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 and/or butane) turns from liquid to gas when it boils.
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.
Temperature – Propane & Butane Turn to GasHow 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
To boil, the liquid LPG draws heat from the steel walls of the gas bottle which, in turn, get heat from the ambient air.
LPG liquid boils and turns back into gas vapour when you release some of the pressure in the gas bottle by turning on your gas appliance.
As with water, the more heat that is applied, the more rapidly it boils, vaporising at a faster rate.
The vapour pressure in the bottle also increases with temperature, as explained below.
So, 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 using the gas.
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.
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.
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 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
18490kg
215L
70
140
175
211
246
281
190kg
499L
106219
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
5003
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.
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!
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
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.
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.
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.
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.