] Two research efforts are currently underway to build steam-filled balloons (see external links below), taking rather different approaches; both of them have succeeded in practical demonstration of steam as a lift gas.
[edit] Low pressure buoyancy
The average density of an aircraft can be reduced, at least in principle, by creating a partial vacuum. To be lighter than air, the envelope of such a device would have to be strong enough to resist crushing by atmospheric pressure yet light enough for the net density of the craft to be less than that of the surrounding air. The concept of an airship supported by the buoyancy of a vacuum has been explored in science fiction but no such device has ever been constructed. Each m³ of pure vacuum would lift by displacement approximately 1.2 kg (see air density), while 1 m³ of hydrogen displaces approximately 1.1kg and 1 m³ of helium lifts by displacement ~1kg meaning that the net difference in lift between vacuum and the lightest gases is too small to pursue vacuum displacement given current technology limitations.
[edit] Derivation
The ideal gas laws (Avogadro's law, Boyle's law, and Charles's law) are used to approximate gas density.
These three laws show that a gas with low density can be achieved by:
- lowering the pressure (Boyle's law);
- raising the temperature (Charles's law); or
- reducing the molecular mass (Avogadro's law).
