LPG is an abbreviation for ‘Liquefied Petroleum Gas’
It is a gas in normal conditions but is stored in cylinders under pressure as a liquid.
LPG is a fossil fuel closely linked to oil. About two thirds of the LPG is extracted directly from the Earth in the same way as ordinary natural gas.
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There are two basic types of LPG Propane and Butane. The difference in their properties means that they are particularly suited to specific uses.
The chemical composition of Propane is C3H8 and Butane is C4H10. The larger Butane molecule gives both gases differing properties
– Propane’s lower boiling point suits outdoor storage and is primarily used for central heating, cooking and numerous commercial applications. Butane, on the other hand doesn’t work in colder conditions, and is best used indoors and is perfect for powering indoor portable heaters, or used in warmer months for outdoor camping and cooking.
Butane having a larger molecule when burnt in air dissipates more heat (higher calorific value than propane and is proportional to the additional carbon atoms contained.
LPG is compressed under pressure into portable cylinders or road tankers which in turn fill static bulk tanks at point of use. When the gas is compressed it easily changes to a liquid state so allowing a large amount of gas (stored energy) to be contained in a relatively small space.
Likewise the reverse process occurs when decompressed or allowed to escape the cylinder or pressure vessel. The liquid gas boils and quickly reverts to its gaseous state (vaporise) and so LPG is often called a vapour.
The gas offtake decreases the gas pressure the pressure the pressurised container. This pressure is regenerated by liquid vaporisation. The vaporisation need energy or heat and this is provided the ambient air. This gives energy to the vessel to maintain the pressure but the heat exchange is not perfect as the temperature of the liquid starts decreases and consequently the pressure decreases.
Therefore cylinders and bulk tanks have recommended offtake rates of vapour withdraw to ensure that the ambient air is sufficient to heat the pressurised container maintaining the required force to power the downstream gas appliances.
The larger the pressurised container, the greater the surface area in contact with the liquid LPG then the more vapour can be generated.
To recap – LPG when in a suitable pressurised tank or cylinder it will remain liquid, occupying only a small space. Release some of the liquid and it will immediately boil and revert to its gaseous state, expanding as it does so. Liquid Propane expands to 250 times its liquid volume when reverting to gas. The reverse applies, as gaseous LPG will ‘shrink’ by 250 times when compressed and liquefaction takes place.
The liquid is boiling, releasing vapour (gas) as it does so just like boiling water releases steam, (vapour) albeit at a much cooler temperature. Propane will do this right down to minus 42 degrees C, which ensures that it will vaporise in all but the coldest climates and conditions.
Butane however will only vaporise readily from temperatures 0 – 2 degrees Celsius, so when stored outdoors appliances in the winter will need a propane supply.
Installers Technical Tip – Commercial propane contains some butane and occasionally remains as a liquid in containers during sustained cold weather conditions. Propane having a lower boiling point preferentially evaporates first. Only when the remaining butane liquid warms up will it start to produce pressure. This can sometimes be seen when an automatic changeover switches to reserve cylinder bank before all the contents has emptied normally in very cold weather conditions. However once the cylinder has warmed then the residual butane will evaporate providing sufficient pressure to pass through the regulator to the appliances.