How Does LPG – Propane Turn from Liquid to Gas?

LPG (Propane) Vaporisation

Did you know that every time you turn on one of your gas appliances, the LPG in your gas bottles starts to boil?

If you could see though the steel, you would also notice that it looks just like water boiling.

The big difference is that it happens at -42°C or -44°F.

This is vaporisation, which is how LPG – propane – goes from liquid to vapour (gas).

LPG – Propane & Butane Turn from Liquid to Gas

LPG (propane and/or butane) turns from liquid to gas when it boils.

However, propane and butane boil at different temperatures.

Propane boils at a lower temperature than butane, making it more suitable for cold climates.

On a cold winter day, outdoor butane cylinders may not reach their boiling temperature, leaving the user with no gas.

How Liquid LPG Gets Converted to Gas – LPG (Propane) Vaporisation

Liquid LPG boils and turns back into gas vapour when you release some of the pressure in the gas bottle, by turning on your gas appliance.

The vapour pressure in the bottle also increases with temperature, as explained below.

How LPG-Propane Boils

To boil, the liquid LPG draws heat from the steel walls of the gas bottle which, in turn, get heat from the ambient air.

As with water, the more heat that is applied, the more rapidly it boils, vaporising at a faster rate.

As the steel of the bottle draws heat from the ambient air heat, cold weather will slow down the rate of vaporisation.

Vaporisation also makes the gas bottle feel colder than the ambient temperature.

The gas bottle gets even colder when you are actually in the process of using the gas.

LPG Boiling Point

Water boils at 100°C or 212°F, becoming a gas (steam).

In contrast, LPG (propane) boils at -42°C or -44°F, becoming gas vapour.

LPG stays liquid because it is under pressure in a gas cylinder.

As a liquid, it looks a lot like water.

It is colourless and odourless in its natural state.

Unlike water, 1 kilogram of LPG does NOT equal 1 litre of liquid LPG.

LPG density or specific gravity is about half that of water, at 0.51.

In Australia, where LPG is propane, 1kg of LPG has a volume of 1.96L.

Conversely, 1L of LPG weighs 0.51kg.

Vaporisation Must Match Consumption

The amount of gas that the appliance or appliances are drawing from the gas bottles must be matched by the rate of vaporisation.

If a gas bottle ices up regularly, it simply means that the vessel is too small for the vaporisation load placed on it.

Switching to a larger vessel can provide a higher rate of vaporisation.

Heat is absorbed through the vessel shell and into the liquid.

This is known as the “wetted area”.

The larger the tank or the fuller the tank, the more gas that can be vaporised at a given temperature.

Vaporisation tables (as shown below) are used to match the required vaporisation rates to the corresponding vessel size.

Vaporisation tables show the maximum continuous vaporisation rates, in MJ/hr, at different ambient temperatures for each available vessel size.

In instances where a larger vessel is not an option, the only alternative is to supply some artificial means of increasing vaporisation.

The units used are very appropriately call vapourisers.

Lower Fill Equals Less Vaporisation

Keeping in mind the concept of “wetted area”, the maximum rate of vaporisation drops as the fill level drops.

Larger gas bottles have higher vaporisation than small gas bottles e.g. 45kg vs 9kg.

If there is less LPG in the vessel, there is less contact area between the liquid LPG and the steel that provides the heat for vaporisation.

Depending on the consumption rate of the attached appliances, this may make no difference at all, if the consumption rate is low.

However, if the consumption rate is high, the vaporisation rate may not keep pace with the consumption.

Depending on the appliance, this starvation may cause the appliance to function poorly or not at all.

LPG (Propane) Vaporisation Table

Vaporisation Table Notes:

1. As a simple rule of thumb, when using vessels of say 2.75 or 5.1kL capacities, simply extrapolate between the two nearest size vessels but biasing your calculations on the conservative side. Always consult your supplier’s technical representative for advice.

2. Always check with your supplier’s technical representative that the above vapourisation rates are correct for the particular vessel you have designated.

3. For sites requiring a high vapourisation rate but it is not cost effective to install larger and/or multiple vessels, consider using a vapouriser.

4. Vessels above 3 tonnes or over 7.5kL will be custom designed by supplier to suit customer needs. Figures provided are only rough estimates, based on previous designs.

Condensation Turns to Ice

Initially, condensation forms when the temperature of the gas bottle or regulator drops below the dew point.

This is exactly the same as the condensation you get on a humid day with a glass of ice water.

Under the right conditions, when you are using gas very rapidly, ice can even form on the gas bottle!

LPG Vapour vs Gas

Let’s also clarify the terminology.

The two terms, vapour and gas, are used interchangeably by most people in reference to LPG.

Vapour (or vapor in American spelling) is the more technically correct term for LPG, as it is in gaseous and liquid equilibrium at room temperature.

It can be turned back into a liquid by increasing the pressure on it without reducing the temperature.

A gas has one defined state at room temperature.

So, vapours are gases however not all gases are vapours

Gas Bottles Contain Liquid and Gas

The LPG gas vapour is held in the top of the bottle and the liquid LPG at the bottom, as shown in the accompanying image.

Almost all of the uses for LPG involve the use of the gas vapour, not the liquefied gas.

LPG Pressure Varies with Temperature

As previously mentioned, when LPG is stored in a gas bottle, it is under pressure.

The term “pressure” refers to the average force per unit of area that the gas exerts on the inside walls of the gas bottle.

(LPG Pressure-Temperature Chart shown)

Pressure is measured in kilopascals (kPa) or pounds per square inch (psi).

“Bar” is yet another unit of measure for pressure.

1 Bar = 100 kPa, so it is metric based but not an SI unit of measure.

LPG pressure can vary greatly based on temperature, as shown in the chart.

The level of fill in the gas bottle comes into play when the LPG is being used, as it affects the rate of vaporisation.

As LPG is a liquefied gas, the pressure inside the cylinder will remain the same from full until the last of the liquid LPG is vaporised.

Then the pressure will fall quickly as the last of the LPG vapour is used, as well.

Final Thoughts

Understanding vaporisation helps explain how LPG turns from liquid to gas.

It is particularly important for larger commercial installations where the rates of gas consumption are higher.

The technical staff matches the gas load to the appropriate vessel size and, if required, a vapouriser.