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  • Last Updated: 27 July 2021

LPG Vaporisation – How Does LPG – Propane Turn from Liquid to Gas?

LPG vaporisation is the process through which liquid LPG turns into gaseous LPG. The LPG liquid boils at -42°C or -44°F, by drawing heat from the walls of the cylinder, and turns into gas. LPG vaporisation happens when some of the cylinder pressure is relieved by turning on an appliance.
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).

Liquid Propane Temperature – What Temp is Liquid Propane

Liquid propane temperature must be below -42°C or -44°F, unless it is under pressure. Propane would be liquid in an open container in -43°C or -45°F or lower ambient temperature. Liquid propane can be warmer when contained under pressure in a gas cylinder.

LPG Gas Temperature When Propane & Butane Turn from Liquid to Gas – Liquid Propane Temperature

It is typically used as a gas (vapour) but there are liquid applications, as well. The pressure and temperature at which it is stored determines which kind you have. At 1 ATM, propane vaporises at-42°C.

LPG (propane and/or butane) turns from liquid to gas when the LPG gas temperature reaching the boiling point. However, propane and butane boil at different gas temperatures and are also referred to as natural gas liquids – NGL.

Propane boils at a lower gas 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.

 

LPG Gas Temperature – Liquid Propane Temperature

Propane & Butane Turn to Gas – Liquid Propane Temperature

LPG – Propane Turns
from Liquid or Gas?

LPG (1atm) Liquid Vapour (Gas)
Propane < -42°C or < -43.6ºF ≥ -42°C or ≥ -43.6ºF
Butane < -0.4°C or < 31.28ºF ≥ -0.4°C or ≥ 31.28ºF

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

Liquid LPG boils (liquid propane turns to gas) 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 – How Does Liquid Propane Turn to Gas

Liquid propane turns to gas by boiling and turning from a liquid to gas vapour, a process called vaporisation. 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 Gas Boiling Point Temperature

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

In contrast, LPG (propane) gas boiling point temperature is -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.

LPG Vaporizer How it Works – How Does a LPG Vaporizer Works

An LPG vaporizer works by adding more heat to the liquid LPG, for a higher vaporisation rate. An LPG vaporizer (vaporiser) works can be nothing more than a serpentine coil of LPG filled tubing, absorbing heat from ambient air or a heated water bath of about 60°C.

A heaterless LPG vaporizer works in much the same way except that the water is at ambient temperature. The liquid LPG is allowed to expand and cool rapidly and then the LPG extracts heat from the ambient temperature water tank. Cooling tower water can also be used for further energy savings.

A direct fired LPG vaporiser heats the propane liquid with a direct flame, which hastens the vaporisation of the propane liquid for use in equipment that has a high vapour requirement. The propane liquid is pumped from the LPG tank to the vaporiser for the increased vaporisation flow.

An LPG (propane) vaporizer is used when the steel walls of the vessel cannot provide sufficent heat to the liquid LPG to meet the required gas vaporisation load.

Lower Fill Equals Less Vaporisation

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

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

LPG Vaporisation Chart of Standard Size Vessels
Nominal LPG Vessel Size Volume in Water Capacity Maximum Continuous Vapourisation Rates for LPG (propane) at Indicated Ambient Temperatures.
(in MJ/hr)
at 30% full
Weight Volume -18˚C -7˚C -1˚C 4˚C 10˚C 16˚C
45kg 108L 46 92 115 138 161 184
90kg 215L 70 140 175 211 246 281
190kg 499L 106 219 274 328 383 438
0.5t 1.35kL 235 469 587 704 821 939
1.0t 2.2kL 327 653 816 980 1143 1306
2.0t 4.3kL 545 1090 1363 1636 1908 2181
2.5t 6.7kL 826 1652 2065 2478 2891 3304
3.0t 7.5kL 921 1841 2302 2762 3222 3683
10t 23kL 1616 3231 4039 4847 5655 6463
13t 33kL 2214 4482 5603 6724 7844 8965
17t 43kL 2502 2503 6300 7505 8756 10006
21t 53kL 3492 6984 8730 10476 12222 13968
25t 62kL 3502 7004 8755 10507 12258 14009
33t 81kL 4503 9006 11257 13509 15760 18011
40t 100kL 5504 11007 13759 16511 19262 22014

© 2013-2017 Elgas Ltd.

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.

Liquefaction: The Conversion of LPG Vapour to LPG Liquid

The conversion of LPG vapour to LPG liquid is called liquefaction, and depends on temperature and pressure of the vapour. The higher the temperature of the vapour, the higher the pressure needed to convert the vapour to liquid.

For propane vapour at 20°C it must be pressurised to about 836 kPa to see it liquefy.  At 50°C, about 1713 kPa pressure is required. The lower the temperature, the easier it is to liquefy the vapour.

For butane vapour at 20°C it must be pressurised to about 115 kPa to see it liquefy.  At 50°C, about 510 kPa pressure is required.

For mixtures of propane and butane, the liquefaction conditions also depend on the composition of the mix, as well as the temperature and pressure of the vapours.

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 Gas 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.

 

 

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The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.